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Redis modules API

Introduction to writing Redis modules

The modules documentation is composed of the following pages:

  • Introduction to Redis modules (this file). An overview about Redis Modules system and API. It’s a good idea to start your reading here.
  • Implementing native data types covers the implementation of native data types into modules.
  • Blocking operations shows how to write blocking commands that will not reply immediately, but will block the client, without blocking the Redis server, and will provide a reply whenever will be possible.
  • Redis modules API reference is generated from module.c top comments of RedisModule functions. It is a good reference in order to understand how each function works.

Redis modules make it possible to extend Redis functionality using external modules, rapidly implementing new Redis commands with features similar to what can be done inside the core itself.

Redis modules are dynamic libraries that can be loaded into Redis at startup, or using the MODULE LOAD command. Redis exports a C API, in the form of a single C header file called redismodule.h. Modules are meant to be written in C, however it will be possible to use C++ or other languages that have C binding functionalities.

Modules are designed in order to be loaded into different versions of Redis, so a given module does not need to be designed, or recompiled, in order to run with a specific version of Redis. For this reason, the module will register to the Redis core using a specific API version. The current API version is “1”.

This document is about an alpha version of Redis modules. API, functionalities and other details may change in the future.

Loading modules

In order to test the module you are developing, you can load the module using the following redis.conf configuration directive:

loadmodule /path/to/mymodule.so

It is also possible to load a module at runtime using the following command:

MODULE LOAD /path/to/mymodule.so

In order to list all loaded modules, use:

MODULE LIST

Finally, you can unload (and later reload if you wish) a module using the following command:

MODULE UNLOAD mymodule

Note that mymodule above is not the filename without the .so suffix, but instead, the name the module used to register itself into the Redis core. The name can be obtained using MODULE LIST. However it is good practice that the filename of the dynamic library is the same as the name the module uses to register itself into the Redis core.

The simplest module you can write

In order to show the different parts of a module, here we’ll show a very simple module that implements a command that outputs a random number.

#include "redismodule.h"
#include <stdlib.h>

int HelloworldRand_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    RedisModule_ReplyWithLongLong(ctx,rand());
    return REDISMODULE_OK;
}

int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    if (RedisModule_Init(ctx,"helloworld",1,REDISMODULE_APIVER_1)
        == REDISMODULE_ERR) return REDISMODULE_ERR;

    if (RedisModule_CreateCommand(ctx,"helloworld.rand",
        HelloworldRand_RedisCommand, "fast random",
        0, 0, 0) == REDISMODULE_ERR)
        return REDISMODULE_ERR;

    return REDISMODULE_OK;
}

The example module has two functions. One implements a command called HELLOWORLD.RAND. This function is specific of that module. However the other function called RedisModule_OnLoad() must be present in each Redis module. It is the entry point for the module to be initialized, register its commands, and potentially other private data structures it uses.

Note that it is a good idea for modules to call commands with the name of the module followed by a dot, and finally the command name, like in the case of HELLOWORLD.RAND. This way it is less likely to have collisions.

Note that if different modules have colliding commands, they’ll not be able to work in Redis at the same time, since the function RedisModule_CreateCommand will fail in one of the modules, so the module loading will abort returning an error condition.

Module initialization

The above example shows the usage of the function RedisModule_Init(). It should be the first function called by the module OnLoad function. The following is the function prototype:

int RedisModule_Init(RedisModuleCtx *ctx, const char *modulename,
                     int module_version, int api_version);

The Init function announces the Redis core that the module has a given name, its version (that is reported by MODULE LIST), and that is willing to use a specific version of the API.

If the API version is wrong, the name is already taken, or there are other similar errors, the function will return REDISMODULE_ERR, and the module OnLoad function should return ASAP with an error.

Before the Init function is called, no other API function can be called, otherwise the module will segfault and the Redis instance will crash.

The second function called, RedisModule_CreateCommand, is used in order to register commands into the Redis core. The following is the prototype:

int RedisModule_CreateCommand(RedisModuleCtx *ctx, const char *name,
                              RedisModuleCmdFunc cmdfunc, const char *strflags,
                              int firstkey, int lastkey, int keystep);

As you can see, most Redis modules API calls all take as first argument the context of the module, so that they have a reference to the module calling it, to the command and client executing a given command, and so forth.

To create a new command, the above function needs the context, the command’s name, a pointer to the function implementing the command, the command’s flags and the positions of key names in the command’s arguments.

The function that implements the command must have the following prototype:

int mycommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc);

The command function arguments are just the context, that will be passed to all the other API calls, the command argument vector, and total number of arguments, as passed by the user.

As you can see, the arguments are provided as pointers to a specific data type, the RedisModuleString. This is an opaque data type you have API functions to access and use, direct access to its fields is never needed.

Zooming into the example command implementation, we can find another call:

int RedisModule_ReplyWithLongLong(RedisModuleCtx *ctx, long long integer);

This function returns an integer to the client that invoked the command, exactly like other Redis commands do, like for example INCR or SCARD.

Module cleanup

In most cases, there is no need for special cleanup. When a module is unloaded, Redis will automatically unregister commands and unsubscribe from notifications. However in the case where a module contains some persistent memory or configuration, a module may include an optional RedisModule_OnUnload function. If a module provides this function, it will be invoked during the module unload process. The following is the function prototype:

int RedisModule_OnUnload(RedisModuleCtx *ctx);

The OnUnload function may prevent module unloading by returning REDISMODULE_ERR. Otherwise, REDISMODULE_OK should be returned.

Setup and dependencies of a Redis module

Redis modules don’t depend on Redis or some other library, nor they need to be compiled with a specific redismodule.h file. In order to create a new module, just copy a recent version of redismodule.h in your source tree, link all the libraries you want, and create a dynamic library having the RedisModule_OnLoad() function symbol exported.

The module will be able to load into different versions of Redis.

A module can be designed to support both newer and older Redis versions where certain API functions are not available in all versions. If an API function is not implemented in the currently running Redis version, the function pointer is set to NULL. This allows the module to check if a function exists before using it:

if (RedisModule_SetCommandInfo != NULL) {
    RedisModule_SetCommandInfo(cmd, &info);
}

In recent versions of redismodule.h, a convenience macro RMAPI_FUNC_SUPPORTED(funcname) is defined. Using the macro or just comparing with NULL is a matter of personal preference.

Passing configuration parameters to Redis modules

When the module is loaded with the MODULE LOAD command, or using the loadmodule directive in the redis.conf file, the user is able to pass configuration parameters to the module by adding arguments after the module file name:

loadmodule mymodule.so foo bar 1234

In the above example the strings foo, bar and 1234 will be passed to the module OnLoad() function in the argv argument as an array of RedisModuleString pointers. The number of arguments passed is into argc.

The way you can access those strings will be explained in the rest of this document. Normally the module will store the module configuration parameters in some static global variable that can be accessed module wide, so that the configuration can change the behavior of different commands.

Working with RedisModuleString objects

The command argument vector argv passed to module commands, and the return value of other module APIs functions, are of type RedisModuleString.

Usually you directly pass module strings to other API calls, however sometimes you may need to directly access the string object.

There are a few functions in order to work with string objects:

const char *RedisModule_StringPtrLen(RedisModuleString *string, size_t *len);

The above function accesses a string by returning its pointer and setting its length in len. You should never write to a string object pointer, as you can see from the const pointer qualifier.

However, if you want, you can create new string objects using the following API:

RedisModuleString *RedisModule_CreateString(RedisModuleCtx *ctx, const char *ptr, size_t len);

The string returned by the above command must be freed using a corresponding call to RedisModule_FreeString():

void RedisModule_FreeString(RedisModuleString *str);

However if you want to avoid having to free strings, the automatic memory management, covered later in this document, can be a good alternative, by doing it for you.

Note that the strings provided via the argument vector argv never need to be freed. You only need to free new strings you create, or new strings returned by other APIs, where it is specified that the returned string must be freed.

Creating strings from numbers or parsing strings as numbers

Creating a new string from an integer is a very common operation, so there is a function to do this:

RedisModuleString *mystr = RedisModule_CreateStringFromLongLong(ctx,10);

Similarly in order to parse a string as a number:

long long myval;
if (RedisModule_StringToLongLong(ctx,argv[1],&myval) == REDISMODULE_OK) {
    /* Do something with 'myval' */
}

Accessing Redis keys from modules

Most Redis modules, in order to be useful, have to interact with the Redis data space (this is not always true, for example an ID generator may never touch Redis keys). Redis modules have two different APIs in order to access the Redis data space, one is a low level API that provides very fast access and a set of functions to manipulate Redis data structures. The other API is more high level, and allows to call Redis commands and fetch the result, similarly to how Lua scripts access Redis.

The high level API is also useful in order to access Redis functionalities that are not available as APIs.

In general modules developers should prefer the low level API, because commands implemented using the low level API run at a speed comparable to the speed of native Redis commands. However there are definitely use cases for the higher level API. For example often the bottleneck could be processing the data and not accessing it.

Also note that sometimes using the low level API is not harder compared to the higher level one.

Calling Redis commands

The high level API to access Redis is the sum of the RedisModule_Call() function, together with the functions needed in order to access the reply object returned by Call().

RedisModule_Call uses a special calling convention, with a format specifier that is used to specify what kind of objects you are passing as arguments to the function.

Redis commands are invoked just using a command name and a list of arguments. However when calling commands, the arguments may originate from different kind of strings: null-terminated C strings, RedisModuleString objects as received from the argv parameter in the command implementation, binary safe C buffers with a pointer and a length, and so forth.

For example if I want to call INCRBY using a first argument (the key) a string received in the argument vector argv, which is an array of RedisModuleString object pointers, and a C string representing the number “10” as second argument (the increment), I’ll use the following function call:

RedisModuleCallReply *reply;
reply = RedisModule_Call(ctx,"INCRBY","sc",argv[1],"10");

The first argument is the context, and the second is always a null terminated C string with the command name. The third argument is the format specifier where each character corresponds to the type of the arguments that will follow. In the above case "sc" means a RedisModuleString object, and a null terminated C string. The other arguments are just the two arguments as specified. In fact argv[1] is a RedisModuleString and "10" is a null terminated C string.

This is the full list of format specifiers:

  • c – Null terminated C string pointer.
  • b – C buffer, two arguments needed: C string pointer and size_t length.
  • s – RedisModuleString as received in argv or by other Redis module APIs returning a RedisModuleString object.
  • l – Long long integer.
  • v – Array of RedisModuleString objects.
  • ! – This modifier just tells the function to replicate the command to replicas and AOF. It is ignored from the point of view of arguments parsing.
  • A – This modifier, when ! is given, tells to suppress AOF propagation: the command will be propagated only to replicas.
  • R – This modifier, when ! is given, tells to suppress replicas propagation: the command will be propagated only to the AOF if enabled.

The function returns a RedisModuleCallReply object on success, on error NULL is returned.

NULL is returned when the command name is invalid, the format specifier uses characters that are not recognized, or when the command is called with the wrong number of arguments. In the above cases the errno var is set to EINVAL. NULL is also returned when, in an instance with Cluster enabled, the target keys are about non local hash slots. In this case errno is set to EPERM.

Working with RedisModuleCallReply objects.

RedisModuleCall returns reply objects that can be accessed using the RedisModule_CallReply* family of functions.

In order to obtain the type or reply (corresponding to one of the data types supported by the Redis protocol), the function RedisModule_CallReplyType() is used:

reply = RedisModule_Call(ctx,"INCRBY","sc",argv[1],"10");
if (RedisModule_CallReplyType(reply) == REDISMODULE_REPLY_INTEGER) {
    long long myval = RedisModule_CallReplyInteger(reply);
    /* Do something with myval. */
}

Valid reply types are:

  • REDISMODULE_REPLY_STRING Bulk string or status replies.
  • REDISMODULE_REPLY_ERROR Errors.
  • REDISMODULE_REPLY_INTEGER Signed 64 bit integers.
  • REDISMODULE_REPLY_ARRAY Array of replies.
  • REDISMODULE_REPLY_NULL NULL reply.

Strings, errors and arrays have an associated length. For strings and errors the length corresponds to the length of the string. For arrays the length is the number of elements. To obtain the reply length the following function is used:

size_t reply_len = RedisModule_CallReplyLength(reply);

In order to obtain the value of an integer reply, the following function is used, as already shown in the example above:

long long reply_integer_val = RedisModule_CallReplyInteger(reply);

Called with a reply object of the wrong type, the above function always returns LLONG_MIN.

Sub elements of array replies are accessed this way:

RedisModuleCallReply *subreply;
subreply = RedisModule_CallReplyArrayElement(reply,idx);

The above function returns NULL if you try to access out of range elements.

Strings and errors (which are like strings but with a different type) can be accessed using in the following way, making sure to never write to the resulting pointer (that is returned as as const pointer so that misusing must be pretty explicit):

size_t len;
char *ptr = RedisModule_CallReplyStringPtr(reply,&len);

If the reply type is not a string or an error, NULL is returned.

RedisCallReply objects are not the same as module string objects (RedisModuleString types). However sometimes you may need to pass replies of type string or integer, to API functions expecting a module string.

When this is the case, you may want to evaluate if using the low level API could be a simpler way to implement your command, or you can use the following function in order to create a new string object from a call reply of type string, error or integer:

RedisModuleString *mystr = RedisModule_CreateStringFromCallReply(myreply);

If the reply is not of the right type, NULL is returned. The returned string object should be released with RedisModule_FreeString() as usually, or by enabling automatic memory management (see corresponding section).

Releasing call reply objects

Reply objects must be freed using RedisModule_FreeCallReply. For arrays, you need to free only the top level reply, not the nested replies. Currently the module implementation provides a protection in order to avoid crashing if you free a nested reply object for error, however this feature is not guaranteed to be here forever, so should not be considered part of the API.

If you use automatic memory management (explained later in this document) you don’t need to free replies (but you still could if you wish to release memory ASAP).

Returning values from Redis commands

Like normal Redis commands, new commands implemented via modules must be able to return values to the caller. The API exports a set of functions for this goal, in order to return the usual types of the Redis protocol, and arrays of such types as elements. Also errors can be returned with any error string and code (the error code is the initial uppercase letters in the error message, like the “BUSY” string in the “BUSY the sever is busy” error message).

All the functions to send a reply to the client are called RedisModule_ReplyWith<something>.

To return an error, use:

RedisModule_ReplyWithError(RedisModuleCtx *ctx, const char *err);

There is a predefined error string for key of wrong type errors:

REDISMODULE_ERRORMSG_WRONGTYPE

Example usage:

RedisModule_ReplyWithError(ctx,"ERR invalid arguments");

We already saw how to reply with a long long in the examples above:

RedisModule_ReplyWithLongLong(ctx,12345);

To reply with a simple string, that can’t contain binary values or newlines, (so it’s suitable to send small words, like “OK”) we use:

RedisModule_ReplyWithSimpleString(ctx,"OK");

It’s possible to reply with “bulk strings” that are binary safe, using two different functions:

int RedisModule_ReplyWithStringBuffer(RedisModuleCtx *ctx, const char *buf, size_t len);

int RedisModule_ReplyWithString(RedisModuleCtx *ctx, RedisModuleString *str);

The first function gets a C pointer and length. The second a RedisModuleString object. Use one or the other depending on the source type you have at hand.

In order to reply with an array, you just need to use a function to emit the array length, followed by as many calls to the above functions as the number of elements of the array are:

RedisModule_ReplyWithArray(ctx,2);
RedisModule_ReplyWithStringBuffer(ctx,"age",3);
RedisModule_ReplyWithLongLong(ctx,22);

To return nested arrays is easy, your nested array element just uses another call to RedisModule_ReplyWithArray() followed by the calls to emit the sub array elements.

Returning arrays with dynamic length

Sometimes it is not possible to know beforehand the number of items of an array. As an example, think of a Redis module implementing a FACTOR command that given a number outputs the prime factors. Instead of factorializing the number, storing the prime factors into an array, and later produce the command reply, a better solution is to start an array reply where the length is not known, and set it later. This is accomplished with a special argument to RedisModule_ReplyWithArray():

RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_LEN);

The above call starts an array reply so we can use other ReplyWith calls in order to produce the array items. Finally in order to set the length, use the following call:

RedisModule_ReplySetArrayLength(ctx, number_of_items);

In the case of the FACTOR command, this translates to some code similar to this:

RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_LEN);
number_of_factors = 0;
while(still_factors) {
    RedisModule_ReplyWithLongLong(ctx, some_factor);
    number_of_factors++;
}
RedisModule_ReplySetArrayLength(ctx, number_of_factors);

Another common use case for this feature is iterating over the arrays of some collection and only returning the ones passing some kind of filtering.

It is possible to have multiple nested arrays with postponed reply. Each call to SetArray() will set the length of the latest corresponding call to ReplyWithArray():

RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_LEN);
... generate 100 elements ...
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_LEN);
... generate 10 elements ...
RedisModule_ReplySetArrayLength(ctx, 10);
RedisModule_ReplySetArrayLength(ctx, 100);

This creates a 100 items array having as last element a 10 items array.

Arity and type checks

Often commands need to check that the number of arguments and type of the key is correct. In order to report a wrong arity, there is a specific function called RedisModule_WrongArity(). The usage is trivial:

if (argc != 2) return RedisModule_WrongArity(ctx);

Checking for the wrong type involves opening the key and checking the type:

RedisModuleKey *key = RedisModule_OpenKey(ctx,argv[1],
    REDISMODULE_READ|REDISMODULE_WRITE);

int keytype = RedisModule_KeyType(key);
if (keytype != REDISMODULE_KEYTYPE_STRING &&
    keytype != REDISMODULE_KEYTYPE_EMPTY)
{
    RedisModule_CloseKey(key);
    return RedisModule_ReplyWithError(ctx,REDISMODULE_ERRORMSG_WRONGTYPE);
}

Note that you often want to proceed with a command both if the key is of the expected type, or if it’s empty.

Low level access to keys

Low level access to keys allow to perform operations on value objects associated to keys directly, with a speed similar to what Redis uses internally to implement the built-in commands.

Once a key is opened, a key pointer is returned that will be used with all the other low level API calls in order to perform operations on the key or its associated value.

Because the API is meant to be very fast, it cannot do too many run-time checks, so the user must be aware of certain rules to follow:

  • Opening the same key multiple times where at least one instance is opened for writing, is undefined and may lead to crashes.
  • While a key is open, it should only be accessed via the low level key API. For example opening a key, then calling DEL on the same key using the RedisModule_Call() API will result into a crash. However it is safe to open a key, perform some operation with the low level API, closing it, then using other APIs to manage the same key, and later opening it again to do some more work.

In order to open a key the RedisModule_OpenKey function is used. It returns a key pointer, that we’ll use with all the next calls to access and modify the value:

RedisModuleKey *key;
key = RedisModule_OpenKey(ctx,argv[1],REDISMODULE_READ);

The second argument is the key name, that must be a RedisModuleString object. The third argument is the mode: REDISMODULE_READ or REDISMODULE_WRITE. It is possible to use | to bitwise OR the two modes to open the key in both modes. Currently a key opened for writing can also be accessed for reading but this is to be considered an implementation detail. The right mode should be used in sane modules.

You can open non existing keys for writing, since the keys will be created when an attempt to write to the key is performed. However when opening keys just for reading, RedisModule_OpenKey will return NULL if the key does not exist.

Once you are done using a key, you can close it with:

RedisModule_CloseKey(key);

Note that if automatic memory management is enabled, you are not forced to close keys. When the module function returns, Redis will take care to close all the keys which are still open.

Getting the key type

In order to obtain the value of a key, use the RedisModule_KeyType() function:

int keytype = RedisModule_KeyType(key);

It returns one of the following values:

REDISMODULE_KEYTYPE_EMPTY
REDISMODULE_KEYTYPE_STRING
REDISMODULE_KEYTYPE_LIST
REDISMODULE_KEYTYPE_HASH
REDISMODULE_KEYTYPE_SET
REDISMODULE_KEYTYPE_ZSET

The above are just the usual Redis key types, with the addition of an empty type, that signals the key pointer is associated with an empty key that does not yet exists.

Creating new keys

To create a new key, open it for writing and then write to it using one of the key writing functions. Example:

RedisModuleKey *key;
key = RedisModule_OpenKey(ctx,argv[1],REDISMODULE_WRITE);
if (RedisModule_KeyType(key) == REDISMODULE_KEYTYPE_EMPTY) {
    RedisModule_StringSet(key,argv[2]);
}

Deleting keys

Just use:

RedisModule_DeleteKey(key);

The function returns REDISMODULE_ERR if the key is not open for writing. Note that after a key gets deleted, it is setup in order to be targeted by new key commands. For example RedisModule_KeyType() will return it is an empty key, and writing to it will create a new key, possibly of another type (depending on the API used).

Managing key expires (TTLs)

To control key expires two functions are provided, that are able to set, modify, get, and unset the time to live associated with a key.

One function is used in order to query the current expire of an open key:

mstime_t RedisModule_GetExpire(RedisModuleKey *key);

The function returns the time to live of the key in milliseconds, or REDISMODULE_NO_EXPIRE as a special value to signal the key has no associated expire or does not exist at all (you can differentiate the two cases checking if the key type is REDISMODULE_KEYTYPE_EMPTY).

In order to change the expire of a key the following function is used instead:

int RedisModule_SetExpire(RedisModuleKey *key, mstime_t expire);

When called on a non existing key, REDISMODULE_ERR is returned, because the function can only associate expires to existing open keys (non existing open keys are only useful in order to create new values with data type specific write operations).

Again the expire time is specified in milliseconds. If the key has currently no expire, a new expire is set. If the key already have an expire, it is replaced with the new value.

If the key has an expire, and the special value REDISMODULE_NO_EXPIRE is used as a new expire, the expire is removed, similarly to the Redis PERSIST command. In case the key was already persistent, no operation is performed.

Obtaining the length of values

There is a single function in order to retrieve the length of the value associated to an open key. The returned length is value-specific, and is the string length for strings, and the number of elements for the aggregated data types (how many elements there is in a list, set, sorted set, hash).

size_t len = RedisModule_ValueLength(key);

If the key does not exist, 0 is returned by the function:

String type API

Setting a new string value, like the Redis SET command does, is performed using:

int RedisModule_StringSet(RedisModuleKey *key, RedisModuleString *str);

The function works exactly like the Redis SET command itself, that is, if there is a prior value (of any type) it will be deleted.

Accessing existing string values is performed using DMA (direct memory access) for speed. The API will return a pointer and a length, so that’s possible to access and, if needed, modify the string directly.

size_t len, j;
char *myptr = RedisModule_StringDMA(key,&len,REDISMODULE_WRITE);
for (j = 0; j < len; j++) myptr[j] = 'A';

In the above example we write directly on the string. Note that if you want to write, you must be sure to ask for WRITE mode.

DMA pointers are only valid if no other operations are performed with the key before using the pointer, after the DMA call.

Sometimes when we want to manipulate strings directly, we need to change their size as well. For this scope, the RedisModule_StringTruncate function is used. Example:

RedisModule_StringTruncate(mykey,1024);

The function truncates, or enlarges the string as needed, padding it with zero bytes if the previous length is smaller than the new length we request. If the string does not exist since key is associated to an open empty key, a string value is created and associated to the key.

Note that every time StringTruncate() is called, we need to re-obtain the DMA pointer again, since the old may be invalid.

List type API

It’s possible to push and pop values from list values:

int RedisModule_ListPush(RedisModuleKey *key, int where, RedisModuleString *ele);
RedisModuleString *RedisModule_ListPop(RedisModuleKey *key, int where);

In both the APIs the where argument specifies if to push or pop from tail or head, using the following macros:

REDISMODULE_LIST_HEAD
REDISMODULE_LIST_TAIL

Elements returned by RedisModule_ListPop() are like strings created with RedisModule_CreateString(), they must be released with RedisModule_FreeString() or by enabling automatic memory management.

Set type API

Work in progress.

Sorted set type API

Documentation missing, please refer to the top comments inside module.c for the following functions:

  • RedisModule_ZsetAdd
  • RedisModule_ZsetIncrby
  • RedisModule_ZsetScore
  • RedisModule_ZsetRem

And for the sorted set iterator:

  • RedisModule_ZsetRangeStop
  • RedisModule_ZsetFirstInScoreRange
  • RedisModule_ZsetLastInScoreRange
  • RedisModule_ZsetFirstInLexRange
  • RedisModule_ZsetLastInLexRange
  • RedisModule_ZsetRangeCurrentElement
  • RedisModule_ZsetRangeNext
  • RedisModule_ZsetRangePrev
  • RedisModule_ZsetRangeEndReached

Hash type API

Documentation missing, please refer to the top comments inside module.c for the following functions:

  • RedisModule_HashSet
  • RedisModule_HashGet

Iterating aggregated values

Work in progress.

Replicating commands

If you want to use module commands exactly like normal Redis commands, in the context of replicated Redis instances, or using the AOF file for persistence, it is important for module commands to handle their replication in a consistent way.

When using the higher level APIs to invoke commands, replication happens automatically if you use the “!” modifier in the format string of RedisModule_Call() as in the following example:

reply = RedisModule_Call(ctx,"INCRBY","!sc",argv[1],"10");

As you can see the format specifier is "!sc". The bang is not parsed as a format specifier, but it internally flags the command as “must replicate”.

If you use the above programming style, there are no problems. However sometimes things are more complex than that, and you use the low level API. In this case, if there are no side effects in the command execution, and it consistently always performs the same work, what is possible to do is to replicate the command verbatim as the user executed it. To do that, you just need to call the following function:

RedisModule_ReplicateVerbatim(ctx);

When you use the above API, you should not use any other replication function since they are not guaranteed to mix well.

However this is not the only option. It’s also possible to exactly tell Redis what commands to replicate as the effect of the command execution, using an API similar to RedisModule_Call() but that instead of calling the command sends it to the AOF / replicas stream. Example:

RedisModule_Replicate(ctx,"INCRBY","cl","foo",my_increment);

It’s possible to call RedisModule_Replicate multiple times, and each will emit a command. All the sequence emitted is wrapped between a MULTI/EXEC transaction, so that the AOF and replication effects are the same as executing a single command.

Note that Call() replication and Replicate() replication have a rule, in case you want to mix both forms of replication (not necessarily a good idea if there are simpler approaches). Commands replicated with Call() are always the first emitted in the final MULTI/EXEC block, while all the commands emitted with Replicate() will follow.

Automatic memory management

Normally when writing programs in the C language, programmers need to manage memory manually. This is why the Redis modules API has functions to release strings, close open keys, free replies, and so forth.

However given that commands are executed in a contained environment and with a set of strict APIs, Redis is able to provide automatic memory management to modules, at the cost of some performance (most of the time, a very low cost).

When automatic memory management is enabled:

  1. You don’t need to close open keys.
  2. You don’t need to free replies.
  3. You don’t need to free RedisModuleString objects.

However you can still do it, if you want. For example, automatic memory management may be active, but inside a loop allocating a lot of strings, you may still want to free strings no longer used.

In order to enable automatic memory management, just call the following function at the start of the command implementation:

RedisModule_AutoMemory(ctx);

Automatic memory management is usually the way to go, however experienced C programmers may not use it in order to gain some speed and memory usage benefit.

Allocating memory into modules

Normal C programs use malloc() and free() in order to allocate and release memory dynamically. While in Redis modules the use of malloc is not technically forbidden, it is a lot better to use the Redis Modules specific functions, that are exact replacements for malloc, free, realloc and strdup. These functions are:

void *RedisModule_Alloc(size_t bytes);
void* RedisModule_Realloc(void *ptr, size_t bytes);
void RedisModule_Free(void *ptr);
void RedisModule_Calloc(size_t nmemb, size_t size);
char *RedisModule_Strdup(const char *str);

They work exactly like their libc equivalent calls, however they use the same allocator Redis uses, and the memory allocated using these functions is reported by the INFO command in the memory section, is accounted when enforcing the maxmemory policy, and in general is a first citizen of the Redis executable. On the contrary, the method allocated inside modules with libc malloc() is transparent to Redis.

Another reason to use the modules functions in order to allocate memory is that, when creating native data types inside modules, the RDB loading functions can return deserialized strings (from the RDB file) directly as RedisModule_Alloc() allocations, so they can be used directly to populate data structures after loading, instead of having to copy them to the data structure.

Pool allocator

Sometimes in commands implementations, it is required to perform many small allocations that will be not retained at the end of the command execution, but are just functional to execute the command itself.

This work can be more easily accomplished using the Redis pool allocator:

void *RedisModule_PoolAlloc(RedisModuleCtx *ctx, size_t bytes);

It works similarly to malloc(), and returns memory aligned to the next power of two of greater or equal to bytes (for a maximum alignment of 8 bytes). However it allocates memory in blocks, so it the overhead of the allocations is small, and more important, the memory allocated is automatically released when the command returns.

So in general short living allocations are a good candidates for the pool allocator.

Writing commands compatible with Redis Cluster

Documentation missing, please check the following functions inside module.c:

RedisModule_IsKeysPositionRequest(ctx);
RedisModule_KeyAtPos(ctx,pos);

1 - Modules API reference

Auto-generated reference for the Redis Modules API

Sections

Heap allocation raw functions

Memory allocated with these functions are taken into account by Redis key eviction algorithms and are reported in Redis memory usage information.

RedisModule_Alloc

void *RedisModule_Alloc(size_t bytes);

Available since: 4.0.0

Use like malloc(). Memory allocated with this function is reported in Redis INFO memory, used for keys eviction according to maxmemory settings and in general is taken into account as memory allocated by Redis. You should avoid using malloc().

RedisModule_Calloc

void *RedisModule_Calloc(size_t nmemb, size_t size);

Available since: 4.0.0

Use like calloc(). Memory allocated with this function is reported in Redis INFO memory, used for keys eviction according to maxmemory settings and in general is taken into account as memory allocated by Redis. You should avoid using calloc() directly.

RedisModule_Realloc

void* RedisModule_Realloc(void *ptr, size_t bytes);

Available since: 4.0.0

Use like realloc() for memory obtained with RedisModule_Alloc().

RedisModule_Free

void RedisModule_Free(void *ptr);

Available since: 4.0.0

Use like free() for memory obtained by RedisModule_Alloc() and RedisModule_Realloc(). However you should never try to free with RedisModule_Free() memory allocated with malloc() inside your module.

RedisModule_Strdup

char *RedisModule_Strdup(const char *str);

Available since: 4.0.0

Like strdup() but returns memory allocated with RedisModule_Alloc().

RedisModule_PoolAlloc

void *RedisModule_PoolAlloc(RedisModuleCtx *ctx, size_t bytes);

Available since: 4.0.0

Return heap allocated memory that will be freed automatically when the module callback function returns. Mostly suitable for small allocations that are short living and must be released when the callback returns anyway. The returned memory is aligned to the architecture word size if at least word size bytes are requested, otherwise it is just aligned to the next power of two, so for example a 3 bytes request is 4 bytes aligned while a 2 bytes request is 2 bytes aligned.

There is no realloc style function since when this is needed to use the pool allocator is not a good idea.

The function returns NULL if bytes is 0.

Commands API

These functions are used to implement custom Redis commands.

For examples, see https://redis.io/topics/modules-intro.

RedisModule_IsKeysPositionRequest

int RedisModule_IsKeysPositionRequest(RedisModuleCtx *ctx);

Available since: 4.0.0

Return non-zero if a module command, that was declared with the flag “getkeys-api”, is called in a special way to get the keys positions and not to get executed. Otherwise zero is returned.

RedisModule_KeyAtPosWithFlags

void RedisModule_KeyAtPosWithFlags(RedisModuleCtx *ctx, int pos, int flags);

When a module command is called in order to obtain the position of keys, since it was flagged as “getkeys-api” during the registration, the command implementation checks for this special call using the RedisModule_IsKeysPositionRequest() API and uses this function in order to report keys.

The supported flags are the ones used by RedisModule_SetCommandInfo, see REDISMODULE_CMD_KEY_*.

The following is an example of how it could be used:

if (RedisModule_IsKeysPositionRequest(ctx)) {
    RedisModule_KeyAtPosWithFlags(ctx, 2, REDISMODULE_CMD_KEY_RO | REDISMODULE_CMD_KEY_ACCESS);
    RedisModule_KeyAtPosWithFlags(ctx, 1, REDISMODULE_CMD_KEY_RW | REDISMODULE_CMD_KEY_UPDATE | REDISMODULE_CMD_KEY_ACCESS);
}

Note: in the example above the get keys API could have been handled by key-specs (preferred). Implementing the getkeys-api is required only when is it not possible to declare key-specs that cover all keys.

RedisModule_KeyAtPos

void RedisModule_KeyAtPos(RedisModuleCtx *ctx, int pos);

Available since: 4.0.0

This API existed before RedisModule_KeyAtPosWithFlags was added, now deprecated and can be used for compatibility with older versions, before key-specs and flags were introduced.

RedisModule_IsChannelsPositionRequest

int RedisModule_IsChannelsPositionRequest(RedisModuleCtx *ctx);

Return non-zero if a module command, that was declared with the flag “getchannels-api”, is called in a special way to get the channel positions and not to get executed. Otherwise zero is returned.

RedisModule_ChannelAtPosWithFlags

void RedisModule_ChannelAtPosWithFlags(RedisModuleCtx *ctx,
                                       int pos,
                                       int flags);

When a module command is called in order to obtain the position of channels, since it was flagged as “getchannels-api” during the registration, the command implementation checks for this special call using the RedisModule_IsChannelsPositionRequest() API and uses this function in order to report the channels.

The supported flags are:

  • REDISMODULE_CMD_CHANNEL_SUBSCRIBE: This command will subscribe to the channel.
  • REDISMODULE_CMD_CHANNEL_UNSUBSCRIBE: This command will unsubscribe from this channel.
  • REDISMODULE_CMD_CHANNEL_PUBLISH: This command will publish to this channel.
  • REDISMODULE_CMD_CHANNEL_PATTERN: Instead of acting on a specific channel, will act on any channel specified by the pattern. This is the same access used by the PSUBSCRIBE and PUNSUBSCRIBE commands available in Redis. Not intended to be used with PUBLISH permissions.

The following is an example of how it could be used:

if (RedisModule_IsChannelsPositionRequest(ctx)) {
    RedisModule_ChannelAtPosWithFlags(ctx, 1, REDISMODULE_CMD_CHANNEL_SUBSCRIBE | REDISMODULE_CMD_CHANNEL_PATTERN);
    RedisModule_ChannelAtPosWithFlags(ctx, 1, REDISMODULE_CMD_CHANNEL_PUBLISH);
}

Note: One usage of declaring channels is for evaluating ACL permissions. In this context, unsubscribing is always allowed, so commands will only be checked against subscribe and publish permissions. This is preferred over using RedisModule_ACLCheckChannelPermissions, since it allows the ACLs to be checked before the command is executed.

RedisModule_CreateCommand

int RedisModule_CreateCommand(RedisModuleCtx *ctx,
                              const char *name,
                              RedisModuleCmdFunc cmdfunc,
                              const char *strflags,
                              int firstkey,
                              int lastkey,
                              int keystep);

Available since: 4.0.0

Register a new command in the Redis server, that will be handled by calling the function pointer ‘cmdfunc’ using the RedisModule calling convention. The function returns REDISMODULE_ERR if the specified command name is already busy or a set of invalid flags were passed, otherwise REDISMODULE_OK is returned and the new command is registered.

This function must be called during the initialization of the module inside the RedisModule_OnLoad() function. Calling this function outside of the initialization function is not defined.

The command function type is the following:

 int MyCommand_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc);

And is supposed to always return REDISMODULE_OK.

The set of flags ‘strflags’ specify the behavior of the command, and should be passed as a C string composed of space separated words, like for example “write deny-oom”. The set of flags are:

  • “write”: The command may modify the data set (it may also read from it).
  • “readonly”: The command returns data from keys but never writes.
  • “admin”: The command is an administrative command (may change replication or perform similar tasks).
  • “deny-oom”: The command may use additional memory and should be denied during out of memory conditions.
  • “deny-script”: Don’t allow this command in Lua scripts.
  • “allow-loading”: Allow this command while the server is loading data. Only commands not interacting with the data set should be allowed to run in this mode. If not sure don’t use this flag.
  • “pubsub”: The command publishes things on Pub/Sub channels.
  • “random”: The command may have different outputs even starting from the same input arguments and key values. Starting from Redis 7.0 this flag has been deprecated. Declaring a command as “random” can be done using command tips, see https://redis.io/topics/command-tips.
  • “allow-stale”: The command is allowed to run on slaves that don’t serve stale data. Don’t use if you don’t know what this means.
  • “no-monitor”: Don’t propagate the command on monitor. Use this if the command has sensible data among the arguments.
  • “no-slowlog”: Don’t log this command in the slowlog. Use this if the command has sensible data among the arguments.
  • “fast”: The command time complexity is not greater than O(log(N)) where N is the size of the collection or anything else representing the normal scalability issue with the command.
  • “getkeys-api”: The command implements the interface to return the arguments that are keys. Used when start/stop/step is not enough because of the command syntax.
  • “no-cluster”: The command should not register in Redis Cluster since is not designed to work with it because, for example, is unable to report the position of the keys, programmatically creates key names, or any other reason.
  • “no-auth”: This command can be run by an un-authenticated client. Normally this is used by a command that is used to authenticate a client.
  • “may-replicate”: This command may generate replication traffic, even though it’s not a write command.
  • “no-mandatory-keys”: All the keys this command may take are optional
  • “blocking”: The command has the potential to block the client.
  • “allow-busy”: Permit the command while the server is blocked either by a script or by a slow module command, see RM_Yield.
  • “getchannels-api”: The command implements the interface to return the arguments that are channels.

The last three parameters specify which arguments of the new command are Redis keys. See https://redis.io/commands/command for more information.

  • firstkey: One-based index of the first argument that’s a key. Position 0 is always the command name itself. 0 for commands with no keys.
  • lastkey: One-based index of the last argument that’s a key. Negative numbers refer to counting backwards from the last argument (-1 means the last argument provided) 0 for commands with no keys.
  • keystep: Step between first and last key indexes. 0 for commands with no keys.

This information is used by ACL, Cluster and the COMMAND command.

NOTE: The scheme described above serves a limited purpose and can only be used to find keys that exist at constant indices. For non-trivial key arguments, you may pass 0,0,0 and use RedisModule_SetCommandInfo to set key specs using a more advanced scheme.

RedisModule_GetCommand

RedisModuleCommand *RedisModule_GetCommand(RedisModuleCtx *ctx,
                                           const char *name);

Get an opaque structure, representing a module command, by command name. This structure is used in some of the command-related APIs.

NULL is returned in case of the following errors:

  • Command not found
  • The command is not a module command
  • The command doesn’t belong to the calling module

RedisModule_CreateSubcommand

int RedisModule_CreateSubcommand(RedisModuleCommand *parent,
                                 const char *name,
                                 RedisModuleCmdFunc cmdfunc,
                                 const char *strflags,
                                 int firstkey,
                                 int lastkey,
                                 int keystep);

Very similar to RedisModule_CreateCommand except that it is used to create a subcommand, associated with another, container, command.

Example: If a module has a configuration command, MODULE.CONFIG, then GET and SET should be individual subcommands, while MODULE.CONFIG is a command, but should not be registered with a valid funcptr:

 if (RedisModule_CreateCommand(ctx,"module.config",NULL,"",0,0,0) == REDISMODULE_ERR)
     return REDISMODULE_ERR;

 RedisModuleCommand *parent = RedisModule_GetCommand(ctx,,"module.config");

 if (RedisModule_CreateSubcommand(parent,"set",cmd_config_set,"",0,0,0) == REDISMODULE_ERR)
    return REDISMODULE_ERR;

 if (RedisModule_CreateSubcommand(parent,"get",cmd_config_get,"",0,0,0) == REDISMODULE_ERR)
    return REDISMODULE_ERR;

Returns REDISMODULE_OK on success and REDISMODULE_ERR in case of the following errors:

  • Error while parsing strflags
  • Command is marked as no-cluster but cluster mode is enabled
  • parent is already a subcommand (we do not allow more than one level of command nesting)
  • parent is a command with an implementation (RedisModuleCmdFunc) (A parent command should be a pure container of subcommands)
  • parent already has a subcommand called name

RedisModule_SetCommandInfo

int RedisModule_SetCommandInfo(RedisModuleCommand *command,
                               const RedisModuleCommandInfo *info);

Set additional command information.

Affects the output of COMMAND, COMMAND INFO and COMMAND DOCS, Cluster, ACL and is used to filter commands with the wrong number of arguments before the call reaches the module code.

This function can be called after creating a command using RedisModule_CreateCommand and fetching the command pointer using RedisModule_GetCommand. The information can only be set once for each command and has the following structure:

typedef struct RedisModuleCommandInfo {
    const RedisModuleCommandInfoVersion *version;
    const char *summary;
    const char *complexity;
    const char *since;
    RedisModuleCommandHistoryEntry *history;
    const char *tips;
    int arity;
    RedisModuleCommandKeySpec *key_specs;
    RedisModuleCommandArg *args;
} RedisModuleCommandInfo;

All fields except version are optional. Explanation of the fields:

  • version: This field enables compatibility with different Redis versions. Always set this field to REDISMODULE_COMMAND_INFO_VERSION.

  • summary: A short description of the command (optional).

  • complexity: Complexity description (optional).

  • since: The version where the command was introduced (optional). Note: The version specified should be the module’s, not Redis version.

  • history: An array of RedisModuleCommandHistoryEntry (optional), which is a struct with the following fields:

      const char *since;
  const char *changes;

    since is a version string and changes is a string describing the changes. The array is terminated by a zeroed entry, i.e. an entry with both strings set to NULL.

  • tips: A string of space-separated tips regarding this command, meant for clients and proxies. See https://redis.io/topics/command-tips.

  • arity: Number of arguments, including the command name itself. A positive number specifies an exact number of arguments and a negative number specifies a minimum number of arguments, so use -N to say >= N. Redis validates a call before passing it to a module, so this can replace an arity check inside the module command implementation. A value of 0 (or an omitted arity field) is equivalent to -2 if the command has sub commands and -1 otherwise.

  • key_specs: An array of RedisModuleCommandKeySpec, terminated by an element memset to zero. This is a scheme that tries to describe the positions of key arguments better than the old RedisModule_CreateCommand arguments firstkey, lastkey, keystep and is needed if those three are not enough to describe the key positions. There are two steps to retrieve key positions: begin search (BS) in which index should find the first key and find keys (FK) which, relative to the output of BS, describes how can we will which arguments are keys. Additionally, there are key specific flags.

    Key-specs cause the triplet (firstkey, lastkey, keystep) given in RM_CreateCommand to be recomputed, but it is still useful to provide these three parameters in RM_CreateCommand, to better support old Redis versions where RM_SetCommandInfo is not available.

    Note that key-specs don’t fully replace the “getkeys-api” (see RM_CreateCommand, RM_IsKeysPositionRequest and RM_KeyAtPosWithFlags) so it may be a good idea to supply both key-specs and implement the getkeys-api.

    A key-spec has the following structure:

      typedef struct RedisModuleCommandKeySpec {
      const char *notes;
      uint64_t flags;
      RedisModuleKeySpecBeginSearchType begin_search_type;
      union {
          struct {
              int pos;
          } index;
          struct {
              const char *keyword;
              int startfrom;
          } keyword;
      } bs;
      RedisModuleKeySpecFindKeysType find_keys_type;
      union {
          struct {
              int lastkey;
              int keystep;
              int limit;
          } range;
          struct {
              int keynumidx;
              int firstkey;
              int keystep;
          } keynum;
      } fk;
  } RedisModuleCommandKeySpec;

    Explanation of the fields of RedisModuleCommandKeySpec:

    • notes: Optional notes or clarifications about this key spec.

    • flags: A bitwise or of key-spec flags described below.

    • begin_search_type: This describes how the first key is discovered. There are two ways to determine the first key:

      • REDISMODULE_KSPEC_BS_UNKNOWN: There is no way to tell where the key args start.
      • REDISMODULE_KSPEC_BS_INDEX: Key args start at a constant index.
      • REDISMODULE_KSPEC_BS_KEYWORD: Key args start just after a specific keyword.

    • bs: This is a union in which the index or keyword branch is used depending on the value of the begin_search_type field.

      • bs.index.pos: The index from which we start the search for keys. (REDISMODULE_KSPEC_BS_INDEX only.)

      • bs.keyword.keyword: The keyword (string) that indicates the beginning of key arguments. (REDISMODULE_KSPEC_BS_KEYWORD only.)

      • bs.keyword.startfrom: An index in argv from which to start searching. Can be negative, which means start search from the end, in reverse. Example: -2 means to start in reverse from the penultimate argument. (REDISMODULE_KSPEC_BS_KEYWORD only.)

    • find_keys_type: After the “begin search”, this describes which arguments are keys. The strategies are:

      • REDISMODULE_KSPEC_BS_UNKNOWN: There is no way to tell where the key args are located.
      • REDISMODULE_KSPEC_FK_RANGE: Keys end at a specific index (or relative to the last argument).
      • REDISMODULE_KSPEC_FK_KEYNUM: There’s an argument that contains the number of key args somewhere before the keys themselves.

      find_keys_type and fk can be omitted if this keyspec describes exactly one key.

    • fk: This is a union in which the range or keynum branch is used depending on the value of the find_keys_type field.

      • fk.range (for REDISMODULE_KSPEC_FK_RANGE): A struct with the following fields:

        • lastkey: Index of the last key relative to the result of the begin search step. Can be negative, in which case it’s not relative. -1 indicates the last argument, -2 one before the last and so on.

        • keystep: How many arguments should we skip after finding a key, in order to find the next one?

        • limit: If lastkey is -1, we use limit to stop the search by a factor. 0 and 1 mean no limit. 2 means 1/2 of the remaining args, 3 means 1/3, and so on.

      • fk.keynum (for REDISMODULE_KSPEC_FK_KEYNUM): A struct with the following fields:

        • keynumidx: Index of the argument containing the number of keys to come, relative to the result of the begin search step.

        • firstkey: Index of the fist key relative to the result of the begin search step. (Usually it’s just after keynumidx, in which case it should be set to keynumidx + 1.)

        • keystep: How many argumentss should we skip after finding a key, in order to find the next one?

    Key-spec flags:

    The first four refer to what the command actually does with the value or metadata of the key, and not necessarily the user data or how it affects it. Each key-spec may must have exactly one of these. Any operation that’s not distinctly deletion, overwrite or read-only would be marked as RW.

    • REDISMODULE_CMD_KEY_RO: Read-Only. Reads the value of the key, but doesn’t necessarily return it.

    • REDISMODULE_CMD_KEY_RW: Read-Write. Modifies the data stored in the value of the key or its metadata.

    • REDISMODULE_CMD_KEY_OW: Overwrite. Overwrites the data stored in the value of the key.

    • REDISMODULE_CMD_KEY_RM: Deletes the key.

    The next four refer to user data inside the value of the key, not the metadata like LRU, type, cardinality. It refers to the logical operation on the user’s data (actual input strings or TTL), being used/returned/copied/changed. It doesn’t refer to modification or returning of metadata (like type, count, presence of data). ACCESS can be combined with one of the write operations INSERT, DELETE or UPDATE. Any write that’s not an INSERT or a DELETE would be UPDATE.

    • REDISMODULE_CMD_KEY_ACCESS: Returns, copies or uses the user data from the value of the key.

    • REDISMODULE_CMD_KEY_UPDATE: Updates data to the value, new value may depend on the old value.

    • REDISMODULE_CMD_KEY_INSERT: Adds data to the value with no chance of modification or deletion of existing data.

    • REDISMODULE_CMD_KEY_DELETE: Explicitly deletes some content from the value of the key.

    Other flags:

    • REDISMODULE_CMD_KEY_NOT_KEY: The key is not actually a key, but should be routed in cluster mode as if it was a key.

    • REDISMODULE_CMD_KEY_INCOMPLETE: The keyspec might not point out all the keys it should cover.

    • REDISMODULE_CMD_KEY_VARIABLE_FLAGS: Some keys might have different flags depending on arguments.

  • args: An array of RedisModuleCommandArg, terminated by an element memset to zero. RedisModuleCommandArg is a structure with at the fields described below.

      typedef struct RedisModuleCommandArg {
      const char *name;
      RedisModuleCommandArgType type;
      int key_spec_index;
      const char *token;
      const char *summary;
      const char *since;
      int flags;
      struct RedisModuleCommandArg *subargs;
  } RedisModuleCommandArg;

    Explanation of the fields:

    • name: Name of the argument.

    • type: The type of the argument. See below for details. The types REDISMODULE_ARG_TYPE_ONEOF and REDISMODULE_ARG_TYPE_BLOCK require an argument to have sub-arguments, i.e. subargs.

    • key_spec_index: If the type is REDISMODULE_ARG_TYPE_KEY you must provide the index of the key-spec associated with this argument. See key_specs above. If the argument is not a key, you may specify -1.

    • token: The token preceding the argument (optional). Example: the argument seconds in SET has a token EX. If the argument consists of only a token (for example NX in SET) the type should be REDISMODULE_ARG_TYPE_PURE_TOKEN and value should be NULL.

    • summary: A short description of the argument (optional).

    • since: The first version which included this argument (optional).

    • flags: A bitwise or of the macros REDISMODULE_CMD_ARG_*. See below.

    • value: The display-value of the argument. This string is what should be displayed when creating the command syntax from the output of COMMAND. If token is not NULL, it should also be displayed.

    Explanation of RedisModuleCommandArgType:

    • REDISMODULE_ARG_TYPE_STRING: String argument.
    • REDISMODULE_ARG_TYPE_INTEGER: Integer argument.
    • REDISMODULE_ARG_TYPE_DOUBLE: Double-precision float argument.
    • REDISMODULE_ARG_TYPE_KEY: String argument representing a keyname.
    • REDISMODULE_ARG_TYPE_PATTERN: String, but regex pattern.
    • REDISMODULE_ARG_TYPE_UNIX_TIME: Integer, but Unix timestamp.
    • REDISMODULE_ARG_TYPE_PURE_TOKEN: Argument doesn’t have a placeholder. It’s just a token without a value. Example: the KEEPTTL option of the SET command.
    • REDISMODULE_ARG_TYPE_ONEOF: Used when the user can choose only one of a few sub-arguments. Requires subargs. Example: the NX and XX options of SET.
    • REDISMODULE_ARG_TYPE_BLOCK: Used when one wants to group together several sub-arguments, usually to apply something on all of them, like making the entire group “optional”. Requires subargs. Example: the LIMIT offset count parameters in ZRANGE.

    Explanation of the command argument flags:

    • REDISMODULE_CMD_ARG_OPTIONAL: The argument is optional (like GET in the SET command).
    • REDISMODULE_CMD_ARG_MULTIPLE: The argument may repeat itself (like key in DEL).
    • REDISMODULE_CMD_ARG_MULTIPLE_TOKEN: The argument may repeat itself, and so does its token (like GET pattern in SORT).

On success REDISMODULE_OK is returned. On error REDISMODULE_ERR is returned and errno is set to EINVAL if invalid info was provided or EEXIST if info has already been set. If the info is invalid, a warning is logged explaining which part of the info is invalid and why.

Module information and time measurement

RedisModule_IsModuleNameBusy

int RedisModule_IsModuleNameBusy(const char *name);

Available since: 4.0.3

Return non-zero if the module name is busy. Otherwise zero is returned.

RedisModule_Milliseconds

long long RedisModule_Milliseconds(void);

Available since: 4.0.0

Return the current UNIX time in milliseconds.

RedisModule_MonotonicMicroseconds

uint64_t RedisModule_MonotonicMicroseconds(void);

Return counter of micro-seconds relative to an arbitrary point in time.

RedisModule_BlockedClientMeasureTimeStart

int RedisModule_BlockedClientMeasureTimeStart(RedisModuleBlockedClient *bc);

Available since: 6.2.0

Mark a point in time that will be used as the start time to calculate the elapsed execution time when RedisModule_BlockedClientMeasureTimeEnd() is called. Within the same command, you can call multiple times RedisModule_BlockedClientMeasureTimeStart() and RedisModule_BlockedClientMeasureTimeEnd() to accumulate independent time intervals to the background duration. This method always return REDISMODULE_OK.

RedisModule_BlockedClientMeasureTimeEnd

int RedisModule_BlockedClientMeasureTimeEnd(RedisModuleBlockedClient *bc);

Available since: 6.2.0

Mark a point in time that will be used as the end time to calculate the elapsed execution time. On success REDISMODULE_OK is returned. This method only returns REDISMODULE_ERR if no start time was previously defined ( meaning RedisModule_BlockedClientMeasureTimeStart was not called ).

RedisModule_Yield

void RedisModule_Yield(RedisModuleCtx *ctx, int flags, const char *busy_reply);

This API allows modules to let Redis process background tasks, and some commands during long blocking execution of a module command. The module can call this API periodically. The flags is a bit mask of these:

  • REDISMODULE_YIELD_FLAG_NONE: No special flags, can perform some background operations, but not process client commands.
  • REDISMODULE_YIELD_FLAG_CLIENTS: Redis can also process client commands.

The busy_reply argument is optional, and can be used to control the verbose error string after the -BUSY error code.

When the REDISMODULE_YIELD_FLAG_CLIENTS is used, Redis will only start processing client commands after the time defined by the busy-reply-threshold config, in which case Redis will start rejecting most commands with -BUSY error, but allow the ones marked with the allow-busy flag to be executed. This API can also be used in thread safe context (while locked), and during loading (in the rdb_load callback, in which case it’ll reject commands with the -LOADING error)

RedisModule_SetModuleOptions

void RedisModule_SetModuleOptions(RedisModuleCtx *ctx, int options);

Available since: 6.0.0

Set flags defining capabilities or behavior bit flags.

REDISMODULE_OPTIONS_HANDLE_IO_ERRORS: Generally, modules don’t need to bother with this, as the process will just terminate if a read error happens, however, setting this flag would allow repl-diskless-load to work if enabled. The module should use RedisModule_IsIOError after reads, before using the data that was read, and in case of error, propagate it upwards, and also be able to release the partially populated value and all it’s allocations.

REDISMODULE_OPTION_NO_IMPLICIT_SIGNAL_MODIFIED: See RedisModule_SignalModifiedKey().

REDISMODULE_OPTIONS_HANDLE_REPL_ASYNC_LOAD: Setting this flag indicates module awareness of diskless async replication (repl-diskless-load=swapdb) and that redis could be serving reads during replication instead of blocking with LOADING status.

RedisModule_SignalModifiedKey

int RedisModule_SignalModifiedKey(RedisModuleCtx *ctx,
                                  RedisModuleString *keyname);

Available since: 6.0.0

Signals that the key is modified from user’s perspective (i.e. invalidate WATCH and client side caching).

This is done automatically when a key opened for writing is closed, unless the option REDISMODULE_OPTION_NO_IMPLICIT_SIGNAL_MODIFIED has been set using RedisModule_SetModuleOptions().

Automatic memory management for modules

RedisModule_AutoMemory

void RedisModule_AutoMemory(RedisModuleCtx *ctx);

Available since: 4.0.0

Enable automatic memory management.

The function must be called as the first function of a command implementation that wants to use automatic memory.

When enabled, automatic memory management tracks and automatically frees keys, call replies and Redis string objects once the command returns. In most cases this eliminates the need of calling the following functions:

  1. RedisModule_CloseKey()
  2. RedisModule_FreeCallReply()
  3. RedisModule_FreeString()

These functions can still be used with automatic memory management enabled, to optimize loops that make numerous allocations for example.

String objects APIs

RedisModule_CreateString

RedisModuleString *RedisModule_CreateString(RedisModuleCtx *ctx,
                                            const char *ptr,
                                            size_t len);

Available since: 4.0.0

Create a new module string object. The returned string must be freed with RedisModule_FreeString(), unless automatic memory is enabled.

The string is created by copying the len bytes starting at ptr. No reference is retained to the passed buffer.

The module context ‘ctx’ is optional and may be NULL if you want to create a string out of the context scope. However in that case, the automatic memory management will not be available, and the string memory must be managed manually.

RedisModule_CreateStringPrintf

RedisModuleString *RedisModule_CreateStringPrintf(RedisModuleCtx *ctx,
                                                  const char *fmt,
                                                  ...);

Available since: 4.0.0

Create a new module string object from a printf format and arguments. The returned string must be freed with RedisModule_FreeString(), unless automatic memory is enabled.

The string is created using the sds formatter function sdscatvprintf().

The passed context ‘ctx’ may be NULL if necessary, see the RedisModule_CreateString() documentation for more info.

RedisModule_CreateStringFromLongLong

RedisModuleString *RedisModule_CreateStringFromLongLong(RedisModuleCtx *ctx,
                                                        long long ll);

Available since: 4.0.0

Like RedisModule_CreatString(), but creates a string starting from a long long integer instead of taking a buffer and its length.

The returned string must be released with RedisModule_FreeString() or by enabling automatic memory management.

The passed context ‘ctx’ may be NULL if necessary, see the RedisModule_CreateString() documentation for more info.

RedisModule_CreateStringFromDouble

RedisModuleString *RedisModule_CreateStringFromDouble(RedisModuleCtx *ctx,
                                                      double d);

Available since: 6.0.0

Like RedisModule_CreatString(), but creates a string starting from a double instead of taking a buffer and its length.

The returned string must be released with RedisModule_FreeString() or by enabling automatic memory management.

RedisModule_CreateStringFromLongDouble

RedisModuleString *RedisModule_CreateStringFromLongDouble(RedisModuleCtx *ctx,
                                                          long double ld,
                                                          int humanfriendly);

Available since: 6.0.0

Like RedisModule_CreatString(), but creates a string starting from a long double.

The returned string must be released with RedisModule_FreeString() or by enabling automatic memory management.

The passed context ‘ctx’ may be NULL if necessary, see the RedisModule_CreateString() documentation for more info.

RedisModule_CreateStringFromString

RedisModuleString *RedisModule_CreateStringFromString(RedisModuleCtx *ctx,
                                                      const RedisModuleString *str);

Available since: 4.0.0

Like RedisModule_CreatString(), but creates a string starting from another RedisModuleString.

The returned string must be released with RedisModule_FreeString() or by enabling automatic memory management.

The passed context ‘ctx’ may be NULL if necessary, see the RedisModule_CreateString() documentation for more info.

RedisModule_CreateStringFromStreamID

RedisModuleString *RedisModule_CreateStringFromStreamID(RedisModuleCtx *ctx,
                                                        const RedisModuleStreamID *id);

Available since: 6.2.0

Creates a string from a stream ID. The returned string must be released with RedisModule_FreeString(), unless automatic memory is enabled.

The passed context ctx may be NULL if necessary. See the RedisModule_CreateString() documentation for more info.

RedisModule_FreeString

void RedisModule_FreeString(RedisModuleCtx *ctx, RedisModuleString *str);

Available since: 4.0.0

Free a module string object obtained with one of the Redis modules API calls that return new string objects.

It is possible to call this function even when automatic memory management is enabled. In that case the string will be released ASAP and removed from the pool of string to release at the end.

If the string was created with a NULL context ‘ctx’, it is also possible to pass ctx as NULL when releasing the string (but passing a context will not create any issue). Strings created with a context should be freed also passing the context, so if you want to free a string out of context later, make sure to create it using a NULL context.

RedisModule_RetainString

void RedisModule_RetainString(RedisModuleCtx *ctx, RedisModuleString *str);

Available since: 4.0.0

Every call to this function, will make the string ‘str’ requiring an additional call to RedisModule_FreeString() in order to really free the string. Note that the automatic freeing of the string obtained enabling modules automatic memory management counts for one RedisModule_FreeString() call (it is just executed automatically).

Normally you want to call this function when, at the same time the following conditions are true:

  1. You have automatic memory management enabled.
  2. You want to create string objects.
  3. Those string objects you create need to live after the callback function(for example a command implementation) creating them returns.

Usually you want this in order to store the created string object into your own data structure, for example when implementing a new data type.

Note that when memory management is turned off, you don’t need any call to RetainString() since creating a string will always result into a string that lives after the callback function returns, if no FreeString() call is performed.

It is possible to call this function with a NULL context.

When strings are going to be retained for an extended duration, it is good practice to also call RedisModule_TrimStringAllocation() in order to optimize memory usage.

Threaded modules that reference retained strings from other threads must explicitly trim the allocation as soon as the string is retained. Not doing so may result with automatic trimming which is not thread safe.

RedisModule_HoldString

RedisModuleString* RedisModule_HoldString(RedisModuleCtx *ctx,
                                          RedisModuleString *str);

Available since: 6.0.7

This function can be used instead of RedisModule_RetainString(). The main difference between the two is that this function will always succeed, whereas RedisModule_RetainString() may fail because of an assertion.

The function returns a pointer to RedisModuleString, which is owned by the caller. It requires a call to RedisModule_FreeString() to free the string when automatic memory management is disabled for the context. When automatic memory management is enabled, you can either call RedisModule_FreeString() or let the automation free it.

This function is more efficient than RedisModule_CreateStringFromString() because whenever possible, it avoids copying the underlying RedisModuleString. The disadvantage of using this function is that it might not be possible to use RedisModule_StringAppendBuffer() on the returned RedisModuleString.

It is possible to call this function with a NULL context.

When strings are going to be held for an extended duration, it is good practice to also call RedisModule_TrimStringAllocation() in order to optimize memory usage.

Threaded modules that reference held strings from other threads must explicitly trim the allocation as soon as the string is held. Not doing so may result with automatic trimming which is not thread safe.

RedisModule_StringPtrLen

const char *RedisModule_StringPtrLen(const RedisModuleString *str,
                                     size_t *len);

Available since: 4.0.0

Given a string module object, this function returns the string pointer and length of the string. The returned pointer and length should only be used for read only accesses and never modified.

RedisModule_StringToLongLong

int RedisModule_StringToLongLong(const RedisModuleString *str, long long *ll);

Available since: 4.0.0

Convert the string into a long long integer, storing it at *ll. Returns REDISMODULE_OK on success. If the string can’t be parsed as a valid, strict long long (no spaces before/after), REDISMODULE_ERR is returned.

RedisModule_StringToDouble

int RedisModule_StringToDouble(const RedisModuleString *str, double *d);

Available since: 4.0.0

Convert the string into a double, storing it at *d. Returns REDISMODULE_OK on success or REDISMODULE_ERR if the string is not a valid string representation of a double value.

RedisModule_StringToLongDouble

int RedisModule_StringToLongDouble(const RedisModuleString *str,
                                   long double *ld);

Available since: 6.0.0

Convert the string into a long double, storing it at *ld. Returns REDISMODULE_OK on success or REDISMODULE_ERR if the string is not a valid string representation of a double value.

RedisModule_StringToStreamID

int RedisModule_StringToStreamID(const RedisModuleString *str,
                                 RedisModuleStreamID *id);

Available since: 6.2.0

Convert the string into a stream ID, storing it at *id. Returns REDISMODULE_OK on success and returns REDISMODULE_ERR if the string is not a valid string representation of a stream ID. The special IDs “+” and “-” are allowed.

RedisModule_StringCompare

int RedisModule_StringCompare(RedisModuleString *a, RedisModuleString *b);

Available since: 4.0.0

Compare two string objects, returning -1, 0 or 1 respectively if a < b, a == b, a > b. Strings are compared byte by byte as two binary blobs without any encoding care / collation attempt.

RedisModule_StringAppendBuffer

int RedisModule_StringAppendBuffer(RedisModuleCtx *ctx,
                                   RedisModuleString *str,
                                   const char *buf,
                                   size_t len);

Available since: 4.0.0

Append the specified buffer to the string ‘str’. The string must be a string created by the user that is referenced only a single time, otherwise REDISMODULE_ERR is returned and the operation is not performed.

RedisModule_TrimStringAllocation

void RedisModule_TrimStringAllocation(RedisModuleString *str);

Trim possible excess memory allocated for a RedisModuleString.

Sometimes a RedisModuleString may have more memory allocated for it than required, typically for argv arguments that were constructed from network buffers. This function optimizes such strings by reallocating their memory, which is useful for strings that are not short lived but retained for an extended duration.

This operation is not thread safe and should only be called when no concurrent access to the string is guaranteed. Using it for an argv string in a module command before the string is potentially available to other threads is generally safe.

Currently, Redis may also automatically trim retained strings when a module command returns. However, doing this explicitly should still be a preferred option:

  1. Future versions of Redis may abandon auto-trimming.
  2. Auto-trimming as currently implemented is not thread safe. A background thread manipulating a recently retained string may end up in a race condition with the auto-trim, which could result with data corruption.

Reply APIs

These functions are used for sending replies to the client.

Most functions always return REDISMODULE_OK so you can use it with ‘return’ in order to return from the command implementation with:

if (... some condition ...)
    return RedisModule_ReplyWithLongLong(ctx,mycount);

Reply with collection functions

After starting a collection reply, the module must make calls to other ReplyWith* style functions in order to emit the elements of the collection. Collection types include: Array, Map, Set and Attribute.

When producing collections with a number of elements that is not known beforehand, the function can be called with a special flag REDISMODULE_POSTPONED_LEN (REDISMODULE_POSTPONED_ARRAY_LEN in the past), and the actual number of elements can be later set with RedisModule_ReplySet*Length() call (which will set the latest “open” count if there are multiple ones).

RedisModule_WrongArity

int RedisModule_WrongArity(RedisModuleCtx *ctx);

Available since: 4.0.0

Send an error about the number of arguments given to the command, citing the command name in the error message. Returns REDISMODULE_OK.

Example:

if (argc != 3) return RedisModule_WrongArity(ctx);

RedisModule_ReplyWithLongLong

int RedisModule_ReplyWithLongLong(RedisModuleCtx *ctx, long long ll);

Available since: 4.0.0

Send an integer reply to the client, with the specified long long value. The function always returns REDISMODULE_OK.

RedisModule_ReplyWithError

int RedisModule_ReplyWithError(RedisModuleCtx *ctx, const char *err);

Available since: 4.0.0

Reply with the error ‘err’.

Note that ‘err’ must contain all the error, including the initial error code. The function only provides the initial “-”, so the usage is, for example:

RedisModule_ReplyWithError(ctx,"ERR Wrong Type");

and not just:

RedisModule_ReplyWithError(ctx,"Wrong Type");

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithSimpleString

int RedisModule_ReplyWithSimpleString(RedisModuleCtx *ctx, const char *msg);

Available since: 4.0.0

Reply with a simple string (+... \r\n in RESP protocol). This replies are suitable only when sending a small non-binary string with small overhead, like “OK” or similar replies.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithArray

int RedisModule_ReplyWithArray(RedisModuleCtx *ctx, long len);

Available since: 4.0.0

Reply with an array type of ‘len’ elements.

After starting an array reply, the module must make len calls to other ReplyWith* style functions in order to emit the elements of the array. See Reply APIs section for more details.

Use RedisModule_ReplySetArrayLength() to set deferred length.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithMap

int RedisModule_ReplyWithMap(RedisModuleCtx *ctx, long len);

Reply with a RESP3 Map type of ‘len’ pairs. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

After starting a map reply, the module must make len*2 calls to other ReplyWith* style functions in order to emit the elements of the map. See Reply APIs section for more details.

If the connected client is using RESP2, the reply will be converted to a flat array.

Use RedisModule_ReplySetMapLength() to set deferred length.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithSet

int RedisModule_ReplyWithSet(RedisModuleCtx *ctx, long len);

Reply with a RESP3 Set type of ‘len’ elements. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

After starting a set reply, the module must make len calls to other ReplyWith* style functions in order to emit the elements of the set. See Reply APIs section for more details.

If the connected client is using RESP2, the reply will be converted to an array type.

Use RedisModule_ReplySetSetLength() to set deferred length.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithAttribute

int RedisModule_ReplyWithAttribute(RedisModuleCtx *ctx, long len);

Add attributes (metadata) to the reply. Should be done before adding the actual reply. see https://github.com/antirez/RESP3/blob/master/spec.md#attribute-type

After starting an attributes reply, the module must make len*2 calls to other ReplyWith* style functions in order to emit the elements of the attribtute map. See Reply APIs section for more details.

Use RedisModule_ReplySetAttributeLength() to set deferred length.

Not supported by RESP2 and will return REDISMODULE_ERR, otherwise the function always returns REDISMODULE_OK.

RedisModule_ReplyWithNullArray

int RedisModule_ReplyWithNullArray(RedisModuleCtx *ctx);

Available since: 6.0.0

Reply to the client with a null array, simply null in RESP3, null array in RESP2.

Note: In RESP3 there’s no difference between Null reply and NullArray reply, so to prevent ambiguity it’s better to avoid using this API and use RedisModule_ReplyWithNull instead.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithEmptyArray

int RedisModule_ReplyWithEmptyArray(RedisModuleCtx *ctx);

Available since: 6.0.0

Reply to the client with an empty array.

The function always returns REDISMODULE_OK.

RedisModule_ReplySetArrayLength

void RedisModule_ReplySetArrayLength(RedisModuleCtx *ctx, long len);

Available since: 4.0.0

When RedisModule_ReplyWithArray() is used with the argument REDISMODULE_POSTPONED_LEN, because we don’t know beforehand the number of items we are going to output as elements of the array, this function will take care to set the array length.

Since it is possible to have multiple array replies pending with unknown length, this function guarantees to always set the latest array length that was created in a postponed way.

For example in order to output an array like [1,[10,20,30]] we could write:

 RedisModule_ReplyWithArray(ctx,REDISMODULE_POSTPONED_LEN);
 RedisModule_ReplyWithLongLong(ctx,1);
 RedisModule_ReplyWithArray(ctx,REDISMODULE_POSTPONED_LEN);
 RedisModule_ReplyWithLongLong(ctx,10);
 RedisModule_ReplyWithLongLong(ctx,20);
 RedisModule_ReplyWithLongLong(ctx,30);
 RedisModule_ReplySetArrayLength(ctx,3); // Set len of 10,20,30 array.
 RedisModule_ReplySetArrayLength(ctx,2); // Set len of top array

Note that in the above example there is no reason to postpone the array length, since we produce a fixed number of elements, but in the practice the code may use an iterator or other ways of creating the output so that is not easy to calculate in advance the number of elements.

RedisModule_ReplySetMapLength

void RedisModule_ReplySetMapLength(RedisModuleCtx *ctx, long len);

Very similar to RedisModule_ReplySetArrayLength except len should exactly half of the number of ReplyWith* functions called in the context of the map. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

RedisModule_ReplySetSetLength

void RedisModule_ReplySetSetLength(RedisModuleCtx *ctx, long len);

Very similar to RedisModule_ReplySetArrayLength Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

RedisModule_ReplySetAttributeLength

void RedisModule_ReplySetAttributeLength(RedisModuleCtx *ctx, long len);

Very similar to RedisModule_ReplySetMapLength Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

RedisModule_ReplyWithStringBuffer

int RedisModule_ReplyWithStringBuffer(RedisModuleCtx *ctx,
                                      const char *buf,
                                      size_t len);

Available since: 4.0.0

Reply with a bulk string, taking in input a C buffer pointer and length.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithCString

int RedisModule_ReplyWithCString(RedisModuleCtx *ctx, const char *buf);

Available since: 5.0.6

Reply with a bulk string, taking in input a C buffer pointer that is assumed to be null-terminated.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithString

int RedisModule_ReplyWithString(RedisModuleCtx *ctx, RedisModuleString *str);

Available since: 4.0.0

Reply with a bulk string, taking in input a RedisModuleString object.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithEmptyString

int RedisModule_ReplyWithEmptyString(RedisModuleCtx *ctx);

Available since: 6.0.0

Reply with an empty string.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithVerbatimStringType

int RedisModule_ReplyWithVerbatimStringType(RedisModuleCtx *ctx,
                                            const char *buf,
                                            size_t len,
                                            const char *ext);

Reply with a binary safe string, which should not be escaped or filtered taking in input a C buffer pointer, length and a 3 character type/extension.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithVerbatimString

int RedisModule_ReplyWithVerbatimString(RedisModuleCtx *ctx,
                                        const char *buf,
                                        size_t len);

Available since: 6.0.0

Reply with a binary safe string, which should not be escaped or filtered taking in input a C buffer pointer and length.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithNull

int RedisModule_ReplyWithNull(RedisModuleCtx *ctx);

Available since: 4.0.0

Reply to the client with a NULL.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithBool

int RedisModule_ReplyWithBool(RedisModuleCtx *ctx, int b);

Reply with a RESP3 Boolean type. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

In RESP3, this is boolean type In RESP2, it’s a string response of “1” and “0” for true and false respectively.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithCallReply

int RedisModule_ReplyWithCallReply(RedisModuleCtx *ctx,
                                   RedisModuleCallReply *reply);

Available since: 4.0.0

Reply exactly what a Redis command returned us with RedisModule_Call(). This function is useful when we use RedisModule_Call() in order to execute some command, as we want to reply to the client exactly the same reply we obtained by the command.

Return:

  • REDISMODULE_OK on success.
  • REDISMODULE_ERR if the given reply is in RESP3 format but the client expects RESP2. In case of an error, it’s the module writer responsibility to translate the reply to RESP2 (or handle it differently by returning an error). Notice that for module writer convenience, it is possible to pass 0 as a parameter to the fmt argument of RM_Call so that the RedisModuleCallReply will return in the same protocol (RESP2 or RESP3) as set in the current client’s context.

RedisModule_ReplyWithDouble

int RedisModule_ReplyWithDouble(RedisModuleCtx *ctx, double d);

Available since: 4.0.0

Reply with a RESP3 Double type. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

Send a string reply obtained converting the double ’d' into a bulk string. This function is basically equivalent to converting a double into a string into a C buffer, and then calling the function RedisModule_ReplyWithStringBuffer() with the buffer and length.

In RESP3 the string is tagged as a double, while in RESP2 it’s just a plain string that the user will have to parse.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithBigNumber

int RedisModule_ReplyWithBigNumber(RedisModuleCtx *ctx,
                                   const char *bignum,
                                   size_t len);

Reply with a RESP3 BigNumber type. Visit https://github.com/antirez/RESP3/blob/master/spec.md for more info about RESP3.

In RESP3, this is a string of length len that is tagged as a BigNumber, however, it’s up to the caller to ensure that it’s a valid BigNumber. In RESP2, this is just a plain bulk string response.

The function always returns REDISMODULE_OK.

RedisModule_ReplyWithLongDouble

int RedisModule_ReplyWithLongDouble(RedisModuleCtx *ctx, long double ld);

Available since: 6.0.0

Send a string reply obtained converting the long double ‘ld’ into a bulk string. This function is basically equivalent to converting a long double into a string into a C buffer, and then calling the function RedisModule_ReplyWithStringBuffer() with the buffer and length. The double string uses human readable formatting (see addReplyHumanLongDouble in networking.c).

The function always returns REDISMODULE_OK.

Commands replication API

RedisModule_Replicate

int RedisModule_Replicate(RedisModuleCtx *ctx,
                          const char *cmdname,
                          const char *fmt,
                          ...);

Available since: 4.0.0

Replicate the specified command and arguments to slaves and AOF, as effect of execution of the calling command implementation.

The replicated commands are always wrapped into the MULTI/EXEC that contains all the commands replicated in a given module command execution. However the commands replicated with RedisModule_Call() are the first items, the ones replicated with RedisModule_Replicate() will all follow before the EXEC.

Modules should try to use one interface or the other.

This command follows exactly the same interface of RedisModule_Call(), so a set of format specifiers must be passed, followed by arguments matching the provided format specifiers.

Please refer to RedisModule_Call() for more information.

Using the special “A” and “R” modifiers, the caller can exclude either the AOF or the replicas from the propagation of the specified command. Otherwise, by default, the command will be propagated in both channels.

Note about calling this function from a thread safe context:

Normally when you call this function from the callback implementing a module command, or any other callback provided by the Redis Module API, Redis will accumulate all the calls to this function in the context of the callback, and will propagate all the commands wrapped in a MULTI/EXEC transaction. However when calling this function from a threaded safe context that can live an undefined amount of time, and can be locked/unlocked in at will, the behavior is different: MULTI/EXEC wrapper is not emitted and the command specified is inserted in the AOF and replication stream immediately.

Return value

The command returns REDISMODULE_ERR if the format specifiers are invalid or the command name does not belong to a known command.

RedisModule_ReplicateVerbatim

int RedisModule_ReplicateVerbatim(RedisModuleCtx *ctx);

Available since: 4.0.0

This function will replicate the command exactly as it was invoked by the client. Note that this function will not wrap the command into a MULTI/EXEC stanza, so it should not be mixed with other replication commands.

Basically this form of replication is useful when you want to propagate the command to the slaves and AOF file exactly as it was called, since the command can just be re-executed to deterministically re-create the new state starting from the old one.

The function always returns REDISMODULE_OK.

DB and Key APIs – Generic API

RedisModule_GetClientId

unsigned long long RedisModule_GetClientId(RedisModuleCtx *ctx);

Available since: 4.0.0

Return the ID of the current client calling the currently active module command. The returned ID has a few guarantees:

  1. The ID is different for each different client, so if the same client executes a module command multiple times, it can be recognized as having the same ID, otherwise the ID will be different.
  2. The ID increases monotonically. Clients connecting to the server later are guaranteed to get IDs greater than any past ID previously seen.

Valid IDs are from 1 to 2^64 - 1. If 0 is returned it means there is no way to fetch the ID in the context the function was currently called.

After obtaining the ID, it is possible to check if the command execution is actually happening in the context of AOF loading, using this macro:

 if (RedisModule_IsAOFClient(RedisModule_GetClientId(ctx)) {
     // Handle it differently.
 }

RedisModule_GetClientUserNameById

RedisModuleString *RedisModule_GetClientUserNameById(RedisModuleCtx *ctx,
                                                     uint64_t id);

Available since: 6.2.1

Return the ACL user name used by the client with the specified client ID. Client ID can be obtained with RedisModule_GetClientId() API. If the client does not exist, NULL is returned and errno is set to ENOENT. If the client isn’t using an ACL user, NULL is returned and errno is set to ENOTSUP

RedisModule_GetClientInfoById

int RedisModule_GetClientInfoById(void *ci, uint64_t id);

Available since: 6.0.0

Return information about the client with the specified ID (that was previously obtained via the RedisModule_GetClientId() API). If the client exists, REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned.

When the client exist and the ci pointer is not NULL, but points to a structure of type RedisModuleClientInfo, previously initialized with the correct REDISMODULE_CLIENTINFO_INITIALIZER, the structure is populated with the following fields:

 uint64_t flags;         // REDISMODULE_CLIENTINFO_FLAG_*
 uint64_t id;            // Client ID
 char addr[46];          // IPv4 or IPv6 address.
 uint16_t port;          // TCP port.
 uint16_t db;            // Selected DB.

Note: the client ID is useless in the context of this call, since we already know, however the same structure could be used in other contexts where we don’t know the client ID, yet the same structure is returned.

With flags having the following meaning:

REDISMODULE_CLIENTINFO_FLAG_SSL          Client using SSL connection.
REDISMODULE_CLIENTINFO_FLAG_PUBSUB       Client in Pub/Sub mode.
REDISMODULE_CLIENTINFO_FLAG_BLOCKED      Client blocked in command.
REDISMODULE_CLIENTINFO_FLAG_TRACKING     Client with keys tracking on.
REDISMODULE_CLIENTINFO_FLAG_UNIXSOCKET   Client using unix domain socket.
REDISMODULE_CLIENTINFO_FLAG_MULTI        Client in MULTI state.

However passing NULL is a way to just check if the client exists in case we are not interested in any additional information.

This is the correct usage when we want the client info structure returned:

 RedisModuleClientInfo ci = REDISMODULE_CLIENTINFO_INITIALIZER;
 int retval = RedisModule_GetClientInfoById(&ci,client_id);
 if (retval == REDISMODULE_OK) {
     printf("Address: %s\n", ci.addr);
 }

RedisModule_PublishMessage

int RedisModule_PublishMessage(RedisModuleCtx *ctx,
                               RedisModuleString *channel,
                               RedisModuleString *message);

Available since: 6.0.0

Publish a message to subscribers (see PUBLISH command).

RedisModule_GetSelectedDb

int RedisModule_GetSelectedDb(RedisModuleCtx *ctx);

Available since: 4.0.0

Return the currently selected DB.

RedisModule_GetContextFlags

int RedisModule_GetContextFlags(RedisModuleCtx *ctx);

Available since: 4.0.3

Return the current context’s flags. The flags provide information on the current request context (whether the client is a Lua script or in a MULTI), and about the Redis instance in general, i.e replication and persistence.

It is possible to call this function even with a NULL context, however in this case the following flags will not be reported:

  • LUA, MULTI, REPLICATED, DIRTY (see below for more info).

Available flags and their meaning:

  • REDISMODULE_CTX_FLAGS_LUA: The command is running in a Lua script

  • REDISMODULE_CTX_FLAGS_MULTI: The command is running inside a transaction

  • REDISMODULE_CTX_FLAGS_REPLICATED: The command was sent over the replication link by the MASTER

  • REDISMODULE_CTX_FLAGS_MASTER: The Redis instance is a master

  • REDISMODULE_CTX_FLAGS_SLAVE: The Redis instance is a slave

  • REDISMODULE_CTX_FLAGS_READONLY: The Redis instance is read-only

  • REDISMODULE_CTX_FLAGS_CLUSTER: The Redis instance is in cluster mode

  • REDISMODULE_CTX_FLAGS_AOF: The Redis instance has AOF enabled

  • REDISMODULE_CTX_FLAGS_RDB: The instance has RDB enabled

  • REDISMODULE_CTX_FLAGS_MAXMEMORY: The instance has Maxmemory set

  • REDISMODULE_CTX_FLAGS_EVICT: Maxmemory is set and has an eviction policy that may delete keys

  • REDISMODULE_CTX_FLAGS_OOM: Redis is out of memory according to the maxmemory setting.

  • REDISMODULE_CTX_FLAGS_OOM_WARNING: Less than 25% of memory remains before reaching the maxmemory level.

  • REDISMODULE_CTX_FLAGS_LOADING: Server is loading RDB/AOF

  • REDISMODULE_CTX_FLAGS_REPLICA_IS_STALE: No active link with the master.

  • REDISMODULE_CTX_FLAGS_REPLICA_IS_CONNECTING: The replica is trying to connect with the master.

  • REDISMODULE_CTX_FLAGS_REPLICA_IS_TRANSFERRING: Master -> Replica RDB transfer is in progress.

  • REDISMODULE_CTX_FLAGS_REPLICA_IS_ONLINE: The replica has an active link with its master. This is the contrary of STALE state.

  • REDISMODULE_CTX_FLAGS_ACTIVE_CHILD: There is currently some background process active (RDB, AUX or module).

  • REDISMODULE_CTX_FLAGS_MULTI_DIRTY: The next EXEC will fail due to dirty CAS (touched keys).

  • REDISMODULE_CTX_FLAGS_IS_CHILD: Redis is currently running inside background child process.

  • REDISMODULE_CTX_FLAGS_RESP3: Indicate the that client attached to this context is using RESP3.

RedisModule_AvoidReplicaTraffic

int RedisModule_AvoidReplicaTraffic();

Available since: 6.0.0

Returns true if a client sent the CLIENT PAUSE command to the server or if Redis Cluster does a manual failover, pausing the clients. This is needed when we have a master with replicas, and want to write, without adding further data to the replication channel, that the replicas replication offset, match the one of the master. When this happens, it is safe to failover the master without data loss.

However modules may generate traffic by calling RedisModule_Call() with the “!” flag, or by calling RedisModule_Replicate(), in a context outside commands execution, for instance in timeout callbacks, threads safe contexts, and so forth. When modules will generate too much traffic, it will be hard for the master and replicas offset to match, because there is more data to send in the replication channel.

So modules may want to try to avoid very heavy background work that has the effect of creating data to the replication channel, when this function returns true. This is mostly useful for modules that have background garbage collection tasks, or that do writes and replicate such writes periodically in timer callbacks or other periodic callbacks.

RedisModule_SelectDb

int RedisModule_SelectDb(RedisModuleCtx *ctx, int newid);

Available since: 4.0.0

Change the currently selected DB. Returns an error if the id is out of range.

Note that the client will retain the currently selected DB even after the Redis command implemented by the module calling this function returns.

If the module command wishes to change something in a different DB and returns back to the original one, it should call RedisModule_GetSelectedDb() before in order to restore the old DB number before returning.

RedisModule_KeyExists

int RedisModule_KeyExists(RedisModuleCtx *ctx, robj *keyname);

Check if a key exists, without affecting its last access time.

This is equivalent to calling RedisModule_OpenKey with the mode REDISMODULE_READ | REDISMODULE_OPEN_KEY_NOTOUCH, then checking if NULL was returned and, if not, calling RedisModule_CloseKey on the opened key.

RedisModule_OpenKey

void *RedisModule_OpenKey(RedisModuleCtx *ctx, robj *keyname, int mode);

Available since: 4.0.0

Return an handle representing a Redis key, so that it is possible to call other APIs with the key handle as argument to perform operations on the key.

The return value is the handle representing the key, that must be closed with RedisModule_CloseKey().

If the key does not exist and WRITE mode is requested, the handle is still returned, since it is possible to perform operations on a yet not existing key (that will be created, for example, after a list push operation). If the mode is just READ instead, and the key does not exist, NULL is returned. However it is still safe to call RedisModule_CloseKey() and RedisModule_KeyType() on a NULL value.

RedisModule_CloseKey

void RedisModule_CloseKey(RedisModuleKey *key);

Available since: 4.0.0

Close a key handle.

RedisModule_KeyType

int RedisModule_KeyType(RedisModuleKey *key);

Available since: 4.0.0

Return the type of the key. If the key pointer is NULL then REDISMODULE_KEYTYPE_EMPTY is returned.

RedisModule_ValueLength

size_t RedisModule_ValueLength(RedisModuleKey *key);

Available since: 4.0.0

Return the length of the value associated with the key. For strings this is the length of the string. For all the other types is the number of elements (just counting keys for hashes).

If the key pointer is NULL or the key is empty, zero is returned.

RedisModule_DeleteKey

int RedisModule_DeleteKey(RedisModuleKey *key);

Available since: 4.0.0

If the key is open for writing, remove it, and setup the key to accept new writes as an empty key (that will be created on demand). On success REDISMODULE_OK is returned. If the key is not open for writing REDISMODULE_ERR is returned.

RedisModule_UnlinkKey

int RedisModule_UnlinkKey(RedisModuleKey *key);

Available since: 4.0.7

If the key is open for writing, unlink it (that is delete it in a non-blocking way, not reclaiming memory immediately) and setup the key to accept new writes as an empty key (that will be created on demand). On success REDISMODULE_OK is returned. If the key is not open for writing REDISMODULE_ERR is returned.

RedisModule_GetExpire

mstime_t RedisModule_GetExpire(RedisModuleKey *key);

Available since: 4.0.0

Return the key expire value, as milliseconds of remaining TTL. If no TTL is associated with the key or if the key is empty, REDISMODULE_NO_EXPIRE is returned.

RedisModule_SetExpire

int RedisModule_SetExpire(RedisModuleKey *key, mstime_t expire);

Available since: 4.0.0

Set a new expire for the key. If the special expire REDISMODULE_NO_EXPIRE is set, the expire is cancelled if there was one (the same as the PERSIST command).

Note that the expire must be provided as a positive integer representing the number of milliseconds of TTL the key should have.

The function returns REDISMODULE_OK on success or REDISMODULE_ERR if the key was not open for writing or is an empty key.

RedisModule_GetAbsExpire

mstime_t RedisModule_GetAbsExpire(RedisModuleKey *key);

Available since: 6.2.2

Return the key expire value, as absolute Unix timestamp. If no TTL is associated with the key or if the key is empty, REDISMODULE_NO_EXPIRE is returned.

RedisModule_SetAbsExpire

int RedisModule_SetAbsExpire(RedisModuleKey *key, mstime_t expire);

Available since: 6.2.2

Set a new expire for the key. If the special expire REDISMODULE_NO_EXPIRE is set, the expire is cancelled if there was one (the same as the PERSIST command).

Note that the expire must be provided as a positive integer representing the absolute Unix timestamp the key should have.

The function returns REDISMODULE_OK on success or REDISMODULE_ERR if the key was not open for writing or is an empty key.

RedisModule_ResetDataset

void RedisModule_ResetDataset(int restart_aof, int async);

Available since: 6.0.0

Performs similar operation to FLUSHALL, and optionally start a new AOF file (if enabled) If restart_aof is true, you must make sure the command that triggered this call is not propagated to the AOF file. When async is set to true, db contents will be freed by a background thread.

RedisModule_DbSize

unsigned long long RedisModule_DbSize(RedisModuleCtx *ctx);

Available since: 6.0.0

Returns the number of keys in the current db.

RedisModule_RandomKey

RedisModuleString *RedisModule_RandomKey(RedisModuleCtx *ctx);

Available since: 6.0.0

Returns a name of a random key, or NULL if current db is empty.

RedisModule_GetKeyNameFromOptCtx

const RedisModuleString *RedisModule_GetKeyNameFromOptCtx(RedisModuleKeyOptCtx *ctx);

Returns the name of the key currently being processed.

RedisModule_GetToKeyNameFromOptCtx

const RedisModuleString *RedisModule_GetToKeyNameFromOptCtx(RedisModuleKeyOptCtx *ctx);

Returns the name of the target key currently being processed.

RedisModule_GetDbIdFromOptCtx

int RedisModule_GetDbIdFromOptCtx(RedisModuleKeyOptCtx *ctx);

Returns the dbid currently being processed.

RedisModule_GetToDbIdFromOptCtx

int RedisModule_GetToDbIdFromOptCtx(RedisModuleKeyOptCtx *ctx);

Returns the target dbid currently being processed.

Key API for String type

See also RedisModule_ValueLength(), which returns the length of a string.

RedisModule_StringSet

int RedisModule_StringSet(RedisModuleKey *key, RedisModuleString *str);

Available since: 4.0.0

If the key is open for writing, set the specified string ‘str’ as the value of the key, deleting the old value if any. On success REDISMODULE_OK is returned. If the key is not open for writing or there is an active iterator, REDISMODULE_ERR is returned.

RedisModule_StringDMA

char *RedisModule_StringDMA(RedisModuleKey *key, size_t *len, int mode);

Available since: 4.0.0

Prepare the key associated string value for DMA access, and returns a pointer and size (by reference), that the user can use to read or modify the string in-place accessing it directly via pointer.

The ‘mode’ is composed by bitwise OR-ing the following flags:

REDISMODULE_READ -- Read access
REDISMODULE_WRITE -- Write access

If the DMA is not requested for writing, the pointer returned should only be accessed in a read-only fashion.

On error (wrong type) NULL is returned.

DMA access rules:

  1. No other key writing function should be called since the moment the pointer is obtained, for all the time we want to use DMA access to read or modify the string.

  2. Each time RedisModule_StringTruncate() is called, to continue with the DMA access, RedisModule_StringDMA() should be called again to re-obtain a new pointer and length.

  3. If the returned pointer is not NULL, but the length is zero, no byte can be touched (the string is empty, or the key itself is empty) so a RedisModule_StringTruncate() call should be used if there is to enlarge the string, and later call StringDMA() again to get the pointer.

RedisModule_StringTruncate

int RedisModule_StringTruncate(RedisModuleKey *key, size_t newlen);

Available since: 4.0.0

If the key is open for writing and is of string type, resize it, padding with zero bytes if the new length is greater than the old one.

After this call, RedisModule_StringDMA() must be called again to continue DMA access with the new pointer.

The function returns REDISMODULE_OK on success, and REDISMODULE_ERR on error, that is, the key is not open for writing, is not a string or resizing for more than 512 MB is requested.

If the key is empty, a string key is created with the new string value unless the new length value requested is zero.

Key API for List type

Many of the list functions access elements by index. Since a list is in essence a doubly-linked list, accessing elements by index is generally an O(N) operation. However, if elements are accessed sequentially or with indices close together, the functions are optimized to seek the index from the previous index, rather than seeking from the ends of the list.

This enables iteration to be done efficiently using a simple for loop:

long n = RM_ValueLength(key);
for (long i = 0; i < n; i++) {
    RedisModuleString *elem = RedisModule_ListGet(key, i);
    // Do stuff...
}

Note that after modifying a list using RedisModule_ListPop, RedisModule_ListSet or RedisModule_ListInsert, the internal iterator is invalidated so the next operation will require a linear seek.

Modifying a list in any another way, for examle using RedisModule_Call(), while a key is open will confuse the internal iterator and may cause trouble if the key is used after such modifications. The key must be reopened in this case.

See also RedisModule_ValueLength(), which returns the length of a list.

RedisModule_ListPush

int RedisModule_ListPush(RedisModuleKey *key,
                         int where,
                         RedisModuleString *ele);

Available since: 4.0.0

Push an element into a list, on head or tail depending on ‘where’ argument (REDISMODULE_LIST_HEAD or REDISMODULE_LIST_TAIL). If the key refers to an empty key opened for writing, the key is created. On success, REDISMODULE_OK is returned. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if key or ele is NULL.
  • ENOTSUP if the key is of another type than list.
  • EBADF if the key is not opened for writing.

Note: Before Redis 7.0, errno was not set by this function.

RedisModule_ListPop

RedisModuleString *RedisModule_ListPop(RedisModuleKey *key, int where);

Available since: 4.0.0

Pop an element from the list, and returns it as a module string object that the user should be free with RedisModule_FreeString() or by enabling automatic memory. The where argument specifies if the element should be popped from the beginning or the end of the list (REDISMODULE_LIST_HEAD or REDISMODULE_LIST_TAIL). On failure, the command returns NULL and sets errno as follows:

  • EINVAL if key is NULL.
  • ENOTSUP if the key is empty or of another type than list.
  • EBADF if the key is not opened for writing.

Note: Before Redis 7.0, errno was not set by this function.

RedisModule_ListGet

RedisModuleString *RedisModule_ListGet(RedisModuleKey *key, long index);

Returns the element at index index in the list stored at key, like the LINDEX command. The element should be free’d using RedisModule_FreeString() or using automatic memory management.

The index is zero-based, so 0 means the first element, 1 the second element and so on. Negative indices can be used to designate elements starting at the tail of the list. Here, -1 means the last element, -2 means the penultimate and so forth.

When no value is found at the given key and index, NULL is returned and errno is set as follows:

  • EINVAL if key is NULL.
  • ENOTSUP if the key is not a list.
  • EBADF if the key is not opened for reading.
  • EDOM if the index is not a valid index in the list.

RedisModule_ListSet

int RedisModule_ListSet(RedisModuleKey *key,
                        long index,
                        RedisModuleString *value);

Replaces the element at index index in the list stored at key.

The index is zero-based, so 0 means the first element, 1 the second element and so on. Negative indices can be used to designate elements starting at the tail of the list. Here, -1 means the last element, -2 means the penultimate and so forth.

On success, REDISMODULE_OK is returned. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if key or value is NULL.
  • ENOTSUP if the key is not a list.
  • EBADF if the key is not opened for writing.
  • EDOM if the index is not a valid index in the list.

RedisModule_ListInsert

int RedisModule_ListInsert(RedisModuleKey *key,
                           long index,
                           RedisModuleString *value);

Inserts an element at the given index.

The index is zero-based, so 0 means the first element, 1 the second element and so on. Negative indices can be used to designate elements starting at the tail of the list. Here, -1 means the last element, -2 means the penultimate and so forth. The index is the element’s index after inserting it.

On success, REDISMODULE_OK is returned. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if key or value is NULL.
  • ENOTSUP if the key of another type than list.
  • EBADF if the key is not opened for writing.
  • EDOM if the index is not a valid index in the list.

RedisModule_ListDelete

int RedisModule_ListDelete(RedisModuleKey *key, long index);

Removes an element at the given index. The index is 0-based. A negative index can also be used, counting from the end of the list.

On success, REDISMODULE_OK is returned. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if key or value is NULL.
  • ENOTSUP if the key is not a list.
  • EBADF if the key is not opened for writing.
  • EDOM if the index is not a valid index in the list.

Key API for Sorted Set type

See also RedisModule_ValueLength(), which returns the length of a sorted set.

RedisModule_ZsetAdd

int RedisModule_ZsetAdd(RedisModuleKey *key,
                        double score,
                        RedisModuleString *ele,
                        int *flagsptr);

Available since: 4.0.0

Add a new element into a sorted set, with the specified ‘score’. If the element already exists, the score is updated.

A new sorted set is created at value if the key is an empty open key setup for writing.

Additional flags can be passed to the function via a pointer, the flags are both used to receive input and to communicate state when the function returns. ‘flagsptr’ can be NULL if no special flags are used.

The input flags are:

REDISMODULE_ZADD_XX: Element must already exist. Do nothing otherwise.
REDISMODULE_ZADD_NX: Element must not exist. Do nothing otherwise.
REDISMODULE_ZADD_GT: If element exists, new score must be greater than the current score. 
                     Do nothing otherwise. Can optionally be combined with XX.
REDISMODULE_ZADD_LT: If element exists, new score must be less than the current score.
                     Do nothing otherwise. Can optionally be combined with XX.

The output flags are:

REDISMODULE_ZADD_ADDED: The new element was added to the sorted set.
REDISMODULE_ZADD_UPDATED: The score of the element was updated.
REDISMODULE_ZADD_NOP: No operation was performed because XX or NX flags.

On success the function returns REDISMODULE_OK. On the following errors REDISMODULE_ERR is returned:

  • The key was not opened for writing.
  • The key is of the wrong type.
  • ‘score’ double value is not a number (NaN).

RedisModule_ZsetIncrby

int RedisModule_ZsetIncrby(RedisModuleKey *key,
                           double score,
                           RedisModuleString *ele,
                           int *flagsptr,
                           double *newscore);

Available since: 4.0.0

This function works exactly like RedisModule_ZsetAdd(), but instead of setting a new score, the score of the existing element is incremented, or if the element does not already exist, it is added assuming the old score was zero.

The input and output flags, and the return value, have the same exact meaning, with the only difference that this function will return REDISMODULE_ERR even when ‘score’ is a valid double number, but adding it to the existing score results into a NaN (not a number) condition.

This function has an additional field ‘newscore’, if not NULL is filled with the new score of the element after the increment, if no error is returned.

RedisModule_ZsetRem

int RedisModule_ZsetRem(RedisModuleKey *key,
                        RedisModuleString *ele,
                        int *deleted);

Available since: 4.0.0

Remove the specified element from the sorted set. The function returns REDISMODULE_OK on success, and REDISMODULE_ERR on one of the following conditions:

  • The key was not opened for writing.
  • The key is of the wrong type.

The return value does NOT indicate the fact the element was really removed (since it existed) or not, just if the function was executed with success.

In order to know if the element was removed, the additional argument ‘deleted’ must be passed, that populates the integer by reference setting it to 1 or 0 depending on the outcome of the operation. The ‘deleted’ argument can be NULL if the caller is not interested to know if the element was really removed.

Empty keys will be handled correctly by doing nothing.

RedisModule_ZsetScore

int RedisModule_ZsetScore(RedisModuleKey *key,
                          RedisModuleString *ele,
                          double *score);

Available since: 4.0.0

On success retrieve the double score associated at the sorted set element ‘ele’ and returns REDISMODULE_OK. Otherwise REDISMODULE_ERR is returned to signal one of the following conditions:

  • There is no such element ‘ele’ in the sorted set.
  • The key is not a sorted set.
  • The key is an open empty key.

Key API for Sorted Set iterator

RedisModule_ZsetRangeStop

void RedisModule_ZsetRangeStop(RedisModuleKey *key);

Available since: 4.0.0

Stop a sorted set iteration.

RedisModule_ZsetRangeEndReached

int RedisModule_ZsetRangeEndReached(RedisModuleKey *key);

Available since: 4.0.0

Return the “End of range” flag value to signal the end of the iteration.

RedisModule_ZsetFirstInScoreRange

int RedisModule_ZsetFirstInScoreRange(RedisModuleKey *key,
                                      double min,
                                      double max,
                                      int minex,
                                      int maxex);

Available since: 4.0.0

Setup a sorted set iterator seeking the first element in the specified range. Returns REDISMODULE_OK if the iterator was correctly initialized otherwise REDISMODULE_ERR is returned in the following conditions:

  1. The value stored at key is not a sorted set or the key is empty.

The range is specified according to the two double values ‘min’ and ‘max’. Both can be infinite using the following two macros:

  • REDISMODULE_POSITIVE_INFINITE for positive infinite value
  • REDISMODULE_NEGATIVE_INFINITE for negative infinite value

‘minex’ and ‘maxex’ parameters, if true, respectively setup a range where the min and max value are exclusive (not included) instead of inclusive.

RedisModule_ZsetLastInScoreRange

int RedisModule_ZsetLastInScoreRange(RedisModuleKey *key,
                                     double min,
                                     double max,
                                     int minex,
                                     int maxex);

Available since: 4.0.0

Exactly like RedisModule_ZsetFirstInScoreRange() but the last element of the range is selected for the start of the iteration instead.

RedisModule_ZsetFirstInLexRange

int RedisModule_ZsetFirstInLexRange(RedisModuleKey *key,
                                    RedisModuleString *min,
                                    RedisModuleString *max);

Available since: 4.0.0

Setup a sorted set iterator seeking the first element in the specified lexicographical range. Returns REDISMODULE_OK if the iterator was correctly initialized otherwise REDISMODULE_ERR is returned in the following conditions:

  1. The value stored at key is not a sorted set or the key is empty.
  2. The lexicographical range ‘min’ and ‘max’ format is invalid.

‘min’ and ‘max’ should be provided as two RedisModuleString objects in the same format as the parameters passed to the ZRANGEBYLEX command. The function does not take ownership of the objects, so they can be released ASAP after the iterator is setup.

RedisModule_ZsetLastInLexRange

int RedisModule_ZsetLastInLexRange(RedisModuleKey *key,
                                   RedisModuleString *min,
                                   RedisModuleString *max);

Available since: 4.0.0

Exactly like RedisModule_ZsetFirstInLexRange() but the last element of the range is selected for the start of the iteration instead.

RedisModule_ZsetRangeCurrentElement

RedisModuleString *RedisModule_ZsetRangeCurrentElement(RedisModuleKey *key,
                                                       double *score);

Available since: 4.0.0

Return the current sorted set element of an active sorted set iterator or NULL if the range specified in the iterator does not include any element.

RedisModule_ZsetRangeNext

int RedisModule_ZsetRangeNext(RedisModuleKey *key);

Available since: 4.0.0

Go to the next element of the sorted set iterator. Returns 1 if there was a next element, 0 if we are already at the latest element or the range does not include any item at all.

RedisModule_ZsetRangePrev

int RedisModule_ZsetRangePrev(RedisModuleKey *key);

Available since: 4.0.0

Go to the previous element of the sorted set iterator. Returns 1 if there was a previous element, 0 if we are already at the first element or the range does not include any item at all.

Key API for Hash type

See also RedisModule_ValueLength(), which returns the number of fields in a hash.

RedisModule_HashSet

int RedisModule_HashSet(RedisModuleKey *key, int flags, ...);

Available since: 4.0.0

Set the field of the specified hash field to the specified value. If the key is an empty key open for writing, it is created with an empty hash value, in order to set the specified field.

The function is variadic and the user must specify pairs of field names and values, both as RedisModuleString pointers (unless the CFIELD option is set, see later). At the end of the field/value-ptr pairs, NULL must be specified as last argument to signal the end of the arguments in the variadic function.

Example to set the hash argv[1] to the value argv[2]:

 RedisModule_HashSet(key,REDISMODULE_HASH_NONE,argv[1],argv[2],NULL);

The function can also be used in order to delete fields (if they exist) by setting them to the specified value of REDISMODULE_HASH_DELETE:

 RedisModule_HashSet(key,REDISMODULE_HASH_NONE,argv[1],
                     REDISMODULE_HASH_DELETE,NULL);

The behavior of the command changes with the specified flags, that can be set to REDISMODULE_HASH_NONE if no special behavior is needed.

REDISMODULE_HASH_NX: The operation is performed only if the field was not
                     already existing in the hash.
REDISMODULE_HASH_XX: The operation is performed only if the field was
                     already existing, so that a new value could be
                     associated to an existing filed, but no new fields
                     are created.
REDISMODULE_HASH_CFIELDS: The field names passed are null terminated C
                          strings instead of RedisModuleString objects.
REDISMODULE_HASH_COUNT_ALL: Include the number of inserted fields in the
                            returned number, in addition to the number of
                            updated and deleted fields. (Added in Redis
                            6.2.)

Unless NX is specified, the command overwrites the old field value with the new one.

When using REDISMODULE_HASH_CFIELDS, field names are reported using normal C strings, so for example to delete the field “foo” the following code can be used:

 RedisModule_HashSet(key,REDISMODULE_HASH_CFIELDS,"foo",
                     REDISMODULE_HASH_DELETE,NULL);

Return value:

The number of fields existing in the hash prior to the call, which have been updated (its old value has been replaced by a new value) or deleted. If the flag REDISMODULE_HASH_COUNT_ALL is set, inserted fields not previously existing in the hash are also counted.

If the return value is zero, errno is set (since Redis 6.2) as follows:

  • EINVAL if any unknown flags are set or if key is NULL.
  • ENOTSUP if the key is associated with a non Hash value.
  • EBADF if the key was not opened for writing.
  • ENOENT if no fields were counted as described under Return value above. This is not actually an error. The return value can be zero if all fields were just created and the COUNT_ALL flag was unset, or if changes were held back due to the NX and XX flags.

NOTICE: The return value semantics of this function are very different between Redis 6.2 and older versions. Modules that use it should determine the Redis version and handle it accordingly.

RedisModule_HashGet

int RedisModule_HashGet(RedisModuleKey *key, int flags, ...);

Available since: 4.0.0

Get fields from an hash value. This function is called using a variable number of arguments, alternating a field name (as a RedisModuleString pointer) with a pointer to a RedisModuleString pointer, that is set to the value of the field if the field exists, or NULL if the field does not exist. At the end of the field/value-ptr pairs, NULL must be specified as last argument to signal the end of the arguments in the variadic function.

This is an example usage:

 RedisModuleString *first, *second;
 RedisModule_HashGet(mykey,REDISMODULE_HASH_NONE,argv[1],&first,
                     argv[2],&second,NULL);

As with RedisModule_HashSet() the behavior of the command can be specified passing flags different than REDISMODULE_HASH_NONE:

REDISMODULE_HASH_CFIELDS: field names as null terminated C strings.

REDISMODULE_HASH_EXISTS: instead of setting the value of the field expecting a RedisModuleString pointer to pointer, the function just reports if the field exists or not and expects an integer pointer as the second element of each pair.

Example of REDISMODULE_HASH_CFIELDS:

 RedisModuleString *username, *hashedpass;
 RedisModule_HashGet(mykey,REDISMODULE_HASH_CFIELDS,"username",&username,"hp",&hashedpass, NULL);

Example of REDISMODULE_HASH_EXISTS:

 int exists;
 RedisModule_HashGet(mykey,REDISMODULE_HASH_EXISTS,argv[1],&exists,NULL);

The function returns REDISMODULE_OK on success and REDISMODULE_ERR if the key is not an hash value.

Memory management:

The returned RedisModuleString objects should be released with RedisModule_FreeString(), or by enabling automatic memory management.

Key API for Stream type

For an introduction to streams, see https://redis.io/topics/streams-intro.

The type RedisModuleStreamID, which is used in stream functions, is a struct with two 64-bit fields and is defined as

typedef struct RedisModuleStreamID {
    uint64_t ms;
    uint64_t seq;
} RedisModuleStreamID;

See also RedisModule_ValueLength(), which returns the length of a stream, and the conversion functions RedisModule_StringToStreamID() and RedisModule_CreateStringFromStreamID().

RedisModule_StreamAdd

int RedisModule_StreamAdd(RedisModuleKey *key,
                          int flags,
                          RedisModuleStreamID *id,
                          RedisModuleString **argv,
                          long numfields);

Available since: 6.2.0

Adds an entry to a stream. Like XADD without trimming.

  • key: The key where the stream is (or will be) stored
  • flags: A bit field of
    • REDISMODULE_STREAM_ADD_AUTOID: Assign a stream ID automatically, like * in the XADD command.
  • id: If the AUTOID flag is set, this is where the assigned ID is returned. Can be NULL if AUTOID is set, if you don’t care to receive the ID. If AUTOID is not set, this is the requested ID.
  • argv: A pointer to an array of size numfields * 2 containing the fields and values.
  • numfields: The number of field-value pairs in argv.

Returns REDISMODULE_OK if an entry has been added. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with invalid arguments
  • ENOTSUP if the key refers to a value of a type other than stream
  • EBADF if the key was not opened for writing
  • EDOM if the given ID was 0-0 or not greater than all other IDs in the stream (only if the AUTOID flag is unset)
  • EFBIG if the stream has reached the last possible ID
  • ERANGE if the elements are too large to be stored.

RedisModule_StreamDelete

int RedisModule_StreamDelete(RedisModuleKey *key, RedisModuleStreamID *id);

Available since: 6.2.0

Deletes an entry from a stream.

  • key: A key opened for writing, with no stream iterator started.
  • id: The stream ID of the entry to delete.

Returns REDISMODULE_OK on success. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with invalid arguments
  • ENOTSUP if the key refers to a value of a type other than stream or if the key is empty
  • EBADF if the key was not opened for writing or if a stream iterator is associated with the key
  • ENOENT if no entry with the given stream ID exists

See also RedisModule_StreamIteratorDelete() for deleting the current entry while iterating using a stream iterator.

RedisModule_StreamIteratorStart

int RedisModule_StreamIteratorStart(RedisModuleKey *key,
                                    int flags,
                                    RedisModuleStreamID *start,
                                    RedisModuleStreamID *end);

Available since: 6.2.0

Sets up a stream iterator.

  • key: The stream key opened for reading using RedisModule_OpenKey().
  • flags:
    • REDISMODULE_STREAM_ITERATOR_EXCLUSIVE: Don’t include start and end in the iterated range.
    • REDISMODULE_STREAM_ITERATOR_REVERSE: Iterate in reverse order, starting from the end of the range.
  • start: The lower bound of the range. Use NULL for the beginning of the stream.
  • end: The upper bound of the range. Use NULL for the end of the stream.

Returns REDISMODULE_OK on success. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with invalid arguments
  • ENOTSUP if the key refers to a value of a type other than stream or if the key is empty
  • EBADF if the key was not opened for writing or if a stream iterator is already associated with the key
  • EDOM if start or end is outside the valid range

Returns REDISMODULE_OK on success and REDISMODULE_ERR if the key doesn’t refer to a stream or if invalid arguments were given.

The stream IDs are retrieved using RedisModule_StreamIteratorNextID() and for each stream ID, the fields and values are retrieved using RedisModule_StreamIteratorNextField(). The iterator is freed by calling RedisModule_StreamIteratorStop().

Example (error handling omitted):

RedisModule_StreamIteratorStart(key, 0, startid_ptr, endid_ptr);
RedisModuleStreamID id;
long numfields;
while (RedisModule_StreamIteratorNextID(key, &id, &numfields) ==
       REDISMODULE_OK) {
    RedisModuleString *field, *value;
    while (RedisModule_StreamIteratorNextField(key, &field, &value) ==
           REDISMODULE_OK) {
        //
        // ... Do stuff ...
        //
        RedisModule_FreeString(ctx, field);
        RedisModule_FreeString(ctx, value);
    }
}
RedisModule_StreamIteratorStop(key);

RedisModule_StreamIteratorStop

int RedisModule_StreamIteratorStop(RedisModuleKey *key);

Available since: 6.2.0

Stops a stream iterator created using RedisModule_StreamIteratorStart() and reclaims its memory.

Returns REDISMODULE_OK on success. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with a NULL key
  • ENOTSUP if the key refers to a value of a type other than stream or if the key is empty
  • EBADF if the key was not opened for writing or if no stream iterator is associated with the key

RedisModule_StreamIteratorNextID

int RedisModule_StreamIteratorNextID(RedisModuleKey *key,
                                     RedisModuleStreamID *id,
                                     long *numfields);

Available since: 6.2.0

Finds the next stream entry and returns its stream ID and the number of fields.

  • key: Key for which a stream iterator has been started using RedisModule_StreamIteratorStart().
  • id: The stream ID returned. NULL if you don’t care.
  • numfields: The number of fields in the found stream entry. NULL if you don’t care.

Returns REDISMODULE_OK and sets *id and *numfields if an entry was found. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with a NULL key
  • ENOTSUP if the key refers to a value of a type other than stream or if the key is empty
  • EBADF if no stream iterator is associated with the key
  • ENOENT if there are no more entries in the range of the iterator

In practice, if RedisModule_StreamIteratorNextID() is called after a successful call to RedisModule_StreamIteratorStart() and with the same key, it is safe to assume that an REDISMODULE_ERR return value means that there are no more entries.

Use RedisModule_StreamIteratorNextField() to retrieve the fields and values. See the example at RedisModule_StreamIteratorStart().

RedisModule_StreamIteratorNextField

int RedisModule_StreamIteratorNextField(RedisModuleKey *key,
                                        RedisModuleString **field_ptr,
                                        RedisModuleString **value_ptr);

Available since: 6.2.0

Retrieves the next field of the current stream ID and its corresponding value in a stream iteration. This function should be called repeatedly after calling RedisModule_StreamIteratorNextID() to fetch each field-value pair.

  • key: Key where a stream iterator has been started.
  • field_ptr: This is where the field is returned.
  • value_ptr: This is where the value is returned.

Returns REDISMODULE_OK and points *field_ptr and *value_ptr to freshly allocated RedisModuleString objects. The string objects are freed automatically when the callback finishes if automatic memory is enabled. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if called with a NULL key
  • ENOTSUP if the key refers to a value of a type other than stream or if the key is empty
  • EBADF if no stream iterator is associated with the key
  • ENOENT if there are no more fields in the current stream entry

In practice, if RedisModule_StreamIteratorNextField() is called after a successful call to RedisModule_StreamIteratorNextID() and with the same key, it is safe to assume that an REDISMODULE_ERR return value means that there are no more fields.

See the example at RedisModule_StreamIteratorStart().

RedisModule_StreamIteratorDelete

int RedisModule_StreamIteratorDelete(RedisModuleKey *key);

Available since: 6.2.0

Deletes the current stream entry while iterating.

This function can be called after RedisModule_StreamIteratorNextID() or after any calls to RedisModule_StreamIteratorNextField().

Returns REDISMODULE_OK on success. On failure, REDISMODULE_ERR is returned and errno is set as follows:

  • EINVAL if key is NULL
  • ENOTSUP if the key is empty or is of another type than stream
  • EBADF if the key is not opened for writing, if no iterator has been started
  • ENOENT if the iterator has no current stream entry

RedisModule_StreamTrimByLength

long long RedisModule_StreamTrimByLength(RedisModuleKey *key,
                                         int flags,
                                         long long length);

Available since: 6.2.0

Trim a stream by length, similar to XTRIM with MAXLEN.

  • key: Key opened for writing.
  • flags: A bitfield of
    • REDISMODULE_STREAM_TRIM_APPROX: Trim less if it improves performance, like XTRIM with ~.
  • length: The number of stream entries to keep after trimming.

Returns the number of entries deleted. On failure, a negative value is returned and errno is set as follows:

  • EINVAL if called with invalid arguments
  • ENOTSUP if the key is empty or of a type other than stream
  • EBADF if the key is not opened for writing

RedisModule_StreamTrimByID

long long RedisModule_StreamTrimByID(RedisModuleKey *key,
                                     int flags,
                                     RedisModuleStreamID *id);

Available since: 6.2.0

Trim a stream by ID, similar to XTRIM with MINID.

  • key: Key opened for writing.
  • flags: A bitfield of
    • REDISMODULE_STREAM_TRIM_APPROX: Trim less if it improves performance, like XTRIM with ~.
  • id: The smallest stream ID to keep after trimming.

Returns the number of entries deleted. On failure, a negative value is returned and errno is set as follows:

  • EINVAL if called with invalid arguments
  • ENOTSUP if the key is empty or of a type other than stream
  • EBADF if the key is not opened for writing

Calling Redis commands from modules

RedisModule_Call() sends a command to Redis. The remaining functions handle the reply.

RedisModule_FreeCallReply

void RedisModule_FreeCallReply(RedisModuleCallReply *reply);

Available since: 4.0.0

Free a Call reply and all the nested replies it contains if it’s an array.

RedisModule_CallReplyType

int RedisModule_CallReplyType(RedisModuleCallReply *reply);

Available since: 4.0.0

Return the reply type as one of the following:

  • REDISMODULE_REPLY_UNKNOWN
  • REDISMODULE_REPLY_STRING
  • REDISMODULE_REPLY_ERROR
  • REDISMODULE_REPLY_INTEGER
  • REDISMODULE_REPLY_ARRAY
  • REDISMODULE_REPLY_NULL
  • REDISMODULE_REPLY_MAP
  • REDISMODULE_REPLY_SET
  • REDISMODULE_REPLY_BOOL
  • REDISMODULE_REPLY_DOUBLE
  • REDISMODULE_REPLY_BIG_NUMBER
  • REDISMODULE_REPLY_VERBATIM_STRING
  • REDISMODULE_REPLY_ATTRIBUTE

RedisModule_CallReplyLength

size_t RedisModule_CallReplyLength(RedisModuleCallReply *reply);

Available since: 4.0.0

Return the reply type length, where applicable.

RedisModule_CallReplyArrayElement

RedisModuleCallReply *RedisModule_CallReplyArrayElement(RedisModuleCallReply *reply,
                                                        size_t idx);

Available since: 4.0.0

Return the ‘idx’-th nested call reply element of an array reply, or NULL if the reply type is wrong or the index is out of range.

RedisModule_CallReplyInteger

long long RedisModule_CallReplyInteger(RedisModuleCallReply *reply);

Available since: 4.0.0

Return the long long of an integer reply.

RedisModule_CallReplyDouble

double RedisModule_CallReplyDouble(RedisModuleCallReply *reply);

Return the double value of a double reply.

RedisModule_CallReplyBigNumber

const char *RedisModule_CallReplyBigNumber(RedisModuleCallReply *reply,
                                           size_t *len);

Return the big number value of a big number reply.

RedisModule_CallReplyVerbatim

const char *RedisModule_CallReplyVerbatim(RedisModuleCallReply *reply,
                                          size_t *len,
                                          const char **format);

Return the value of an verbatim string reply, An optional output argument can be given to get verbatim reply format.

RedisModule_CallReplyBool

int RedisModule_CallReplyBool(RedisModuleCallReply *reply);

Return the Boolean value of a Boolean reply.

RedisModule_CallReplySetElement

RedisModuleCallReply *RedisModule_CallReplySetElement(RedisModuleCallReply *reply,
                                                      size_t idx);

Return the ‘idx’-th nested call reply element of a set reply, or NULL if the reply type is wrong or the index is out of range.

RedisModule_CallReplyMapElement

int RedisModule_CallReplyMapElement(RedisModuleCallReply *reply,
                                    size_t idx,
                                    RedisModuleCallReply **key,
                                    RedisModuleCallReply **val);

Retrieve the ‘idx’-th key and value of a map reply.

Returns:

  • REDISMODULE_OK on success.
  • REDISMODULE_ERR if idx out of range or if the reply type is wrong.

The key and value arguments are used to return by reference, and may be NULL if not required.

RedisModule_CallReplyAttribute

RedisModuleCallReply *RedisModule_CallReplyAttribute(RedisModuleCallReply *reply);

Return the attribute of the given reply, or NULL if no attribute exists.

RedisModule_CallReplyAttributeElement

int RedisModule_CallReplyAttributeElement(RedisModuleCallReply *reply,
                                          size_t idx,
                                          RedisModuleCallReply **key,
                                          RedisModuleCallReply **val);

Retrieve the ‘idx’-th key and value of a attribute reply.

Returns:

  • REDISMODULE_OK on success.
  • REDISMODULE_ERR if idx out of range or if the reply type is wrong.

The key and value arguments are used to return by reference, and may be NULL if not required.

RedisModule_CallReplyStringPtr

const char *RedisModule_CallReplyStringPtr(RedisModuleCallReply *reply,
                                           size_t *len);

Available since: 4.0.0

Return the pointer and length of a string or error reply.

RedisModule_CreateStringFromCallReply

RedisModuleString *RedisModule_CreateStringFromCallReply(RedisModuleCallReply *reply);

Available since: 4.0.0

Return a new string object from a call reply of type string, error or integer. Otherwise (wrong reply type) return NULL.

RedisModule_Call

RedisModuleCallReply *RedisModule_Call(RedisModuleCtx *ctx,
                                       const char *cmdname,
                                       const char *fmt,
                                       ...);

Available since: 4.0.0

Exported API to call any Redis command from modules.

  • cmdname: The Redis command to call.

  • fmt: A format specifier string for the command’s arguments. Each of the arguments should be specified by a valid type specification. The format specifier can also contain the modifiers !, A, 3 and R which don’t have a corresponding argument.

    • b – The argument is a buffer and is immediately followed by another argument that is the buffer’s length.
    • c – The argument is a pointer to a plain C string (null-terminated).
    • l – The argument is long long integer.
    • s – The argument is a RedisModuleString.
    • v – The argument(s) is a vector of RedisModuleString.
    • ! – Sends the Redis command and its arguments to replicas and AOF.
    • A – Suppress AOF propagation, send only to replicas (requires !).
    • R – Suppress replicas propagation, send only to AOF (requires !).
    • 3 – Return a RESP3 reply. This will change the command reply. e.g., HGETALL returns a map instead of a flat array.
    • 0 – Return the reply in auto mode, i.e. the reply format will be the same as the client attached to the given RedisModuleCtx. This will probably used when you want to pass the reply directly to the client.
    • C – Check if command can be executed according to ACL rules.

  • …: The actual arguments to the Redis command.

On success a RedisModuleCallReply object is returned, otherwise NULL is returned and errno is set to the following values:

  • EBADF: wrong format specifier.
  • EINVAL: wrong command arity.
  • ENOENT: command does not exist.
  • EPERM: operation in Cluster instance with key in non local slot.
  • EROFS: operation in Cluster instance when a write command is sent in a readonly state.
  • ENETDOWN: operation in Cluster instance when cluster is down.
  • ENOTSUP: No ACL user for the specified module context
  • EACCES: Command cannot be executed, according to ACL rules

Example code fragment:

 reply = RedisModule_Call(ctx,"INCRBY","sc",argv[1],"10");
 if (RedisModule_CallReplyType(reply) == REDISMODULE_REPLY_INTEGER) {
   long long myval = RedisModule_CallReplyInteger(reply);
   // Do something with myval.
 }

This API is documented here: https://redis.io/topics/modules-intro

RedisModule_CallReplyProto

const char *RedisModule_CallReplyProto(RedisModuleCallReply *reply,
                                       size_t *len);

Available since: 4.0.0

Return a pointer, and a length, to the protocol returned by the command that returned the reply object.

Modules data types

When String DMA or using existing data structures is not enough, it is possible to create new data types from scratch and export them to Redis. The module must provide a set of callbacks for handling the new values exported (for example in order to provide RDB saving/loading, AOF rewrite, and so forth). In this section we define this API.

RedisModule_CreateDataType

moduleType *RedisModule_CreateDataType(RedisModuleCtx *ctx,
                                       const char *name,
                                       int encver,
                                       void *typemethods_ptr);

Available since: 4.0.0

Register a new data type exported by the module. The parameters are the following. Please for in depth documentation check the modules API documentation, especially https://redis.io/topics/modules-native-types.

  • name: A 9 characters data type name that MUST be unique in the Redis Modules ecosystem. Be creative… and there will be no collisions. Use the charset A-Z a-z 9-0, plus the two “-_” characters. A good idea is to use, for example <typename>-<vendor>. For example “tree-AntZ” may mean “Tree data structure by @antirez”. To use both lower case and upper case letters helps in order to prevent collisions.

  • encver: Encoding version, which is, the version of the serialization that a module used in order to persist data. As long as the “name” matches, the RDB loading will be dispatched to the type callbacks whatever ‘encver’ is used, however the module can understand if the encoding it must load are of an older version of the module. For example the module “tree-AntZ” initially used encver=0. Later after an upgrade, it started to serialize data in a different format and to register the type with encver=1. However this module may still load old data produced by an older version if the rdb_load callback is able to check the encver value and act accordingly. The encver must be a positive value between 0 and 1023.

  • typemethods_ptr is a pointer to a RedisModuleTypeMethods structure that should be populated with the methods callbacks and structure version, like in the following example:

      RedisModuleTypeMethods tm = {
      .version = REDISMODULE_TYPE_METHOD_VERSION,
      .rdb_load = myType_RDBLoadCallBack,
      .rdb_save = myType_RDBSaveCallBack,
      .aof_rewrite = myType_AOFRewriteCallBack,
      .free = myType_FreeCallBack,

      // Optional fields
      .digest = myType_DigestCallBack,
      .mem_usage = myType_MemUsageCallBack,
      .aux_load = myType_AuxRDBLoadCallBack,
      .aux_save = myType_AuxRDBSaveCallBack,
      .free_effort = myType_FreeEffortCallBack,
      .unlink = myType_UnlinkCallBack,
      .copy = myType_CopyCallback,
      .defrag = myType_DefragCallback

      // Enhanced optional fields
      .mem_usage2 = myType_MemUsageCallBack2,
      .free_effort2 = myType_FreeEffortCallBack2,
      .unlink2 = myType_UnlinkCallBack2,
      .copy2 = myType_CopyCallback2,
  }

  • rdb_load: A callback function pointer that loads data from RDB files.

  • rdb_save: A callback function pointer that saves data to RDB files.

  • aof_rewrite: A callback function pointer that rewrites data as commands.

  • digest: A callback function pointer that is used for DEBUG DIGEST.

  • free: A callback function pointer that can free a type value.

  • aux_save: A callback function pointer that saves out of keyspace data to RDB files. ‘when’ argument is either REDISMODULE_AUX_BEFORE_RDB or REDISMODULE_AUX_AFTER_RDB.

  • aux_load: A callback function pointer that loads out of keyspace data from RDB files. Similar to aux_save, returns REDISMODULE_OK on success, and ERR otherwise.

  • free_effort: A callback function pointer that used to determine whether the module’s memory needs to be lazy reclaimed. The module should return the complexity involved by freeing the value. for example: how many pointers are gonna be freed. Note that if it returns 0, we’ll always do an async free.

  • unlink: A callback function pointer that used to notifies the module that the key has been removed from the DB by redis, and may soon be freed by a background thread. Note that it won’t be called on FLUSHALL/FLUSHDB (both sync and async), and the module can use the RedisModuleEvent_FlushDB to hook into that.

  • copy: A callback function pointer that is used to make a copy of the specified key. The module is expected to perform a deep copy of the specified value and return it. In addition, hints about the names of the source and destination keys is provided. A NULL return value is considered an error and the copy operation fails. Note: if the target key exists and is being overwritten, the copy callback will be called first, followed by a free callback to the value that is being replaced.

  • defrag: A callback function pointer that is used to request the module to defrag a key. The module should then iterate pointers and call the relevant RedisModule_Defrag*() functions to defragment pointers or complex types. The module should continue iterating as long as RedisModule_DefragShouldStop() returns a zero value, and return a zero value if finished or non-zero value if more work is left to be done. If more work needs to be done, RedisModule_DefragCursorSet() and RedisModule_DefragCursorGet() can be used to track this work across different calls. Normally, the defrag mechanism invokes the callback without a time limit, so RedisModule_DefragShouldStop() always returns zero. The “late defrag” mechanism which has a time limit and provides cursor support is used only for keys that are determined to have significant internal complexity. To determine this, the defrag mechanism uses the free_effort callback and the ‘active-defrag-max-scan-fields’ config directive. NOTE: The value is passed as a void** and the function is expected to update the pointer if the top-level value pointer is defragmented and consequently changes.

  • mem_usage2: Similar to mem_usage, but provides the RedisModuleKeyOptCtx parameter so that meta information such as key name and db id can be obtained, and the sample_size for size estimation (see MEMORY USAGE command).

  • free_effort2: Similar to free_effort, but provides the RedisModuleKeyOptCtx parameter so that meta information such as key name and db id can be obtained.

  • unlink2: Similar to unlink, but provides the RedisModuleKeyOptCtx parameter so that meta information such as key name and db id can be obtained.

  • copy2: Similar to copy, but provides the RedisModuleKeyOptCtx parameter so that meta information such as key names and db ids can be obtained.

Note: the module name “AAAAAAAAA” is reserved and produces an error, it happens to be pretty lame as well.

If there is already a module registering a type with the same name, and if the module name or encver is invalid, NULL is returned. Otherwise the new type is registered into Redis, and a reference of type RedisModuleType is returned: the caller of the function should store this reference into a global variable to make future use of it in the modules type API, since a single module may register multiple types. Example code fragment:

 static RedisModuleType *BalancedTreeType;

 int RedisModule_OnLoad(RedisModuleCtx *ctx) {
     // some code here ...
     BalancedTreeType = RM_CreateDataType(...);
 }

RedisModule_ModuleTypeSetValue

int RedisModule_ModuleTypeSetValue(RedisModuleKey *key,
                                   moduleType *mt,
                                   void *value);

Available since: 4.0.0

If the key is open for writing, set the specified module type object as the value of the key, deleting the old value if any. On success REDISMODULE_OK is returned. If the key is not open for writing or there is an active iterator, REDISMODULE_ERR is returned.

RedisModule_ModuleTypeGetType

moduleType *RedisModule_ModuleTypeGetType(RedisModuleKey *key);

Available since: 4.0.0

Assuming RedisModule_KeyType() returned REDISMODULE_KEYTYPE_MODULE on the key, returns the module type pointer of the value stored at key.

If the key is NULL, is not associated with a module type, or is empty, then NULL is returned instead.

RedisModule_ModuleTypeGetValue

void *RedisModule_ModuleTypeGetValue(RedisModuleKey *key);

Available since: 4.0.0

Assuming RedisModule_KeyType() returned REDISMODULE_KEYTYPE_MODULE on the key, returns the module type low-level value stored at key, as it was set by the user via RedisModule_ModuleTypeSetValue().

If the key is NULL, is not associated with a module type, or is empty, then NULL is returned instead.

RDB loading and saving functions

RedisModule_IsIOError

int RedisModule_IsIOError(RedisModuleIO *io);

Available since: 6.0.0

Returns true if any previous IO API failed. for Load* APIs the REDISMODULE_OPTIONS_HANDLE_IO_ERRORS flag must be set with RedisModule_SetModuleOptions first.

RedisModule_SaveUnsigned

void RedisModule_SaveUnsigned(RedisModuleIO *io, uint64_t value);

Available since: 4.0.0

Save an unsigned 64 bit value into the RDB file. This function should only be called in the context of the rdb_save method of modules implementing new data types.

RedisModule_LoadUnsigned

uint64_t RedisModule_LoadUnsigned(RedisModuleIO *io);

Available since: 4.0.0

Load an unsigned 64 bit value from the RDB file. This function should only be called in the context of the rdb_load method of modules implementing new data types.

RedisModule_SaveSigned

void RedisModule_SaveSigned(RedisModuleIO *io, int64_t value);

Available since: 4.0.0

Like RedisModule_SaveUnsigned() but for signed 64 bit values.

RedisModule_LoadSigned

int64_t RedisModule_LoadSigned(RedisModuleIO *io);

Available since: 4.0.0

Like RedisModule_LoadUnsigned() but for signed 64 bit values.

RedisModule_SaveString

void RedisModule_SaveString(RedisModuleIO *io, RedisModuleString *s);

Available since: 4.0.0

In the context of the rdb_save method of a module type, saves a string into the RDB file taking as input a RedisModuleString.

The string can be later loaded with RedisModule_LoadString() or other Load family functions expecting a serialized string inside the RDB file.

RedisModule_SaveStringBuffer

void RedisModule_SaveStringBuffer(RedisModuleIO *io,
                                  const char *str,
                                  size_t len);

Available since: 4.0.0

Like RedisModule_SaveString() but takes a raw C pointer and length as input.

RedisModule_LoadString

RedisModuleString *RedisModule_LoadString(RedisModuleIO *io);

Available since: 4.0.0

In the context of the rdb_load method of a module data type, loads a string from the RDB file, that was previously saved with RedisModule_SaveString() functions family.

The returned string is a newly allocated RedisModuleString object, and the user should at some point free it with a call to RedisModule_FreeString().

If the data structure does not store strings as RedisModuleString objects, the similar function RedisModule_LoadStringBuffer() could be used instead.

RedisModule_LoadStringBuffer

char *RedisModule_LoadStringBuffer(RedisModuleIO *io, size_t *lenptr);

Available since: 4.0.0

Like RedisModule_LoadString() but returns an heap allocated string that was allocated with RedisModule_Alloc(), and can be resized or freed with RedisModule_Realloc() or RedisModule_Free().

The size of the string is stored at ‘*lenptr’ if not NULL. The returned string is not automatically NULL terminated, it is loaded exactly as it was stored inside the RDB file.

RedisModule_SaveDouble

void RedisModule_SaveDouble(RedisModuleIO *io, double value);

Available since: 4.0.0

In the context of the rdb_save method of a module data type, saves a double value to the RDB file. The double can be a valid number, a NaN or infinity. It is possible to load back the value with RedisModule_LoadDouble().

RedisModule_LoadDouble

double RedisModule_LoadDouble(RedisModuleIO *io);

Available since: 4.0.0

In the context of the rdb_save method of a module data type, loads back the double value saved by RedisModule_SaveDouble().

RedisModule_SaveFloat

void RedisModule_SaveFloat(RedisModuleIO *io, float value);

Available since: 4.0.0

In the context of the rdb_save method of a module data type, saves a float value to the RDB file. The float can be a valid number, a NaN or infinity. It is possible to load back the value with RedisModule_LoadFloat().

RedisModule_LoadFloat

float RedisModule_LoadFloat(RedisModuleIO *io);

Available since: 4.0.0

In the context of the rdb_save method of a module data type, loads back the float value saved by RedisModule_SaveFloat().

RedisModule_SaveLongDouble

void RedisModule_SaveLongDouble(RedisModuleIO *io, long double value);

Available since: 6.0.0

In the context of the rdb_save method of a module data type, saves a long double value to the RDB file. The double can be a valid number, a NaN or infinity. It is possible to load back the value with RedisModule_LoadLongDouble().

RedisModule_LoadLongDouble

long double RedisModule_LoadLongDouble(RedisModuleIO *io);

Available since: 6.0.0

In the context of the rdb_save method of a module data type, loads back the long double value saved by RedisModule_SaveLongDouble().

Key digest API (DEBUG DIGEST interface for modules types)

RedisModule_DigestAddStringBuffer

void RedisModule_DigestAddStringBuffer(RedisModuleDigest *md,
                                       const char *ele,
                                       size_t len);

Available since: 4.0.0

Add a new element to the digest. This function can be called multiple times one element after the other, for all the elements that constitute a given data structure. The function call must be followed by the call to RedisModule_DigestEndSequence eventually, when all the elements that are always in a given order are added. See the Redis Modules data types documentation for more info. However this is a quick example that uses Redis data types as an example.

To add a sequence of unordered elements (for example in the case of a Redis Set), the pattern to use is:

foreach element {
    AddElement(element);
    EndSequence();
}

Because Sets are not ordered, so every element added has a position that does not depend from the other. However if instead our elements are ordered in pairs, like field-value pairs of an Hash, then one should use:

foreach key,value {
    AddElement(key);
    AddElement(value);
    EndSequence();
}

Because the key and value will be always in the above order, while instead the single key-value pairs, can appear in any position into a Redis hash.

A list of ordered elements would be implemented with:

foreach element {
    AddElement(element);
}
EndSequence();

RedisModule_DigestAddLongLong

void RedisModule_DigestAddLongLong(RedisModuleDigest *md, long long ll);

Available since: 4.0.0

Like RedisModule_DigestAddStringBuffer() but takes a long long as input that gets converted into a string before adding it to the digest.

RedisModule_DigestEndSequence

void RedisModule_DigestEndSequence(RedisModuleDigest *md);

Available since: 4.0.0

See the documentation for RedisModule_DigestAddElement().

RedisModule_LoadDataTypeFromStringEncver

void *RedisModule_LoadDataTypeFromStringEncver(const RedisModuleString *str,
                                               const moduleType *mt,
                                               int encver);

Decode a serialized representation of a module data type ‘mt’, in a specific encoding version ‘encver’ from string ‘str’ and return a newly allocated value, or NULL if decoding failed.

This call basically reuses the ‘rdb_load’ callback which module data types implement in order to allow a module to arbitrarily serialize/de-serialize keys, similar to how the Redis ‘DUMP’ and ‘RESTORE’ commands are implemented.

Modules should generally use the REDISMODULE_OPTIONS_HANDLE_IO_ERRORS flag and make sure the de-serialization code properly checks and handles IO errors (freeing allocated buffers and returning a NULL).

If this is NOT done, Redis will handle corrupted (or just truncated) serialized data by producing an error message and terminating the process.

RedisModule_LoadDataTypeFromString

void *RedisModule_LoadDataTypeFromString(const RedisModuleString *str,
                                         const moduleType *mt);

Available since: 6.0.0

Similar to RedisModule_LoadDataTypeFromStringEncver, original version of the API, kept for backward compatibility.

RedisModule_SaveDataTypeToString

RedisModuleString *RedisModule_SaveDataTypeToString(RedisModuleCtx *ctx,
                                                    void *data,
                                                    const moduleType *mt);

Available since: 6.0.0

Encode a module data type ‘mt’ value ‘data’ into serialized form, and return it as a newly allocated RedisModuleString.

This call basically reuses the ‘rdb_save’ callback which module data types implement in order to allow a module to arbitrarily serialize/de-serialize keys, similar to how the Redis ‘DUMP’ and ‘RESTORE’ commands are implemented.

RedisModule_GetKeyNameFromDigest

const RedisModuleString *RedisModule_GetKeyNameFromDigest(RedisModuleDigest *dig);

Returns the name of the key currently being processed.

RedisModule_GetDbIdFromDigest

int RedisModule_GetDbIdFromDigest(RedisModuleDigest *dig);

Returns the database id of the key currently being processed.

AOF API for modules data types

RedisModule_EmitAOF

void RedisModule_EmitAOF(RedisModuleIO *io,
                         const char *cmdname,
                         const char *fmt,
                         ...);

Available since: 4.0.0

Emits a command into the AOF during the AOF rewriting process. This function is only called in the context of the aof_rewrite method of data types exported by a module. The command works exactly like RedisModule_Call() in the way the parameters are passed, but it does not return anything as the error handling is performed by Redis itself.

IO context handling

RedisModule_GetKeyNameFromIO

const RedisModuleString *RedisModule_GetKeyNameFromIO(RedisModuleIO *io);

Available since: 5.0.5

Returns the name of the key currently being processed. There is no guarantee that the key name is always available, so this may return NULL.

RedisModule_GetKeyNameFromModuleKey

const RedisModuleString *RedisModule_GetKeyNameFromModuleKey(RedisModuleKey *key);

Available since: 6.0.0

Returns a RedisModuleString with the name of the key from RedisModuleKey.

RedisModule_GetDbIdFromModuleKey

int RedisModule_GetDbIdFromModuleKey(RedisModuleKey *key);

Returns a database id of the key from RedisModuleKey.

RedisModule_GetDbIdFromIO

int RedisModule_GetDbIdFromIO(RedisModuleIO *io);

Returns the database id of the key currently being processed. There is no guarantee that this info is always available, so this may return -1.

Logging

RedisModule_Log

void RedisModule_Log(RedisModuleCtx *ctx,
                     const char *levelstr,
                     const char *fmt,
                     ...);

Available since: 4.0.0

Produces a log message to the standard Redis log, the format accepts printf-alike specifiers, while level is a string describing the log level to use when emitting the log, and must be one of the following:

  • “debug” (REDISMODULE_LOGLEVEL_DEBUG)
  • “verbose” (REDISMODULE_LOGLEVEL_VERBOSE)
  • “notice” (REDISMODULE_LOGLEVEL_NOTICE)
  • “warning” (REDISMODULE_LOGLEVEL_WARNING)

If the specified log level is invalid, verbose is used by default. There is a fixed limit to the length of the log line this function is able to emit, this limit is not specified but is guaranteed to be more than a few lines of text.

The ctx argument may be NULL if cannot be provided in the context of the caller for instance threads or callbacks, in which case a generic “module” will be used instead of the module name.

RedisModule_LogIOError

void RedisModule_LogIOError(RedisModuleIO *io,
                            const char *levelstr,
                            const char *fmt,
                            ...);

Available since: 4.0.0

Log errors from RDB / AOF serialization callbacks.

This function should be used when a callback is returning a critical error to the caller since cannot load or save the data for some critical reason.

RedisModule__Assert

void RedisModule__Assert(const char *estr, const char *file, int line);

Available since: 6.0.0

Redis-like assert function.

The macro RedisModule_Assert(expression) is recommended, rather than calling this function directly.

A failed assertion will shut down the server and produce logging information that looks identical to information generated by Redis itself.

RedisModule_LatencyAddSample

void RedisModule_LatencyAddSample(const char *event, mstime_t latency);

Available since: 6.0.0

Allows adding event to the latency monitor to be observed by the LATENCY command. The call is skipped if the latency is smaller than the configured latency-monitor-threshold.

Blocking clients from modules

For a guide about blocking commands in modules, see https://redis.io/topics/modules-blocking-ops.

RedisModule_BlockClient

RedisModuleBlockedClient *RedisModule_BlockClient(RedisModuleCtx *ctx,
                                                  RedisModuleCmdFunc reply_callback,
                                                  RedisModuleCmdFunc timeout_callback,
                                                  void (*free_privdata)(RedisModuleCtx*, void*),
                                                  long long timeout_ms);

Available since: 4.0.0

Block a client in the context of a blocking command, returning an handle which will be used, later, in order to unblock the client with a call to RedisModule_UnblockClient(). The arguments specify callback functions and a timeout after which the client is unblocked.

The callbacks are called in the following contexts:

reply_callback:   called after a successful RedisModule_UnblockClient()
                  call in order to reply to the client and unblock it.

timeout_callback: called when the timeout is reached or if `CLIENT UNBLOCK`
                  is invoked, in order to send an error to the client.

free_privdata:    called in order to free the private data that is passed
                  by RedisModule_UnblockClient() call.

Note: RedisModule_UnblockClient should be called for every blocked client, even if client was killed, timed-out or disconnected. Failing to do so will result in memory leaks.

There are some cases where RedisModule_BlockClient() cannot be used:

  1. If the client is a Lua script.
  2. If the client is executing a MULTI block.

In these cases, a call to RedisModule_BlockClient() will not block the client, but instead produce a specific error reply.

A module that registers a timeout_callback function can also be unblocked using the CLIENT UNBLOCK command, which will trigger the timeout callback. If a callback function is not registered, then the blocked client will be treated as if it is not in a blocked state and CLIENT UNBLOCK will return a zero value.

Measuring background time: By default the time spent in the blocked command is not account for the total command duration. To include such time you should use RedisModule_BlockedClientMeasureTimeStart() and RedisModule_BlockedClientMeasureTimeEnd() one, or multiple times within the blocking command background work.

RedisModule_BlockClientOnKeys

RedisModuleBlockedClient *RedisModule_BlockClientOnKeys(RedisModuleCtx *ctx,
                                                        RedisModuleCmdFunc reply_callback,
                                                        RedisModuleCmdFunc timeout_callback,
                                                        void (*free_privdata)(RedisModuleCtx*, void*),
                                                        long long timeout_ms,
                                                        RedisModuleString **keys,
                                                        int numkeys,
                                                        void *privdata);

Available since: 6.0.0

This call is similar to RedisModule_BlockClient(), however in this case we don’t just block the client, but also ask Redis to unblock it automatically once certain keys become “ready”, that is, contain more data.

Basically this is similar to what a typical Redis command usually does, like BLPOP or BZPOPMAX: the client blocks if it cannot be served ASAP, and later when the key receives new data (a list push for instance), the client is unblocked and served.

However in the case of this module API, when the client is unblocked?

  1. If you block on a key of a type that has blocking operations associated, like a list, a sorted set, a stream, and so forth, the client may be unblocked once the relevant key is targeted by an operation that normally unblocks the native blocking operations for that type. So if we block on a list key, an RPUSH command may unblock our client and so forth.
  2. If you are implementing your native data type, or if you want to add new unblocking conditions in addition to “1”, you can call the modules API RedisModule_SignalKeyAsReady().

Anyway we can’t be sure if the client should be unblocked just because the key is signaled as ready: for instance a successive operation may change the key, or a client in queue before this one can be served, modifying the key as well and making it empty again. So when a client is blocked with RedisModule_BlockClientOnKeys() the reply callback is not called after RedisModule_UnblockClient() is called, but every time a key is signaled as ready: if the reply callback can serve the client, it returns REDISMODULE_OK and the client is unblocked, otherwise it will return REDISMODULE_ERR and we’ll try again later.

The reply callback can access the key that was signaled as ready by calling the API RedisModule_GetBlockedClientReadyKey(), that returns just the string name of the key as a RedisModuleString object.

Thanks to this system we can setup complex blocking scenarios, like unblocking a client only if a list contains at least 5 items or other more fancy logics.

Note that another difference with RedisModule_BlockClient(), is that here we pass the private data directly when blocking the client: it will be accessible later in the reply callback. Normally when blocking with RedisModule_BlockClient() the private data to reply to the client is passed when calling RedisModule_UnblockClient() but here the unblocking is performed by Redis itself, so we need to have some private data before hand. The private data is used to store any information about the specific unblocking operation that you are implementing. Such information will be freed using the free_privdata callback provided by the user.

However the reply callback will be able to access the argument vector of the command, so the private data is often not needed.

Note: Under normal circumstances RedisModule_UnblockClient should not be called for clients that are blocked on keys (Either the key will become ready or a timeout will occur). If for some reason you do want to call RedisModule_UnblockClient it is possible: Client will be handled as if it were timed-out (You must implement the timeout callback in that case).

RedisModule_SignalKeyAsReady

void RedisModule_SignalKeyAsReady(RedisModuleCtx *ctx, RedisModuleString *key);

Available since: 6.0.0

This function is used in order to potentially unblock a client blocked on keys with RedisModule_BlockClientOnKeys(). When this function is called, all the clients blocked for this key will get their reply_callback called.

Note: The function has no effect if the signaled key doesn’t exist.

RedisModule_UnblockClient

int RedisModule_UnblockClient(RedisModuleBlockedClient *bc, void *privdata);

Available since: 4.0.0

Unblock a client blocked by RedisModule_BlockedClient. This will trigger the reply callbacks to be called in order to reply to the client. The ‘privdata’ argument will be accessible by the reply callback, so the caller of this function can pass any value that is needed in order to actually reply to the client.

A common usage for ‘privdata’ is a thread that computes something that needs to be passed to the client, included but not limited some slow to compute reply or some reply obtained via networking.

Note 1: this function can be called from threads spawned by the module.

Note 2: when we unblock a client that is blocked for keys using the API RedisModule_BlockClientOnKeys(), the privdata argument here is not used. Unblocking a client that was blocked for keys using this API will still require the client to get some reply, so the function will use the “timeout” handler in order to do so (The privdata provided in RedisModule_BlockClientOnKeys() is accessible from the timeout callback via RedisModule_GetBlockedClientPrivateData).

RedisModule_AbortBlock

int RedisModule_AbortBlock(RedisModuleBlockedClient *bc);

Available since: 4.0.0

Abort a blocked client blocking operation: the client will be unblocked without firing any callback.

RedisModule_SetDisconnectCallback

void RedisModule_SetDisconnectCallback(RedisModuleBlockedClient *bc,
                                       RedisModuleDisconnectFunc callback);

Available since: 5.0.0

Set a callback that will be called if a blocked client disconnects before the module has a chance to call RedisModule_UnblockClient()

Usually what you want to do there, is to cleanup your module state so that you can call RedisModule_UnblockClient() safely, otherwise the client will remain blocked forever if the timeout is large.

Notes:

  1. It is not safe to call Reply* family functions here, it is also useless since the client is gone.

  2. This callback is not called if the client disconnects because of a timeout. In such a case, the client is unblocked automatically and the timeout callback is called.

RedisModule_IsBlockedReplyRequest

int RedisModule_IsBlockedReplyRequest(RedisModuleCtx *ctx);

Available since: 4.0.0

Return non-zero if a module command was called in order to fill the reply for a blocked client.

RedisModule_IsBlockedTimeoutRequest

int RedisModule_IsBlockedTimeoutRequest(RedisModuleCtx *ctx);

Available since: 4.0.0

Return non-zero if a module command was called in order to fill the reply for a blocked client that timed out.

RedisModule_GetBlockedClientPrivateData

void *RedisModule_GetBlockedClientPrivateData(RedisModuleCtx *ctx);

Available since: 4.0.0

Get the private data set by RedisModule_UnblockClient()

RedisModule_GetBlockedClientReadyKey

RedisModuleString *RedisModule_GetBlockedClientReadyKey(RedisModuleCtx *ctx);

Available since: 6.0.0

Get the key that is ready when the reply callback is called in the context of a client blocked by RedisModule_BlockClientOnKeys().

RedisModule_GetBlockedClientHandle

RedisModuleBlockedClient *RedisModule_GetBlockedClientHandle(RedisModuleCtx *ctx);

Available since: 5.0.0

Get the blocked client associated with a given context. This is useful in the reply and timeout callbacks of blocked clients, before sometimes the module has the blocked client handle references around, and wants to cleanup it.

RedisModule_BlockedClientDisconnected

int RedisModule_BlockedClientDisconnected(RedisModuleCtx *ctx);

Available since: 5.0.0

Return true if when the free callback of a blocked client is called, the reason for the client to be unblocked is that it disconnected while it was blocked.

Thread Safe Contexts

RedisModule_GetThreadSafeContext

RedisModuleCtx *RedisModule_GetThreadSafeContext(RedisModuleBlockedClient *bc);

Available since: 4.0.0

Return a context which can be used inside threads to make Redis context calls with certain modules APIs. If ‘bc’ is not NULL then the module will be bound to a blocked client, and it will be possible to use the RedisModule_Reply* family of functions to accumulate a reply for when the client will be unblocked. Otherwise the thread safe context will be detached by a specific client.

To call non-reply APIs, the thread safe context must be prepared with:

RedisModule_ThreadSafeContextLock(ctx);
... make your call here ...
RedisModule_ThreadSafeContextUnlock(ctx);

This is not needed when using RedisModule_Reply* functions, assuming that a blocked client was used when the context was created, otherwise no RedisModule_Reply* call should be made at all.

NOTE: If you’re creating a detached thread safe context (bc is NULL), consider using RM_GetDetachedThreadSafeContext which will also retain the module ID and thus be more useful for logging.

RedisModule_GetDetachedThreadSafeContext

RedisModuleCtx *RedisModule_GetDetachedThreadSafeContext(RedisModuleCtx *ctx);

Available since: 6.0.9

Return a detached thread safe context that is not associated with any specific blocked client, but is associated with the module’s context.

This is useful for modules that wish to hold a global context over a long term, for purposes such as logging.

RedisModule_FreeThreadSafeContext

void RedisModule_FreeThreadSafeContext(RedisModuleCtx *ctx);

Available since: 4.0.0

Release a thread safe context.

RedisModule_ThreadSafeContextLock

void RedisModule_ThreadSafeContextLock(RedisModuleCtx *ctx);

Available since: 4.0.0

Acquire the server lock before executing a thread safe API call. This is not needed for RedisModule_Reply* calls when there is a blocked client connected to the thread safe context.

RedisModule_ThreadSafeContextTryLock

int RedisModule_ThreadSafeContextTryLock(RedisModuleCtx *ctx);

Available since: 6.0.8

Similar to RedisModule_ThreadSafeContextLock but this function would not block if the server lock is already acquired.

If successful (lock acquired) REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set accordingly.

RedisModule_ThreadSafeContextUnlock

void RedisModule_ThreadSafeContextUnlock(RedisModuleCtx *ctx);

Available since: 4.0.0

Release the server lock after a thread safe API call was executed.

Module Keyspace Notifications API

RedisModule_SubscribeToKeyspaceEvents

int RedisModule_SubscribeToKeyspaceEvents(RedisModuleCtx *ctx,
                                          int types,
                                          RedisModuleNotificationFunc callback);

Available since: 4.0.9

Subscribe to keyspace notifications. This is a low-level version of the keyspace-notifications API. A module can register callbacks to be notified when keyspace events occur.

Notification events are filtered by their type (string events, set events, etc), and the subscriber callback receives only events that match a specific mask of event types.

When subscribing to notifications with RedisModule_SubscribeToKeyspaceEvents the module must provide an event type-mask, denoting the events the subscriber is interested in. This can be an ORed mask of any of the following flags:

  • REDISMODULE_NOTIFY_GENERIC: Generic commands like DEL, EXPIRE, RENAME
  • REDISMODULE_NOTIFY_STRING: String events
  • REDISMODULE_NOTIFY_LIST: List events
  • REDISMODULE_NOTIFY_SET: Set events
  • REDISMODULE_NOTIFY_HASH: Hash events
  • REDISMODULE_NOTIFY_ZSET: Sorted Set events
  • REDISMODULE_NOTIFY_EXPIRED: Expiration events
  • REDISMODULE_NOTIFY_EVICTED: Eviction events
  • REDISMODULE_NOTIFY_STREAM: Stream events
  • REDISMODULE_NOTIFY_MODULE: Module types events
  • REDISMODULE_NOTIFY_KEYMISS: Key-miss events
  • REDISMODULE_NOTIFY_ALL: All events (Excluding REDISMODULE_NOTIFY_KEYMISS)
  • REDISMODULE_NOTIFY_LOADED: A special notification available only for modules, indicates that the key was loaded from persistence. Notice, when this event fires, the given key can not be retained, use RM_CreateStringFromString instead.

We do not distinguish between key events and keyspace events, and it is up to the module to filter the actions taken based on the key.

The subscriber signature is:

int (*RedisModuleNotificationFunc) (RedisModuleCtx *ctx, int type,
                                    const char *event,
                                    RedisModuleString *key);

type is the event type bit, that must match the mask given at registration time. The event string is the actual command being executed, and key is the relevant Redis key.

Notification callback gets executed with a redis context that can not be used to send anything to the client, and has the db number where the event occurred as its selected db number.

Notice that it is not necessary to enable notifications in redis.conf for module notifications to work.

Warning: the notification callbacks are performed in a synchronous manner, so notification callbacks must to be fast, or they would slow Redis down. If you need to take long actions, use threads to offload them.

See https://redis.io/topics/notifications for more information.

RedisModule_GetNotifyKeyspaceEvents

int RedisModule_GetNotifyKeyspaceEvents();

Available since: 6.0.0

Get the configured bitmap of notify-keyspace-events (Could be used for additional filtering in RedisModuleNotificationFunc)

RedisModule_NotifyKeyspaceEvent

int RedisModule_NotifyKeyspaceEvent(RedisModuleCtx *ctx,
                                    int type,
                                    const char *event,
                                    RedisModuleString *key);

Available since: 6.0.0

Expose notifyKeyspaceEvent to modules

Modules Cluster API

RedisModule_RegisterClusterMessageReceiver

void RedisModule_RegisterClusterMessageReceiver(RedisModuleCtx *ctx,
                                                uint8_t type,
                                                RedisModuleClusterMessageReceiver callback);

Available since: 5.0.0

Register a callback receiver for cluster messages of type ‘type’. If there was already a registered callback, this will replace the callback function with the one provided, otherwise if the callback is set to NULL and there is already a callback for this function, the callback is unregistered (so this API call is also used in order to delete the receiver).

RedisModule_SendClusterMessage

int RedisModule_SendClusterMessage(RedisModuleCtx *ctx,
                                   const char *target_id,
                                   uint8_t type,
                                   const char *msg,
                                   uint32_t len);

Available since: 5.0.0

Send a message to all the nodes in the cluster if target is NULL, otherwise at the specified target, which is a REDISMODULE_NODE_ID_LEN bytes node ID, as returned by the receiver callback or by the nodes iteration functions.

The function returns REDISMODULE_OK if the message was successfully sent, otherwise if the node is not connected or such node ID does not map to any known cluster node, REDISMODULE_ERR is returned.

RedisModule_GetClusterNodesList

char **RedisModule_GetClusterNodesList(RedisModuleCtx *ctx, size_t *numnodes);

Available since: 5.0.0

Return an array of string pointers, each string pointer points to a cluster node ID of exactly REDISMODULE_NODE_ID_LEN bytes (without any null term). The number of returned node IDs is stored into *numnodes. However if this function is called by a module not running an a Redis instance with Redis Cluster enabled, NULL is returned instead.

The IDs returned can be used with RedisModule_GetClusterNodeInfo() in order to get more information about single node.

The array returned by this function must be freed using the function RedisModule_FreeClusterNodesList().

Example:

size_t count, j;
char **ids = RedisModule_GetClusterNodesList(ctx,&count);
for (j = 0; j < count; j++) {
    RedisModule_Log(ctx,"notice","Node %.*s",
        REDISMODULE_NODE_ID_LEN,ids[j]);
}
RedisModule_FreeClusterNodesList(ids);

RedisModule_FreeClusterNodesList

void RedisModule_FreeClusterNodesList(char **ids);

Available since: 5.0.0

Free the node list obtained with RedisModule_GetClusterNodesList.

RedisModule_GetMyClusterID

const char *RedisModule_GetMyClusterID(void);

Available since: 5.0.0

Return this node ID (REDISMODULE_CLUSTER_ID_LEN bytes) or NULL if the cluster is disabled.

RedisModule_GetClusterSize

size_t RedisModule_GetClusterSize(void);

Available since: 5.0.0

Return the number of nodes in the cluster, regardless of their state (handshake, noaddress, …) so that the number of active nodes may actually be smaller, but not greater than this number. If the instance is not in cluster mode, zero is returned.

RedisModule_GetClusterNodeInfo

int RedisModule_GetClusterNodeInfo(RedisModuleCtx *ctx,
                                   const char *id,
                                   char *ip,
                                   char *master_id,
                                   int *port,
                                   int *flags);

Available since: 5.0.0

Populate the specified info for the node having as ID the specified ‘id’, then returns REDISMODULE_OK. Otherwise if the node ID does not exist from the POV of this local node, REDISMODULE_ERR is returned.

The arguments ip, master_id, port and flags can be NULL in case we don’t need to populate back certain info. If an ip and master_id (only populated if the instance is a slave) are specified, they point to buffers holding at least REDISMODULE_NODE_ID_LEN bytes. The strings written back as ip and master_id are not null terminated.

The list of flags reported is the following:

  • REDISMODULE_NODE_MYSELF: This node
  • REDISMODULE_NODE_MASTER: The node is a master
  • REDISMODULE_NODE_SLAVE: The node is a replica
  • REDISMODULE_NODE_PFAIL: We see the node as failing
  • REDISMODULE_NODE_FAIL: The cluster agrees the node is failing
  • REDISMODULE_NODE_NOFAILOVER: The slave is configured to never failover

RedisModule_SetClusterFlags

void RedisModule_SetClusterFlags(RedisModuleCtx *ctx, uint64_t flags);

Available since: 5.0.0

Set Redis Cluster flags in order to change the normal behavior of Redis Cluster, especially with the goal of disabling certain functions. This is useful for modules that use the Cluster API in order to create a different distributed system, but still want to use the Redis Cluster message bus. Flags that can be set:

  • CLUSTER_MODULE_FLAG_NO_FAILOVER
  • CLUSTER_MODULE_FLAG_NO_REDIRECTION

With the following effects:

  • NO_FAILOVER: prevent Redis Cluster slaves from failing over a dead master. Also disables the replica migration feature.

  • NO_REDIRECTION: Every node will accept any key, without trying to perform partitioning according to the Redis Cluster algorithm. Slots information will still be propagated across the cluster, but without effect.

Modules Timers API

Module timers are an high precision “green timers” abstraction where every module can register even millions of timers without problems, even if the actual event loop will just have a single timer that is used to awake the module timers subsystem in order to process the next event.

All the timers are stored into a radix tree, ordered by expire time, when the main Redis event loop timer callback is called, we try to process all the timers already expired one after the other. Then we re-enter the event loop registering a timer that will expire when the next to process module timer will expire.

Every time the list of active timers drops to zero, we unregister the main event loop timer, so that there is no overhead when such feature is not used.

RedisModule_CreateTimer

RedisModuleTimerID RedisModule_CreateTimer(RedisModuleCtx *ctx,
                                           mstime_t period,
                                           RedisModuleTimerProc callback,
                                           void *data);

Available since: 5.0.0

Create a new timer that will fire after period milliseconds, and will call the specified function using data as argument. The returned timer ID can be used to get information from the timer or to stop it before it fires. Note that for the common use case of a repeating timer (Re-registration of the timer inside the RedisModuleTimerProc callback) it matters when this API is called: If it is called at the beginning of ‘callback’ it means the event will triggered every ‘period’. If it is called at the end of ‘callback’ it means there will ‘period’ milliseconds gaps between events. (If the time it takes to execute ‘callback’ is negligible the two statements above mean the same)

RedisModule_StopTimer

int RedisModule_StopTimer(RedisModuleCtx *ctx,
                          RedisModuleTimerID id,
                          void **data);

Available since: 5.0.0

Stop a timer, returns REDISMODULE_OK if the timer was found, belonged to the calling module, and was stopped, otherwise REDISMODULE_ERR is returned. If not NULL, the data pointer is set to the value of the data argument when the timer was created.

RedisModule_GetTimerInfo

int RedisModule_GetTimerInfo(RedisModuleCtx *ctx,
                             RedisModuleTimerID id,
                             uint64_t *remaining,
                             void **data);

Available since: 5.0.0

Obtain information about a timer: its remaining time before firing (in milliseconds), and the private data pointer associated with the timer. If the timer specified does not exist or belongs to a different module no information is returned and the function returns REDISMODULE_ERR, otherwise REDISMODULE_OK is returned. The arguments remaining or data can be NULL if the caller does not need certain information.

Modules EventLoop API

RedisModule_EventLoopAdd

int RedisModule_EventLoopAdd(int fd,
                             int mask,
                             RedisModuleEventLoopFunc func,
                             void *user_data);

Add a pipe / socket event to the event loop.

  • mask must be one of the following values:

    • REDISMODULE_EVENTLOOP_READABLE
    • REDISMODULE_EVENTLOOP_WRITABLE
    • REDISMODULE_EVENTLOOP_READABLE | REDISMODULE_EVENTLOOP_WRITABLE

On success REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set to the following values:

  • ERANGE: fd is negative or higher than maxclients Redis config.
  • EINVAL: callback is NULL or mask value is invalid.

errno might take other values in case of an internal error.

Example:

void onReadable(int fd, void *user_data, int mask) {
    char buf[32];
    int bytes = read(fd,buf,sizeof(buf));
    printf("Read %d bytes \n", bytes);
}
RM_EventLoopAdd(fd, REDISMODULE_EVENTLOOP_READABLE, onReadable, NULL);

RedisModule_EventLoopDel

int RedisModule_EventLoopDel(int fd, int mask);

Delete a pipe / socket event from the event loop.

  • mask must be one of the following values:

    • REDISMODULE_EVENTLOOP_READABLE
    • REDISMODULE_EVENTLOOP_WRITABLE
    • REDISMODULE_EVENTLOOP_READABLE | REDISMODULE_EVENTLOOP_WRITABLE

On success REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set to the following values:

  • ERANGE: fd is negative or higher than maxclients Redis config.
  • EINVAL: mask value is invalid.

RedisModule_EventLoopAddOneShot

int RedisModule_EventLoopAddOneShot(RedisModuleEventLoopOneShotFunc func,
                                    void *user_data);

This function can be called from other threads to trigger callback on Redis main thread. On success REDISMODULE_OK is returned. If func is NULL REDISMODULE_ERR is returned and errno is set to EINVAL.

Modules ACL API

Implements a hook into the authentication and authorization within Redis.

RedisModule_CreateModuleUser

RedisModuleUser *RedisModule_CreateModuleUser(const char *name);

Available since: 6.0.0

Creates a Redis ACL user that the module can use to authenticate a client. After obtaining the user, the module should set what such user can do using the RedisModule_SetUserACL() function. Once configured, the user can be used in order to authenticate a connection, with the specified ACL rules, using the RedisModule_AuthClientWithUser() function.

Note that:

  • Users created here are not listed by the ACL command.
  • Users created here are not checked for duplicated name, so it’s up to the module calling this function to take care of not creating users with the same name.
  • The created user can be used to authenticate multiple Redis connections.

The caller can later free the user using the function RedisModule_FreeModuleUser(). When this function is called, if there are still clients authenticated with this user, they are disconnected. The function to free the user should only be used when the caller really wants to invalidate the user to define a new one with different capabilities.

RedisModule_FreeModuleUser

int RedisModule_FreeModuleUser(RedisModuleUser *user);

Available since: 6.0.0

Frees a given user and disconnects all of the clients that have been authenticated with it. See RedisModule_CreateModuleUser for detailed usage.

RedisModule_SetModuleUserACL

int RedisModule_SetModuleUserACL(RedisModuleUser *user, const char* acl);

Available since: 6.0.0

Sets the permissions of a user created through the redis module interface. The syntax is the same as ACL SETUSER, so refer to the documentation in acl.c for more information. See RedisModule_CreateModuleUser for detailed usage.

Returns REDISMODULE_OK on success and REDISMODULE_ERR on failure and will set an errno describing why the operation failed.

RedisModule_GetCurrentUserName

RedisModuleString *RedisModule_GetCurrentUserName(RedisModuleCtx *ctx);

Retrieve the user name of the client connection behind the current context. The user name can be used later, in order to get a RedisModuleUser. See more information in RedisModule_GetModuleUserFromUserName.

The returned string must be released with RedisModule_FreeString() or by enabling automatic memory management.

RedisModule_GetModuleUserFromUserName

RedisModuleUser *RedisModule_GetModuleUserFromUserName(RedisModuleString *name);

A RedisModuleUser can be used to check if command, key or channel can be executed or accessed according to the ACLs rules associated with that user. When a Module wants to do ACL checks on a general ACL user (not created by RedisModule_CreateModuleUser), it can get the RedisModuleUser from this API, based on the user name retrieved by RedisModule_GetCurrentUserName.

Since a general ACL user can be deleted at any time, this RedisModuleUser should be used only in the context where this function was called. In order to do ACL checks out of that context, the Module can store the user name, and call this API at any other context.

Returns NULL if the user is disabled or the user does not exist. The caller should later free the user using the function RedisModule_FreeModuleUser().

RedisModule_ACLCheckCommandPermissions

int RedisModule_ACLCheckCommandPermissions(RedisModuleUser *user,
                                           RedisModuleString **argv,
                                           int argc);

Checks if the command can be executed by the user, according to the ACLs associated with it.

On success a REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set to the following values:

  • ENOENT: Specified command does not exist.
  • EACCES: Command cannot be executed, according to ACL rules

RedisModule_ACLCheckKeyPermissions

int RedisModule_ACLCheckKeyPermissions(RedisModuleUser *user,
                                       RedisModuleString *key,
                                       int flags);

Check if the key can be accessed by the user according to the ACLs attached to the user and the flags representing the key access. The flags are the same that are used in the keyspec for logical operations. These flags are documented in RedisModule_SetCommandInfo as the REDISMODULE_CMD_KEY_ACCESS, REDISMODULE_CMD_KEY_UPDATE, REDISMODULE_CMD_KEY_INSERT, and REDISMODULE_CMD_KEY_DELETE flags.

If no flags are supplied, the user is still required to have some access to the key for this command to return successfully.

If the user is able to access the key then REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set to one of the following values:

  • EINVAL: The provided flags are invalid.
  • EACCESS: The user does not have permission to access the key.

RedisModule_ACLCheckChannelPermissions

int RedisModule_ACLCheckChannelPermissions(RedisModuleUser *user,
                                           RedisModuleString *ch,
                                           int flags);

Check if the pubsub channel can be accessed by the user based off of the given access flags. See RedisModule_ChannelAtPosWithFlags for more information about the possible flags that can be passed in.

If the user is able to acecss the pubsub channel then REDISMODULE_OK is returned, otherwise REDISMODULE_ERR is returned and errno is set to one of the following values:

  • EINVAL: The provided flags are invalid.
  • EACCESS: The user does not have permission to access the pubsub channel.

RedisModule_ACLAddLogEntry

void RedisModule_ACLAddLogEntry(RedisModuleCtx *ctx,
                                RedisModuleUser *user,
                                RedisModuleString *object);

Adds a new entry in the ACL log. Returns REDISMODULE_OK on success and REDISMODULE_ERR on error.

For more information about ACL log, please refer to https://redis.io/commands/acl-log

RedisModule_AuthenticateClientWithUser

int RedisModule_AuthenticateClientWithUser(RedisModuleCtx *ctx,
                                           RedisModuleUser *module_user,
                                           RedisModuleUserChangedFunc callback,
                                           void *privdata,
                                           uint64_t *client_id);

Available since: 6.0.0

Authenticate the current context’s user with the provided redis acl user. Returns REDISMODULE_ERR if the user is disabled.

See authenticateClientWithUser for information about callback, client_id, and general usage for authentication.

RedisModule_AuthenticateClientWithACLUser

int RedisModule_AuthenticateClientWithACLUser(RedisModuleCtx *ctx,
                                              const char *name,
                                              size_t len,
                                              RedisModuleUserChangedFunc callback,
                                              void *privdata,
                                              uint64_t *client_id);

Available since: 6.0.0

Authenticate the current context’s user with the provided redis acl user. Returns REDISMODULE_ERR if the user is disabled or the user does not exist.

See authenticateClientWithUser for information about callback, client_id, and general usage for authentication.

RedisModule_DeauthenticateAndCloseClient

int RedisModule_DeauthenticateAndCloseClient(RedisModuleCtx *ctx,
                                             uint64_t client_id);

Available since: 6.0.0

Deauthenticate and close the client. The client resources will not be be immediately freed, but will be cleaned up in a background job. This is the recommended way to deauthenticate a client since most clients can’t handle users becoming deauthenticated. Returns REDISMODULE_ERR when the client doesn’t exist and REDISMODULE_OK when the operation was successful.

The client ID is returned from the RedisModule_AuthenticateClientWithUser and RedisModule_AuthenticateClientWithACLUser APIs, but can be obtained through the CLIENT api or through server events.

This function is not thread safe, and must be executed within the context of a command or thread safe context.

RedisModule_GetClientCertificate

RedisModuleString *RedisModule_GetClientCertificate(RedisModuleCtx *ctx,
                                                    uint64_t client_id);

Available since: 6.0.9

Return the X.509 client-side certificate used by the client to authenticate this connection.

The return value is an allocated RedisModuleString that is a X.509 certificate encoded in PEM (Base64) format. It should be freed (or auto-freed) by the caller.

A NULL value is returned in the following conditions:

  • Connection ID does not exist
  • Connection is not a TLS connection
  • Connection is a TLS connection but no client certificate was used

Modules Dictionary API

Implements a sorted dictionary (actually backed by a radix tree) with the usual get / set / del / num-items API, together with an iterator capable of going back and forth.

RedisModule_CreateDict

RedisModuleDict *RedisModule_CreateDict(RedisModuleCtx *ctx);

Available since: 5.0.0

Create a new dictionary. The ‘ctx’ pointer can be the current module context or NULL, depending on what you want. Please follow the following rules:

  1. Use a NULL context if you plan to retain a reference to this dictionary that will survive the time of the module callback where you created it.
  2. Use a NULL context if no context is available at the time you are creating the dictionary (of course…).
  3. However use the current callback context as ‘ctx’ argument if the dictionary time to live is just limited to the callback scope. In this case, if enabled, you can enjoy the automatic memory management that will reclaim the dictionary memory, as well as the strings returned by the Next / Prev dictionary iterator calls.

RedisModule_FreeDict

void RedisModule_FreeDict(RedisModuleCtx *ctx, RedisModuleDict *d);

Available since: 5.0.0

Free a dictionary created with RedisModule_CreateDict(). You need to pass the context pointer ‘ctx’ only if the dictionary was created using the context instead of passing NULL.

RedisModule_DictSize

uint64_t RedisModule_DictSize(RedisModuleDict *d);

Available since: 5.0.0

Return the size of the dictionary (number of keys).

RedisModule_DictSetC

int RedisModule_DictSetC(RedisModuleDict *d,
                         void *key,
                         size_t keylen,
                         void *ptr);

Available since: 5.0.0

Store the specified key into the dictionary, setting its value to the pointer ‘ptr’. If the key was added with success, since it did not already exist, REDISMODULE_OK is returned. Otherwise if the key already exists the function returns REDISMODULE_ERR.

RedisModule_DictReplaceC

int RedisModule_DictReplaceC(RedisModuleDict *d,
                             void *key,
                             size_t keylen,
                             void *ptr);

Available since: 5.0.0

Like RedisModule_DictSetC() but will replace the key with the new value if the key already exists.

RedisModule_DictSet

int RedisModule_DictSet(RedisModuleDict *d, RedisModuleString *key, void *ptr);

Available since: 5.0.0

Like RedisModule_DictSetC() but takes the key as a RedisModuleString.

RedisModule_DictReplace

int RedisModule_DictReplace(RedisModuleDict *d,
                            RedisModuleString *key,
                            void *ptr);

Available since: 5.0.0

Like RedisModule_DictReplaceC() but takes the key as a RedisModuleString.

RedisModule_DictGetC

void *RedisModule_DictGetC(RedisModuleDict *d,
                           void *key,
                           size_t keylen,
                           int *nokey);

Available since: 5.0.0

Return the value stored at the specified key. The function returns NULL both in the case the key does not exist, or if you actually stored NULL at key. So, optionally, if the ‘nokey’ pointer is not NULL, it will be set by reference to 1 if the key does not exist, or to 0 if the key exists.

RedisModule_DictGet

void *RedisModule_DictGet(RedisModuleDict *d,
                          RedisModuleString *key,
                          int *nokey);

Available since: 5.0.0

Like RedisModule_DictGetC() but takes the key as a RedisModuleString.

RedisModule_DictDelC

int RedisModule_DictDelC(RedisModuleDict *d,
                         void *key,
                         size_t keylen,
                         void *oldval);

Available since: 5.0.0

Remove the specified key from the dictionary, returning REDISMODULE_OK if the key was found and deleted, or REDISMODULE_ERR if instead there was no such key in the dictionary. When the operation is successful, if ‘oldval’ is not NULL, then ‘*oldval’ is set to the value stored at the key before it was deleted. Using this feature it is possible to get a pointer to the value (for instance in order to release it), without having to call RedisModule_DictGet() before deleting the key.

RedisModule_DictDel

int RedisModule_DictDel(RedisModuleDict *d,
                        RedisModuleString *key,
                        void *oldval);

Available since: 5.0.0

Like RedisModule_DictDelC() but gets the key as a RedisModuleString.

RedisModule_DictIteratorStartC

RedisModuleDictIter *RedisModule_DictIteratorStartC(RedisModuleDict *d,
                                                    const char *op,
                                                    void *key,
                                                    size_t keylen);

Available since: 5.0.0

Return an iterator, setup in order to start iterating from the specified key by applying the operator ‘op’, which is just a string specifying the comparison operator to use in order to seek the first element. The operators available are:

  • ^ – Seek the first (lexicographically smaller) key.
  • $ – Seek the last (lexicographically bigger) key.
  • > – Seek the first element greater than the specified key.
  • >= – Seek the first element greater or equal than the specified key.
  • < – Seek the first element smaller than the specified key.
  • <= – Seek the first element smaller or equal than the specified key.
  • == – Seek the first element matching exactly the specified key.

Note that for ^ and $ the passed key is not used, and the user may just pass NULL with a length of 0.

If the element to start the iteration cannot be seeked based on the key and operator passed, RedisModule_DictNext() / Prev() will just return REDISMODULE_ERR at the first call, otherwise they’ll produce elements.

RedisModule_DictIteratorStart

RedisModuleDictIter *RedisModule_DictIteratorStart(RedisModuleDict *d,
                                                   const char *op,
                                                   RedisModuleString *key);

Available since: 5.0.0

Exactly like RedisModule_DictIteratorStartC, but the key is passed as a RedisModuleString.

RedisModule_DictIteratorStop

void RedisModule_DictIteratorStop(RedisModuleDictIter *di);

Available since: 5.0.0

Release the iterator created with RedisModule_DictIteratorStart(). This call is mandatory otherwise a memory leak is introduced in the module.

RedisModule_DictIteratorReseekC

int RedisModule_DictIteratorReseekC(RedisModuleDictIter *di,
                                    const char *op,
                                    void *key,
                                    size_t keylen);

Available since: 5.0.0

After its creation with RedisModule_DictIteratorStart(), it is possible to change the currently selected element of the iterator by using this API call. The result based on the operator and key is exactly like the function RedisModule_DictIteratorStart(), however in this case the return value is just REDISMODULE_OK in case the seeked element was found, or REDISMODULE_ERR in case it was not possible to seek the specified element. It is possible to reseek an iterator as many times as you want.

RedisModule_DictIteratorReseek

int RedisModule_DictIteratorReseek(RedisModuleDictIter *di,
                                   const char *op,
                                   RedisModuleString *key);

Available since: 5.0.0

Like RedisModule_DictIteratorReseekC() but takes the key as as a RedisModuleString.

RedisModule_DictNextC

void *RedisModule_DictNextC(RedisModuleDictIter *di,
                            size_t *keylen,
                            void **dataptr);

Available since: 5.0.0

Return the current item of the dictionary iterator di and steps to the next element. If the iterator already yield the last element and there are no other elements to return, NULL is returned, otherwise a pointer to a string representing the key is provided, and the *keylen length is set by reference (if keylen is not NULL). The *dataptr, if not NULL is set to the value of the pointer stored at the returned key as auxiliary data (as set by the RedisModule_DictSet API).

Usage example:

 ... create the iterator here ...
 char *key;
 void *data;
 while((key = RedisModule_DictNextC(iter,&keylen,&data)) != NULL) {
     printf("%.*s %p\n", (int)keylen, key, data);
 }

The returned pointer is of type void because sometimes it makes sense to cast it to a char* sometimes to an unsigned char* depending on the fact it contains or not binary data, so this API ends being more comfortable to use.

The validity of the returned pointer is until the next call to the next/prev iterator step. Also the pointer is no longer valid once the iterator is released.

RedisModule_DictPrevC

void *RedisModule_DictPrevC(RedisModuleDictIter *di,
                            size_t *keylen,
                            void **dataptr);

Available since: 5.0.0

This function is exactly like RedisModule_DictNext() but after returning the currently selected element in the iterator, it selects the previous element (lexicographically smaller) instead of the next one.

RedisModule_DictNext

RedisModuleString *RedisModule_DictNext(RedisModuleCtx *ctx,
                                        RedisModuleDictIter *di,
                                        void **dataptr);

Available since: 5.0.0

Like RedisModuleNextC(), but instead of returning an internally allocated buffer and key length, it returns directly a module string object allocated in the specified context ‘ctx’ (that may be NULL exactly like for the main API RedisModule_CreateString).

The returned string object should be deallocated after use, either manually or by using a context that has automatic memory management active.

RedisModule_DictPrev

RedisModuleString *RedisModule_DictPrev(RedisModuleCtx *ctx,
                                        RedisModuleDictIter *di,
                                        void **dataptr);

Available since: 5.0.0

Like RedisModule_DictNext() but after returning the currently selected element in the iterator, it selects the previous element (lexicographically smaller) instead of the next one.

RedisModule_DictCompareC

int RedisModule_DictCompareC(RedisModuleDictIter *di,
                             const char *op,
                             void *key,
                             size_t keylen);

Available since: 5.0.0

Compare the element currently pointed by the iterator to the specified element given by key/keylen, according to the operator ‘op’ (the set of valid operators are the same valid for RedisModule_DictIteratorStart). If the comparison is successful the command returns REDISMODULE_OK otherwise REDISMODULE_ERR is returned.

This is useful when we want to just emit a lexicographical range, so in the loop, as we iterate elements, we can also check if we are still on range.

The function return REDISMODULE_ERR if the iterator reached the end of elements condition as well.

RedisModule_DictCompare

int RedisModule_DictCompare(RedisModuleDictIter *di,
                            const char *op,
                            RedisModuleString *key);

Available since: 5.0.0

Like RedisModule_DictCompareC but gets the key to compare with the current iterator key as a RedisModuleString.

Modules Info fields

RedisModule_InfoAddSection

int RedisModule_InfoAddSection(RedisModuleInfoCtx *ctx, const char *name);

Available since: 6.0.0

Used to start a new section, before adding any fields. the section name will be prefixed by <modulename>_ and must only include A-Z,a-z,0-9. NULL or empty string indicates the default section (only <modulename>) is used. When return value is REDISMODULE_ERR, the section should and will be skipped.

RedisModule_InfoBeginDictField

int RedisModule_InfoBeginDictField(RedisModuleInfoCtx *ctx, const char *name);

Available since: 6.0.0

Starts a dict field, similar to the ones in INFO KEYSPACE. Use normal RedisModule_InfoAddField* functions to add the items to this field, and terminate with RedisModule_InfoEndDictField.

RedisModule_InfoEndDictField

int RedisModule_InfoEndDictField(RedisModuleInfoCtx *ctx);

Available since: 6.0.0

Ends a dict field, see RedisModule_InfoBeginDictField

RedisModule_InfoAddFieldString

int RedisModule_InfoAddFieldString(RedisModuleInfoCtx *ctx,
                                   const char *field,
                                   RedisModuleString *value);

Available since: 6.0.0

Used by RedisModuleInfoFunc to add info fields. Each field will be automatically prefixed by <modulename>_. Field names or values must not include \r\n or :.

RedisModule_InfoAddFieldCString

int RedisModule_InfoAddFieldCString(RedisModuleInfoCtx *ctx,
                                    const char *field,
                                    const char *value);

Available since: 6.0.0

See RedisModule_InfoAddFieldString().

RedisModule_InfoAddFieldDouble

int RedisModule_InfoAddFieldDouble(RedisModuleInfoCtx *ctx,
                                   const char *field,
                                   double value);

Available since: 6.0.0

See RedisModule_InfoAddFieldString().

RedisModule_InfoAddFieldLongLong

int RedisModule_InfoAddFieldLongLong(RedisModuleInfoCtx *ctx,
                                     const char *field,
                                     long long value);

Available since: 6.0.0

See RedisModule_InfoAddFieldString().

RedisModule_InfoAddFieldULongLong

int RedisModule_InfoAddFieldULongLong(RedisModuleInfoCtx *ctx,
                                      const char *field,
                                      unsigned long long value);

Available since: 6.0.0

See RedisModule_InfoAddFieldString().

RedisModule_RegisterInfoFunc

int RedisModule_RegisterInfoFunc(RedisModuleCtx *ctx, RedisModuleInfoFunc cb);

Available since: 6.0.0

Registers callback for the INFO command. The callback should add INFO fields by calling the RedisModule_InfoAddField*() functions.

RedisModule_GetServerInfo

RedisModuleServerInfoData *RedisModule_GetServerInfo(RedisModuleCtx *ctx,
                                                     const char *section);

Available since: 6.0.0

Get information about the server similar to the one that returns from the INFO command. This function takes an optional ‘section’ argument that may be NULL. The return value holds the output and can be used with RedisModule_ServerInfoGetField and alike to get the individual fields. When done, it needs to be freed with RedisModule_FreeServerInfo or with the automatic memory management mechanism if enabled.

RedisModule_FreeServerInfo

void RedisModule_FreeServerInfo(RedisModuleCtx *ctx,
                                RedisModuleServerInfoData *data);

Available since: 6.0.0

Free data created with RedisModule_GetServerInfo(). You need to pass the context pointer ‘ctx’ only if the dictionary was created using the context instead of passing NULL.

RedisModule_ServerInfoGetField

RedisModuleString *RedisModule_ServerInfoGetField(RedisModuleCtx *ctx,
                                                  RedisModuleServerInfoData *data,
                                                  const char* field);

Available since: 6.0.0

Get the value of a field from data collected with RedisModule_GetServerInfo(). You need to pass the context pointer ‘ctx’ only if you want to use auto memory mechanism to release the returned string. Return value will be NULL if the field was not found.

RedisModule_ServerInfoGetFieldC

const char *RedisModule_ServerInfoGetFieldC(RedisModuleServerInfoData *data,
                                            const char* field);

Available since: 6.0.0

Similar to RedisModule_ServerInfoGetField, but returns a char* which should not be freed but the caller.

RedisModule_ServerInfoGetFieldSigned

long long RedisModule_ServerInfoGetFieldSigned(RedisModuleServerInfoData *data,
                                               const char* field,
                                               int *out_err);

Available since: 6.0.0

Get the value of a field from data collected with RedisModule_GetServerInfo(). If the field is not found, or is not numerical or out of range, return value will be 0, and the optional out_err argument will be set to REDISMODULE_ERR.

RedisModule_ServerInfoGetFieldUnsigned

unsigned long long RedisModule_ServerInfoGetFieldUnsigned(RedisModuleServerInfoData *data,
                                                          const char* field,
                                                          int *out_err);

Available since: 6.0.0

Get the value of a field from data collected with RedisModule_GetServerInfo(). If the field is not found, or is not numerical or out of range, return value will be 0, and the optional out_err argument will be set to REDISMODULE_ERR.

RedisModule_ServerInfoGetFieldDouble

double RedisModule_ServerInfoGetFieldDouble(RedisModuleServerInfoData *data,
                                            const char* field,
                                            int *out_err);

Available since: 6.0.0

Get the value of a field from data collected with RedisModule_GetServerInfo(). If the field is not found, or is not a double, return value will be 0, and the optional out_err argument will be set to REDISMODULE_ERR.

Modules utility APIs

RedisModule_GetRandomBytes

void RedisModule_GetRandomBytes(unsigned char *dst, size_t len);

Available since: 5.0.0

Return random bytes using SHA1 in counter mode with a /dev/urandom initialized seed. This function is fast so can be used to generate many bytes without any effect on the operating system entropy pool. Currently this function is not thread safe.

RedisModule_GetRandomHexChars

void RedisModule_GetRandomHexChars(char *dst, size_t len);

Available since: 5.0.0

Like RedisModule_GetRandomBytes() but instead of setting the string to random bytes the string is set to random characters in the in the hex charset [0-9a-f].

Modules API exporting / importing

RedisModule_ExportSharedAPI

int RedisModule_ExportSharedAPI(RedisModuleCtx *ctx,
                                const char *apiname,
                                void *func);

Available since: 5.0.4

This function is called by a module in order to export some API with a given name. Other modules will be able to use this API by calling the symmetrical function RedisModule_GetSharedAPI() and casting the return value to the right function pointer.

The function will return REDISMODULE_OK if the name is not already taken, otherwise REDISMODULE_ERR will be returned and no operation will be performed.

IMPORTANT: the apiname argument should be a string literal with static lifetime. The API relies on the fact that it will always be valid in the future.

RedisModule_GetSharedAPI

void *RedisModule_GetSharedAPI(RedisModuleCtx *ctx, const char *apiname);

Available since: 5.0.4

Request an exported API pointer. The return value is just a void pointer that the caller of this function will be required to cast to the right function pointer, so this is a private contract between modules.

If the requested API is not available then NULL is returned. Because modules can be loaded at different times with different order, this function calls should be put inside some module generic API registering step, that is called every time a module attempts to execute a command that requires external APIs: if some API cannot be resolved, the command should return an error.

Here is an example:

int ... myCommandImplementation() {
   if (getExternalAPIs() == 0) {
        reply with an error here if we cannot have the APIs
   }
   // Use the API:
   myFunctionPointer(foo);
}

And the function registerAPI() is:

int getExternalAPIs(void) {
    static int api_loaded = 0;
    if (api_loaded != 0) return 1; // APIs already resolved.

    myFunctionPointer = RedisModule_GetOtherModuleAPI("...");
    if (myFunctionPointer == NULL) return 0;

    return 1;
}

Module Command Filter API

RedisModule_RegisterCommandFilter

RedisModuleCommandFilter *RedisModule_RegisterCommandFilter(RedisModuleCtx *ctx,
                                                            RedisModuleCommandFilterFunc callback,
                                                            int flags);

Available since: 5.0.5

Register a new command filter function.

Command filtering makes it possible for modules to extend Redis by plugging into the execution flow of all commands.

A registered filter gets called before Redis executes any command. This includes both core Redis commands and commands registered by any module. The filter applies in all execution paths including:

  1. Invocation by a client.
  2. Invocation through RedisModule_Call() by any module.
  3. Invocation through Lua ‘redis.`call()``.
  4. Replication of a command from a master.

The filter executes in a special filter context, which is different and more limited than a RedisModuleCtx. Because the filter affects any command, it must be implemented in a very efficient way to reduce the performance impact on Redis. All Redis Module API calls that require a valid context (such as RedisModule_Call(), RedisModule_OpenKey(), etc.) are not supported in a filter context.

The RedisModuleCommandFilterCtx can be used to inspect or modify the executed command and its arguments. As the filter executes before Redis begins processing the command, any change will affect the way the command is processed. For example, a module can override Redis commands this way:

  1. Register a MODULE.SET command which implements an extended version of the Redis SET command.
  2. Register a command filter which detects invocation of SET on a specific pattern of keys. Once detected, the filter will replace the first argument from SET to MODULE.SET.
  3. When filter execution is complete, Redis considers the new command name and therefore executes the module’s own command.

Note that in the above use case, if MODULE.SET itself uses RedisModule_Call() the filter will be applied on that call as well. If that is not desired, the REDISMODULE_CMDFILTER_NOSELF flag can be set when registering the filter.

The REDISMODULE_CMDFILTER_NOSELF flag prevents execution flows that originate from the module’s own RM_Call() from reaching the filter. This flag is effective for all execution flows, including nested ones, as long as the execution begins from the module’s command context or a thread-safe context that is associated with a blocking command.

Detached thread-safe contexts are not associated with the module and cannot be protected by this flag.

If multiple filters are registered (by the same or different modules), they are executed in the order of registration.

RedisModule_UnregisterCommandFilter

int RedisModule_UnregisterCommandFilter(RedisModuleCtx *ctx,
                                        RedisModuleCommandFilter *filter);

Available since: 5.0.5

Unregister a command filter.

RedisModule_CommandFilterArgsCount

int RedisModule_CommandFilterArgsCount(RedisModuleCommandFilterCtx *fctx);

Available since: 5.0.5

Return the number of arguments a filtered command has. The number of arguments include the command itself.

RedisModule_CommandFilterArgGet

RedisModuleString *RedisModule_CommandFilterArgGet(RedisModuleCommandFilterCtx *fctx,
                                                   int pos);

Available since: 5.0.5

Return the specified command argument. The first argument (position 0) is the command itself, and the rest are user-provided args.

RedisModule_CommandFilterArgInsert

int RedisModule_CommandFilterArgInsert(RedisModuleCommandFilterCtx *fctx,
                                       int pos,
                                       RedisModuleString *arg);

Available since: 5.0.5

Modify the filtered command by inserting a new argument at the specified position. The specified RedisModuleString argument may be used by Redis after the filter context is destroyed, so it must not be auto-memory allocated, freed or used elsewhere.

RedisModule_CommandFilterArgReplace

int RedisModule_CommandFilterArgReplace(RedisModuleCommandFilterCtx *fctx,
                                        int pos,
                                        RedisModuleString *arg);

Available since: 5.0.5

Modify the filtered command by replacing an existing argument with a new one. The specified RedisModuleString argument may be used by Redis after the filter context is destroyed, so it must not be auto-memory allocated, freed or used elsewhere.

RedisModule_CommandFilterArgDelete

int RedisModule_CommandFilterArgDelete(RedisModuleCommandFilterCtx *fctx,
                                       int pos);

Available since: 5.0.5

Modify the filtered command by deleting an argument at the specified position.

RedisModule_MallocSize

size_t RedisModule_MallocSize(void* ptr);

Available since: 6.0.0

For a given pointer allocated via RedisModule_Alloc() or RedisModule_Realloc(), return the amount of memory allocated for it. Note that this may be different (larger) than the memory we allocated with the allocation calls, since sometimes the underlying allocator will allocate more memory.

RedisModule_GetUsedMemoryRatio

float RedisModule_GetUsedMemoryRatio();

Available since: 6.0.0

Return the a number between 0 to 1 indicating the amount of memory currently used, relative to the Redis “maxmemory” configuration.

  • 0 - No memory limit configured.
  • Between 0 and 1 - The percentage of the memory used normalized in 0-1 range.
  • Exactly 1 - Memory limit reached.
  • Greater 1 - More memory used than the configured limit.

Scanning keyspace and hashes

RedisModule_ScanCursorCreate

RedisModuleScanCursor *RedisModule_ScanCursorCreate();

Available since: 6.0.0

Create a new cursor to be used with RedisModule_Scan

RedisModule_ScanCursorRestart

void RedisModule_ScanCursorRestart(RedisModuleScanCursor *cursor);

Available since: 6.0.0

Restart an existing cursor. The keys will be rescanned.

RedisModule_ScanCursorDestroy

void RedisModule_ScanCursorDestroy(RedisModuleScanCursor *cursor);

Available since: 6.0.0

Destroy the cursor struct.

RedisModule_Scan

int RedisModule_Scan(RedisModuleCtx *ctx,
                     RedisModuleScanCursor *cursor,
                     RedisModuleScanCB fn,
                     void *privdata);

Available since: 6.0.0

Scan API that allows a module to scan all the keys and value in the selected db.

Callback for scan implementation.

void scan_callback(RedisModuleCtx *ctx, RedisModuleString *keyname,
                   RedisModuleKey *key, void *privdata);
  • ctx: the redis module context provided to for the scan.
  • keyname: owned by the caller and need to be retained if used after this function.
  • key: holds info on the key and value, it is provided as best effort, in some cases it might be NULL, in which case the user should (can) use RedisModule_OpenKey() (and CloseKey too). when it is provided, it is owned by the caller and will be free when the callback returns.
  • privdata: the user data provided to RedisModule_Scan().

The way it should be used:

 RedisModuleCursor *c = RedisModule_ScanCursorCreate();
 while(RedisModule_Scan(ctx, c, callback, privateData));
 RedisModule_ScanCursorDestroy(c);

It is also possible to use this API from another thread while the lock is acquired during the actual call to RedisModule_Scan:

 RedisModuleCursor *c = RedisModule_ScanCursorCreate();
 RedisModule_ThreadSafeContextLock(ctx);
 while(RedisModule_Scan(ctx, c, callback, privateData)){
     RedisModule_ThreadSafeContextUnlock(ctx);
     // do some background job
     RedisModule_ThreadSafeContextLock(ctx);
 }
 RedisModule_ScanCursorDestroy(c);

The function will return 1 if there are more elements to scan and 0 otherwise, possibly setting errno if the call failed.

It is also possible to restart an existing cursor using RedisModule_ScanCursorRestart.

IMPORTANT: This API is very similar to the Redis SCAN command from the point of view of the guarantees it provides. This means that the API may report duplicated keys, but guarantees to report at least one time every key that was there from the start to the end of the scanning process.

NOTE: If you do database changes within the callback, you should be aware that the internal state of the database may change. For instance it is safe to delete or modify the current key, but may not be safe to delete any other key. Moreover playing with the Redis keyspace while iterating may have the effect of returning more duplicates. A safe pattern is to store the keys names you want to modify elsewhere, and perform the actions on the keys later when the iteration is complete. However this can cost a lot of memory, so it may make sense to just operate on the current key when possible during the iteration, given that this is safe.

RedisModule_ScanKey

int RedisModule_ScanKey(RedisModuleKey *key,
                        RedisModuleScanCursor *cursor,
                        RedisModuleScanKeyCB fn,
                        void *privdata);

Available since: 6.0.0

Scan api that allows a module to scan the elements in a hash, set or sorted set key

Callback for scan implementation.

void scan_callback(RedisModuleKey *key, RedisModuleString* field, RedisModuleString* value, void *privdata);
  • key - the redis key context provided to for the scan.
  • field - field name, owned by the caller and need to be retained if used after this function.
  • value - value string or NULL for set type, owned by the caller and need to be retained if used after this function.
  • privdata - the user data provided to RedisModule_ScanKey.

The way it should be used:

 RedisModuleCursor *c = RedisModule_ScanCursorCreate();
 RedisModuleKey *key = RedisModule_OpenKey(...)
 while(RedisModule_ScanKey(key, c, callback, privateData));
 RedisModule_CloseKey(key);
 RedisModule_ScanCursorDestroy(c);

It is also possible to use this API from another thread while the lock is acquired during the actual call to RedisModule_ScanKey, and re-opening the key each time:

 RedisModuleCursor *c = RedisModule_ScanCursorCreate();
 RedisModule_ThreadSafeContextLock(ctx);
 RedisModuleKey *key = RedisModule_OpenKey(...)
 while(RedisModule_ScanKey(ctx, c, callback, privateData)){
     RedisModule_CloseKey(key);
     RedisModule_ThreadSafeContextUnlock(ctx);
     // do some background job
     RedisModule_ThreadSafeContextLock(ctx);
     RedisModuleKey *key = RedisModule_OpenKey(...)
 }
 RedisModule_CloseKey(key);
 RedisModule_ScanCursorDestroy(c);

The function will return 1 if there are more elements to scan and 0 otherwise, possibly setting errno if the call failed. It is also possible to restart an existing cursor using RedisModule_ScanCursorRestart.

NOTE: Certain operations are unsafe while iterating the object. For instance while the API guarantees to return at least one time all the elements that are present in the data structure consistently from the start to the end of the iteration (see HSCAN and similar commands documentation), the more you play with the elements, the more duplicates you may get. In general deleting the current element of the data structure is safe, while removing the key you are iterating is not safe.

Module fork API

RedisModule_Fork

int RedisModule_Fork(RedisModuleForkDoneHandler cb, void *user_data);

Available since: 6.0.0

Create a background child process with the current frozen snapshot of the main process where you can do some processing in the background without affecting / freezing the traffic and no need for threads and GIL locking. Note that Redis allows for only one concurrent fork. When the child wants to exit, it should call RedisModule_ExitFromChild. If the parent wants to kill the child it should call RedisModule_KillForkChild The done handler callback will be executed on the parent process when the child existed (but not when killed) Return: -1 on failure, on success the parent process will get a positive PID of the child, and the child process will get 0.

RedisModule_SendChildHeartbeat

void RedisModule_SendChildHeartbeat(double progress);

Available since: 6.2.0

The module is advised to call this function from the fork child once in a while, so that it can report progress and COW memory to the parent which will be reported in INFO. The progress argument should between 0 and 1, or -1 when not available.

RedisModule_ExitFromChild

int RedisModule_ExitFromChild(int retcode);

Available since: 6.0.0

Call from the child process when you want to terminate it. retcode will be provided to the done handler executed on the parent process.

RedisModule_KillForkChild

int RedisModule_KillForkChild(int child_pid);

Available since: 6.0.0

Can be used to kill the forked child process from the parent process. child_pid would be the return value of RedisModule_Fork.

Server hooks implementation

RedisModule_SubscribeToServerEvent

int RedisModule_SubscribeToServerEvent(RedisModuleCtx *ctx,
                                       RedisModuleEvent event,
                                       RedisModuleEventCallback callback);

Available since: 6.0.0

Register to be notified, via a callback, when the specified server event happens. The callback is called with the event as argument, and an additional argument which is a void pointer and should be cased to a specific type that is event-specific (but many events will just use NULL since they do not have additional information to pass to the callback).

If the callback is NULL and there was a previous subscription, the module will be unsubscribed. If there was a previous subscription and the callback is not null, the old callback will be replaced with the new one.

The callback must be of this type:

int (*RedisModuleEventCallback)(RedisModuleCtx *ctx,
                                RedisModuleEvent eid,
                                uint64_t subevent,
                                void *data);

The ‘ctx’ is a normal Redis module context that the callback can use in order to call other modules APIs. The ‘eid’ is the event itself, this is only useful in the case the module subscribed to multiple events: using the ‘id’ field of this structure it is possible to check if the event is one of the events we registered with this callback. The ‘subevent’ field depends on the event that fired.

Finally the ‘data’ pointer may be populated, only for certain events, with more relevant data.

Here is a list of events you can use as ‘eid’ and related sub events:

  • RedisModuleEvent_ReplicationRoleChanged:

    This event is called when the instance switches from master to replica or the other way around, however the event is also called when the replica remains a replica but starts to replicate with a different master.

    The following sub events are available:

    • REDISMODULE_SUBEVENT_REPLROLECHANGED_NOW_MASTER
    • REDISMODULE_SUBEVENT_REPLROLECHANGED_NOW_REPLICA

    The ‘data’ field can be casted by the callback to a RedisModuleReplicationInfo structure with the following fields:

      int master; // true if master, false if replica
  char *masterhost; // master instance hostname for NOW_REPLICA
  int masterport; // master instance port for NOW_REPLICA
  char *replid1; // Main replication ID
  char *replid2; // Secondary replication ID
  uint64_t repl1_offset; // Main replication offset
  uint64_t repl2_offset; // Offset of replid2 validity

  • RedisModuleEvent_Persistence

    This event is called when RDB saving or AOF rewriting starts and ends. The following sub events are available:

    • REDISMODULE_SUBEVENT_PERSISTENCE_RDB_START
    • REDISMODULE_SUBEVENT_PERSISTENCE_AOF_START
    • REDISMODULE_SUBEVENT_PERSISTENCE_SYNC_RDB_START
    • REDISMODULE_SUBEVENT_PERSISTENCE_SYNC_AOF_START
    • REDISMODULE_SUBEVENT_PERSISTENCE_ENDED
    • REDISMODULE_SUBEVENT_PERSISTENCE_FAILED

    The above events are triggered not just when the user calls the relevant commands like BGSAVE, but also when a saving operation or AOF rewriting occurs because of internal server triggers. The SYNC_RDB_START sub events are happening in the foreground due to SAVE command, FLUSHALL, or server shutdown, and the other RDB and AOF sub events are executed in a background fork child, so any action the module takes can only affect the generated AOF or RDB, but will not be reflected in the parent process and affect connected clients and commands. Also note that the AOF_START sub event may end up saving RDB content in case of an AOF with rdb-preamble.

  • RedisModuleEvent_FlushDB

    The FLUSHALL, FLUSHDB or an internal flush (for instance because of replication, after the replica synchronization) happened. The following sub events are available:

    • REDISMODULE_SUBEVENT_FLUSHDB_START
    • REDISMODULE_SUBEVENT_FLUSHDB_END

    The data pointer can be casted to a RedisModuleFlushInfo structure with the following fields:

      int32_t async;  // True if the flush is done in a thread.
                  // See for instance FLUSHALL ASYNC.
                  // In this case the END callback is invoked
                  // immediately after the database is put
                  // in the free list of the thread.
  int32_t dbnum;  // Flushed database number, -1 for all the DBs
                  // in the case of the FLUSHALL operation.

    The start event is called before the operation is initiated, thus allowing the callback to call DBSIZE or other operation on the yet-to-free keyspace.

  • RedisModuleEvent_Loading

    Called on loading operations: at startup when the server is started, but also after a first synchronization when the replica is loading the RDB file from the master. The following sub events are available:

    • REDISMODULE_SUBEVENT_LOADING_RDB_START
    • REDISMODULE_SUBEVENT_LOADING_AOF_START
    • REDISMODULE_SUBEVENT_LOADING_REPL_START
    • REDISMODULE_SUBEVENT_LOADING_ENDED
    • REDISMODULE_SUBEVENT_LOADING_FAILED

    Note that AOF loading may start with an RDB data in case of rdb-preamble, in which case you’ll only receive an AOF_START event.

  • RedisModuleEvent_ClientChange

    Called when a client connects or disconnects. The data pointer can be casted to a RedisModuleClientInfo structure, documented in RedisModule_GetClientInfoById(). The following sub events are available:

    • REDISMODULE_SUBEVENT_CLIENT_CHANGE_CONNECTED
    • REDISMODULE_SUBEVENT_CLIENT_CHANGE_DISCONNECTED

  • RedisModuleEvent_Shutdown

    The server is shutting down. No subevents are available.

  • RedisModuleEvent_ReplicaChange

    This event is called when the instance (that can be both a master or a replica) get a new online replica, or lose a replica since it gets disconnected. The following sub events are available:

    • REDISMODULE_SUBEVENT_REPLICA_CHANGE_ONLINE
    • REDISMODULE_SUBEVENT_REPLICA_CHANGE_OFFLINE

    No additional information is available so far: future versions of Redis will have an API in order to enumerate the replicas connected and their state.

  • RedisModuleEvent_CronLoop

    This event is called every time Redis calls the serverCron() function in order to do certain bookkeeping. Modules that are required to do operations from time to time may use this callback. Normally Redis calls this function 10 times per second, but this changes depending on the “hz” configuration. No sub events are available.

    The data pointer can be casted to a RedisModuleCronLoop structure with the following fields:

      int32_t hz;  // Approximate number of events per second.

  • RedisModuleEvent_MasterLinkChange

    This is called for replicas in order to notify when the replication link becomes functional (up) with our master, or when it goes down. Note that the link is not considered up when we just connected to the master, but only if the replication is happening correctly. The following sub events are available:

    • REDISMODULE_SUBEVENT_MASTER_LINK_UP
    • REDISMODULE_SUBEVENT_MASTER_LINK_DOWN

  • RedisModuleEvent_ModuleChange

    This event is called when a new module is loaded or one is unloaded. The following sub events are available:

    • REDISMODULE_SUBEVENT_MODULE_LOADED
    • REDISMODULE_SUBEVENT_MODULE_UNLOADED

    The data pointer can be casted to a RedisModuleModuleChange structure with the following fields:

      const char* module_name;  // Name of module loaded or unloaded.
  int32_t module_version;  // Module version.

  • RedisModuleEvent_LoadingProgress

    This event is called repeatedly called while an RDB or AOF file is being loaded. The following sub events are available:

    • REDISMODULE_SUBEVENT_LOADING_PROGRESS_RDB
    • REDISMODULE_SUBEVENT_LOADING_PROGRESS_AOF

    The data pointer can be casted to a RedisModuleLoadingProgress structure with the following fields:

      int32_t hz;  // Approximate number of events per second.
  int32_t progress;  // Approximate progress between 0 and 1024,
                     // or -1 if unknown.

  • RedisModuleEvent_SwapDB

    This event is called when a SWAPDB command has been successfully Executed. For this event call currently there is no subevents available.

    The data pointer can be casted to a RedisModuleSwapDbInfo structure with the following fields:

      int32_t dbnum_first;    // Swap Db first dbnum
  int32_t dbnum_second;   // Swap Db second dbnum

  • RedisModuleEvent_ReplBackup

    WARNING: Replication Backup events are deprecated since Redis 7.0 and are never fired. See RedisModuleEvent_ReplAsyncLoad for understanding how Async Replication Loading events are now triggered when repl-diskless-load is set to swapdb.

    Called when repl-diskless-load config is set to swapdb, And redis needs to backup the the current database for the possibility to be restored later. A module with global data and maybe with aux_load and aux_save callbacks may need to use this notification to backup / restore / discard its globals. The following sub events are available:

    • REDISMODULE_SUBEVENT_REPL_BACKUP_CREATE
    • REDISMODULE_SUBEVENT_REPL_BACKUP_RESTORE
    • REDISMODULE_SUBEVENT_REPL_BACKUP_DISCARD

  • RedisModuleEvent_ReplAsyncLoad

    Called when repl-diskless-load config is set to swapdb and a replication with a master of same data set history (matching replication ID) occurs. In which case redis serves current data set while loading new database in memory from socket. Modules must have declared they support this mechanism in order to activate it, through REDISMODULE_OPTIONS_HANDLE_REPL_ASYNC_LOAD flag. The following sub events are available:

    • REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_STARTED
    • REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_ABORTED
    • REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_COMPLETED

  • RedisModuleEvent_ForkChild

    Called when a fork child (AOFRW, RDBSAVE, module fork…) is born/dies The following sub events are available:

    • REDISMODULE_SUBEVENT_FORK_CHILD_BORN
    • REDISMODULE_SUBEVENT_FORK_CHILD_DIED

  • RedisModuleEvent_EventLoop

    Called on each event loop iteration, once just before the event loop goes to sleep or just after it wakes up. The following sub events are available:

    • REDISMODULE_SUBEVENT_EVENTLOOP_BEFORE_SLEEP
    • REDISMODULE_SUBEVENT_EVENTLOOP_AFTER_SLEEP

The function returns REDISMODULE_OK if the module was successfully subscribed for the specified event. If the API is called from a wrong context or unsupported event is given then REDISMODULE_ERR is returned.

RedisModule_IsSubEventSupported

int RedisModule_IsSubEventSupported(RedisModuleEvent event, int64_t subevent);

Available since: 6.0.9

For a given server event and subevent, return zero if the subevent is not supported and non-zero otherwise.

Key eviction API

RedisModule_SetLRU

int RedisModule_SetLRU(RedisModuleKey *key, mstime_t lru_idle);

Available since: 6.0.0

Set the key last access time for LRU based eviction. not relevant if the servers’s maxmemory policy is LFU based. Value is idle time in milliseconds. returns REDISMODULE_OK if the LRU was updated, REDISMODULE_ERR otherwise.

RedisModule_GetLRU

int RedisModule_GetLRU(RedisModuleKey *key, mstime_t *lru_idle);

Available since: 6.0.0

Gets the key last access time. Value is idletime in milliseconds or -1 if the server’s eviction policy is LFU based. returns REDISMODULE_OK if when key is valid.

RedisModule_SetLFU

int RedisModule_SetLFU(RedisModuleKey *key, long long lfu_freq);

Available since: 6.0.0

Set the key access frequency. only relevant if the server’s maxmemory policy is LFU based. The frequency is a logarithmic counter that provides an indication of the access frequencyonly (must be <= 255). returns REDISMODULE_OK if the LFU was updated, REDISMODULE_ERR otherwise.

RedisModule_GetLFU

int RedisModule_GetLFU(RedisModuleKey *key, long long *lfu_freq);

Available since: 6.0.0

Gets the key access frequency or -1 if the server’s eviction policy is not LFU based. returns REDISMODULE_OK if when key is valid.

Miscellaneous APIs

RedisModule_GetContextFlagsAll

int RedisModule_GetContextFlagsAll();

Available since: 6.0.9

Returns the full ContextFlags mask, using the return value the module can check if a certain set of flags are supported by the redis server version in use. Example:

   int supportedFlags = RM_GetContextFlagsAll();
   if (supportedFlags & REDISMODULE_CTX_FLAGS_MULTI) {
         // REDISMODULE_CTX_FLAGS_MULTI is supported
   } else{
         // REDISMODULE_CTX_FLAGS_MULTI is not supported
   }

RedisModule_GetKeyspaceNotificationFlagsAll

int RedisModule_GetKeyspaceNotificationFlagsAll();

Available since: 6.0.9

Returns the full KeyspaceNotification mask, using the return value the module can check if a certain set of flags are supported by the redis server version in use. Example:

   int supportedFlags = RM_GetKeyspaceNotificationFlagsAll();
   if (supportedFlags & REDISMODULE_NOTIFY_LOADED) {
         // REDISMODULE_NOTIFY_LOADED is supported
   } else{
         // REDISMODULE_NOTIFY_LOADED is not supported
   }

RedisModule_GetServerVersion

int RedisModule_GetServerVersion();

Available since: 6.0.9

Return the redis version in format of 0x00MMmmpp. Example for 6.0.7 the return value will be 0x00060007.

RedisModule_GetTypeMethodVersion

int RedisModule_GetTypeMethodVersion();

Available since: 6.2.0

Return the current redis-server runtime value of REDISMODULE_TYPE_METHOD_VERSION. You can use that when calling RedisModule_CreateDataType to know which fields of RedisModuleTypeMethods are gonna be supported and which will be ignored.

RedisModule_ModuleTypeReplaceValue

int RedisModule_ModuleTypeReplaceValue(RedisModuleKey *key,
                                       moduleType *mt,
                                       void *new_value,
                                       void **old_value);

Available since: 6.0.0

Replace the value assigned to a module type.

The key must be open for writing, have an existing value, and have a moduleType that matches the one specified by the caller.

Unlike RedisModule_ModuleTypeSetValue() which will free the old value, this function simply swaps the old value with the new value.

The function returns REDISMODULE_OK on success, REDISMODULE_ERR on errors such as:

  1. Key is not opened for writing.
  2. Key is not a module data type key.
  3. Key is a module datatype other than ‘mt’.

If old_value is non-NULL, the old value is returned by reference.

RedisModule_GetCommandKeysWithFlags

int *RedisModule_GetCommandKeysWithFlags(RedisModuleCtx *ctx,
                                         RedisModuleString **argv,
                                         int argc,
                                         int *num_keys,
                                         int **out_flags);

For a specified command, parse its arguments and return an array that contains the indexes of all key name arguments. This function is essentially a more efficient way to do COMMAND GETKEYS.

The out_flags argument is optional, and can be set to NULL. When provided it is filled with REDISMODULE_CMD_KEY_ flags in matching indexes with the key indexes of the returned array.

A NULL return value indicates the specified command has no keys, or an error condition. Error conditions are indicated by setting errno as follows:

  • ENOENT: Specified command does not exist.
  • EINVAL: Invalid command arity specified.

NOTE: The returned array is not a Redis Module object so it does not get automatically freed even when auto-memory is used. The caller must explicitly call RedisModule_Free() to free it, same as the out_flags pointer if used.

RedisModule_GetCommandKeys

int *RedisModule_GetCommandKeys(RedisModuleCtx *ctx,
                                RedisModuleString **argv,
                                int argc,
                                int *num_keys);

Available since: 6.0.9

Identinal to RedisModule_GetCommandKeysWithFlags when flags are not needed.

RedisModule_GetCurrentCommandName

const char *RedisModule_GetCurrentCommandName(RedisModuleCtx *ctx);

Available since: 6.2.5

Return the name of the command currently running

Defrag API

RedisModule_RegisterDefragFunc

int RedisModule_RegisterDefragFunc(RedisModuleCtx *ctx,
                                   RedisModuleDefragFunc cb);

Available since: 6.2.0

Register a defrag callback for global data, i.e. anything that the module may allocate that is not tied to a specific data type.

RedisModule_DefragShouldStop

int RedisModule_DefragShouldStop(RedisModuleDefragCtx *ctx);

Available since: 6.2.0

When the data type defrag callback iterates complex structures, this function should be called periodically. A zero (false) return indicates the callback may continue its work. A non-zero value (true) indicates it should stop.

When stopped, the callback may use RedisModule_DefragCursorSet() to store its position so it can later use RedisModule_DefragCursorGet() to resume defragging.

When stopped and more work is left to be done, the callback should return 1. Otherwise, it should return 0.

NOTE: Modules should consider the frequency in which this function is called, so it generally makes sense to do small batches of work in between calls.

RedisModule_DefragCursorSet

int RedisModule_DefragCursorSet(RedisModuleDefragCtx *ctx,
                                unsigned long cursor);

Available since: 6.2.0

Store an arbitrary cursor value for future re-use.

This should only be called if RedisModule_DefragShouldStop() has returned a non-zero value and the defrag callback is about to exit without fully iterating its data type.

This behavior is reserved to cases where late defrag is performed. Late defrag is selected for keys that implement the free_effort callback and return a free_effort value that is larger than the defrag ‘active-defrag-max-scan-fields’ configuration directive.

Smaller keys, keys that do not implement free_effort or the global defrag callback are not called in late-defrag mode. In those cases, a call to this function will return REDISMODULE_ERR.

The cursor may be used by the module to represent some progress into the module’s data type. Modules may also store additional cursor-related information locally and use the cursor as a flag that indicates when traversal of a new key begins. This is possible because the API makes a guarantee that concurrent defragmentation of multiple keys will not be performed.

RedisModule_DefragCursorGet

int RedisModule_DefragCursorGet(RedisModuleDefragCtx *ctx,
                                unsigned long *cursor);

Available since: 6.2.0

Fetch a cursor value that has been previously stored using RedisModule_DefragCursorSet().

If not called for a late defrag operation, REDISMODULE_ERR will be returned and the cursor should be ignored. See RedisModule_DefragCursorSet() for more details on defrag cursors.

RedisModule_DefragAlloc

void *RedisModule_DefragAlloc(RedisModuleDefragCtx *ctx, void *ptr);

Available since: 6.2.0

Defrag a memory allocation previously allocated by RedisModule_Alloc, RedisModule_Calloc, etc. The defragmentation process involves allocating a new memory block and copying the contents to it, like realloc().

If defragmentation was not necessary, NULL is returned and the operation has no other effect.

If a non-NULL value is returned, the caller should use the new pointer instead of the old one and update any reference to the old pointer, which must not be used again.

RedisModule_DefragRedisModuleString

RedisModuleString *RedisModule_DefragRedisModuleString(RedisModuleDefragCtx *ctx,
                                                       RedisModuleString *str);

Available since: 6.2.0

Defrag a RedisModuleString previously allocated by RedisModule_Alloc, RedisModule_Calloc, etc. See RedisModule_DefragAlloc() for more information on how the defragmentation process works.

NOTE: It is only possible to defrag strings that have a single reference. Typically this means strings retained with RedisModule_RetainString or RedisModule_HoldString may not be defragmentable. One exception is command argvs which, if retained by the module, will end up with a single reference (because the reference on the Redis side is dropped as soon as the command callback returns).

RedisModule_GetKeyNameFromDefragCtx

const RedisModuleString *RedisModule_GetKeyNameFromDefragCtx(RedisModuleDefragCtx *ctx);

Returns the name of the key currently being processed. There is no guarantee that the key name is always available, so this may return NULL.

RedisModule_GetDbIdFromDefragCtx

int RedisModule_GetDbIdFromDefragCtx(RedisModuleDefragCtx *ctx);

Returns the database id of the key currently being processed. There is no guarantee that this info is always available, so this may return -1.

Function index

2 - Redis modules and blocking commands

How to implement blocking commands in a Redis module

Redis has a few blocking commands among the built-in set of commands. One of the most used is BLPOP (or the symmetric BRPOP) which blocks waiting for elements arriving in a list.

The interesting fact about blocking commands is that they do not block the whole server, but just the client calling them. Usually the reason to block is that we expect some external event to happen: this can be some change in the Redis data structures like in the BLPOP case, a long computation happening in a thread, to receive some data from the network, and so forth.

Redis modules have the ability to implement blocking commands as well, this documentation shows how the API works and describes a few patterns that can be used in order to model blocking commands.

NOTE: This API is currently experimental, so it can only be used if the macro REDISMODULE_EXPERIMENTAL_API is defined. This is required because these calls are still not in their final stage of design, so may change in the future, certain parts may be deprecated and so forth.

To use this part of the modules API include the modules header like that:

#define REDISMODULE_EXPERIMENTAL_API
#include "redismodule.h"

How blocking and resuming works.

Note: You may want to check the helloblock.c example in the Redis source tree inside the src/modules directory, for a simple to understand example on how the blocking API is applied.

In Redis modules, commands are implemented by callback functions that are invoked by the Redis core when the specific command is called by the user. Normally the callback terminates its execution sending some reply to the client. Using the following function instead, the function implementing the module command may request that the client is put into the blocked state:

RedisModuleBlockedClient *RedisModule_BlockClient(RedisModuleCtx *ctx, RedisModuleCmdFunc reply_callback, RedisModuleCmdFunc timeout_callback, void (*free_privdata)(void*), long long timeout_ms);

The function returns a RedisModuleBlockedClient object, which is later used in order to unblock the client. The arguments have the following meaning:

  • ctx is the command execution context as usually in the rest of the API.
  • reply_callback is the callback, having the same prototype of a normal command function, that is called when the client is unblocked in order to return a reply to the client.
  • timeout_callback is the callback, having the same prototype of a normal command function that is called when the client reached the ms timeout.
  • free_privdata is the callback that is called in order to free the private data. Private data is a pointer to some data that is passed between the API used to unblock the client, to the callback that will send the reply to the client. We’ll see how this mechanism works later in this document.
  • ms is the timeout in milliseconds. When the timeout is reached, the timeout callback is called and the client is automatically aborted.

Once a client is blocked, it can be unblocked with the following API:

int RedisModule_UnblockClient(RedisModuleBlockedClient *bc, void *privdata);

The function takes as argument the blocked client object returned by the previous call to RedisModule_BlockClient(), and unblock the client. Immediately before the client gets unblocked, the reply_callback function specified when the client was blocked is called: this function will have access to the privdata pointer used here.

IMPORTANT: The above function is thread safe, and can be called from within a thread doing some work in order to implement the command that blocked the client.

The privdata data will be freed automatically using the free_privdata callback when the client is unblocked. This is useful since the reply callback may never be called in case the client timeouts or disconnects from the server, so it’s important that it’s up to an external function to have the responsibility to free the data passed if needed.

To better understand how the API works, we can imagine writing a command that blocks a client for one second, and then send as reply “Hello!”.

Note: arity checks and other non important things are not implemented int his command, in order to take the example simple.

int Example_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv,
                         int argc)
{
    RedisModuleBlockedClient *bc =
        RedisModule_BlockClient(ctx,reply_func,timeout_func,NULL,0);

    pthread_t tid;
    pthread_create(&tid,NULL,threadmain,bc);

    return REDISMODULE_OK;
}

void *threadmain(void *arg) {
    RedisModuleBlockedClient *bc = arg;

    sleep(1); /* Wait one second and unblock. */
    RedisModule_UnblockClient(bc,NULL);
}

The above command blocks the client ASAP, spawning a thread that will wait a second and will unblock the client. Let’s check the reply and timeout callbacks, which are in our case very similar, since they just reply the client with a different reply type.

int reply_func(RedisModuleCtx *ctx, RedisModuleString **argv,
               int argc)
{
    return RedisModule_ReplyWithSimpleString(ctx,"Hello!");
}

int timeout_func(RedisModuleCtx *ctx, RedisModuleString **argv,
               int argc)
{
    return RedisModule_ReplyWithNull(ctx);
}

The reply callback just sends the “Hello!” string to the client. The important bit here is that the reply callback is called when the client is unblocked from the thread.

The timeout command returns NULL, as it often happens with actual Redis blocking commands timing out.

Passing reply data when unblocking

The above example is simple to understand but lacks an important real world aspect of an actual blocking command implementation: often the reply function will need to know what to reply to the client, and this information is often provided as the client is unblocked.

We could modify the above example so that the thread generates a random number after waiting one second. You can think at it as an actually expansive operation of some kind. Then this random number can be passed to the reply function so that we return it to the command caller. In order to make this working, we modify the functions as follow:

void *threadmain(void *arg) {
    RedisModuleBlockedClient *bc = arg;

    sleep(1); /* Wait one second and unblock. */

    long *mynumber = RedisModule_Alloc(sizeof(long));
    *mynumber = rand();
    RedisModule_UnblockClient(bc,mynumber);
}

As you can see, now the unblocking call is passing some private data, that is the mynumber pointer, to the reply callback. In order to obtain this private data, the reply callback will use the following function:

void *RedisModule_GetBlockedClientPrivateData(RedisModuleCtx *ctx);

So our reply callback is modified like that:

int reply_func(RedisModuleCtx *ctx, RedisModuleString **argv,
               int argc)
{
    long *mynumber = RedisModule_GetBlockedClientPrivateData(ctx);
    /* IMPORTANT: don't free mynumber here, but in the
     * free privdata callback. */
    return RedisModule_ReplyWithLongLong(ctx,mynumber);
}

Note that we also need to pass a free_privdata function when blocking the client with RedisModule_BlockClient(), since the allocated long value must be freed. Our callback will look like the following:

void free_privdata(void *privdata) {
    RedisModule_Free(privdata);
}

NOTE: It is important to stress that the private data is best freed in the free_privdata callback because the reply function may not be called if the client disconnects or timeout.

Also note that the private data is also accessible from the timeout callback, always using the GetBlockedClientPrivateData() API.

Aborting the blocking of a client

One problem that sometimes arises is that we need to allocate resources in order to implement the non blocking command. So we block the client, then, for example, try to create a thread, but the thread creation function returns an error. What to do in such a condition in order to recover? We don’t want to take the client blocked, nor we want to call UnblockClient() because this will trigger the reply callback to be called.

In this case the best thing to do is to use the following function:

int RedisModule_AbortBlock(RedisModuleBlockedClient *bc);

Practically this is how to use it:

int Example_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv,
                         int argc)
{
    RedisModuleBlockedClient *bc =
        RedisModule_BlockClient(ctx,reply_func,timeout_func,NULL,0);

    pthread_t tid;
    if (pthread_create(&tid,NULL,threadmain,bc) != 0) {
        RedisModule_AbortBlock(bc);
        RedisModule_ReplyWithError(ctx,"Sorry can't create a thread");
    }

    return REDISMODULE_OK;
}

The client will be unblocked but the reply callback will not be called.

Implementing the command, reply and timeout callback using a single function

The following functions can be used in order to implement the reply and callback with the same function that implements the primary command function:

int RedisModule_IsBlockedReplyRequest(RedisModuleCtx *ctx);
int RedisModule_IsBlockedTimeoutRequest(RedisModuleCtx *ctx);

So I could rewrite the example command without using a separated reply and timeout callback:

int Example_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv,
                         int argc)
{
    if (RedisModule_IsBlockedReplyRequest(ctx)) {
        long *mynumber = RedisModule_GetBlockedClientPrivateData(ctx);
        return RedisModule_ReplyWithLongLong(ctx,mynumber);
    } else if (RedisModule_IsBlockedTimeoutRequest) {
        return RedisModule_ReplyWithNull(ctx);
    }

    RedisModuleBlockedClient *bc =
        RedisModule_BlockClient(ctx,reply_func,timeout_func,NULL,0);

    pthread_t tid;
    if (pthread_create(&tid,NULL,threadmain,bc) != 0) {
        RedisModule_AbortBlock(bc);
        RedisModule_ReplyWithError(ctx,"Sorry can't create a thread");
    }

    return REDISMODULE_OK;
}

Functionally is the same but there are people that will prefer the less verbose implementation that concentrates most of the command logic in a single function.

Working on copies of data inside a thread

An interesting pattern in order to work with threads implementing the slow part of a command, is to work with a copy of the data, so that while some operation is performed in a key, the user continues to see the old version. However when the thread terminated its work, the representations are swapped and the new, processed version, is used.

An example of this approach is the Neural Redis module where neural networks are trained in different threads while the user can still execute and inspect their older versions.

Future work

An API is work in progress right now in order to allow Redis modules APIs to be called in a safe way from threads, so that the threaded command can access the data space and do incremental operations.

There is no ETA for this feature but it may appear in the course of the Redis 4.0 release at some point.

3 - Modules API for native types

How to use native types in a Redis module

Redis modules can access Redis built-in data structures both at high level, by calling Redis commands, and at low level, by manipulating the data structures directly.

By using these capabilities in order to build new abstractions on top of existing Redis data structures, or by using strings DMA in order to encode modules data structures into Redis strings, it is possible to create modules that feel like they are exporting new data types. However, for more complex problems, this is not enough, and the implementation of new data structures inside the module is needed.

We call the ability of Redis modules to implement new data structures that feel like native Redis ones native types support. This document describes the API exported by the Redis modules system in order to create new data structures and handle the serialization in RDB files, the rewriting process in AOF, the type reporting via the TYPE command, and so forth.

Overview of native types

A module exporting a native type is composed of the following main parts:

  • The implementation of some kind of new data structure and of commands operating on the new data structure.
  • A set of callbacks that handle: RDB saving, RDB loading, AOF rewriting, releasing of a value associated with a key, calculation of a value digest (hash) to be used with the DEBUG DIGEST command.
  • A 9 characters name that is unique to each module native data type.
  • An encoding version, used to persist into RDB files a module-specific data version, so that a module will be able to load older representations from RDB files.

While to handle RDB loading, saving and AOF rewriting may look complex as a first glance, the modules API provide very high level function for handling all this, without requiring the user to handle read/write errors, so in practical terms, writing a new data structure for Redis is a simple task.

A very easy to understand but complete example of native type implementation is available inside the Redis distribution in the /modules/hellotype.c file. The reader is encouraged to read the documentation by looking at this example implementation to see how things are applied in the practice.

Registering a new data type

In order to register a new native type into the Redis core, the module needs to declare a global variable that will hold a reference to the data type. The API to register the data type will return a data type reference that will be stored in the global variable.

static RedisModuleType *MyType;
#define MYTYPE_ENCODING_VERSION 0

int RedisModule_OnLoad(RedisModuleCtx *ctx) {
RedisModuleTypeMethods tm = {
    .version = REDISMODULE_TYPE_METHOD_VERSION,
    .rdb_load = MyTypeRDBLoad,
    .rdb_save = MyTypeRDBSave,
    .aof_rewrite = MyTypeAOFRewrite,
    .free = MyTypeFree
};

    MyType = RedisModule_CreateDataType(ctx, "MyType-AZ",
	MYTYPE_ENCODING_VERSION, &tm);
    if (MyType == NULL) return REDISMODULE_ERR;
}

As you can see from the example above, a single API call is needed in order to register the new type. However a number of function pointers are passed as arguments. Certain are optionals while some are mandatory. The above set of methods must be passed, while .digest and .mem_usage are optional and are currently not actually supported by the modules internals, so for now you can just ignore them.

The ctx argument is the context that we receive in the OnLoad function. The type name is a 9 character name in the character set that includes from A-Z, a-z, 0-9, plus the underscore _ and minus - characters.

Note that this name must be unique for each data type in the Redis ecosystem, so be creative, use both lower-case and upper case if it makes sense, and try to use the convention of mixing the type name with the name of the author of the module, to create a 9 character unique name.

NOTE: It is very important that the name is exactly 9 chars or the registration of the type will fail. Read more to understand why.

For example if I’m building a b-tree data structure and my name is antirez I’ll call my type btree1-az. The name, converted to a 64 bit integer, is stored inside the RDB file when saving the type, and will be used when the RDB data is loaded in order to resolve what module can load the data. If Redis finds no matching module, the integer is converted back to a name in order to provide some clue to the user about what module is missing in order to load the data.

The type name is also used as a reply for the TYPE command when called with a key holding the registered type.

The encver argument is the encoding version used by the module to store data inside the RDB file. For example I can start with an encoding version of 0, but later when I release version 2.0 of my module, I can switch encoding to something better. The new module will register with an encoding version of 1, so when it saves new RDB files, the new version will be stored on disk. However when loading RDB files, the module rdb_load method will be called even if there is data found for a different encoding version (and the encoding version is passed as argument to rdb_load), so that the module can still load old RDB files.

The last argument is a structure used in order to pass the type methods to the registration function: rdb_load, rdb_save, aof_rewrite, digest and free and mem_usage are all callbacks with the following prototypes and uses:

typedef void *(*RedisModuleTypeLoadFunc)(RedisModuleIO *rdb, int encver);
typedef void (*RedisModuleTypeSaveFunc)(RedisModuleIO *rdb, void *value);
typedef void (*RedisModuleTypeRewriteFunc)(RedisModuleIO *aof, RedisModuleString *key, void *value);
typedef size_t (*RedisModuleTypeMemUsageFunc)(void *value);
typedef void (*RedisModuleTypeDigestFunc)(RedisModuleDigest *digest, void *value);
typedef void (*RedisModuleTypeFreeFunc)(void *value);
  • rdb_load is called when loading data from the RDB file. It loads data in the same format as rdb_save produces.
  • rdb_save is called when saving data to the RDB file.
  • aof_rewrite is called when the AOF is being rewritten, and the module needs to tell Redis what is the sequence of commands to recreate the content of a given key.
  • digest is called when DEBUG DIGEST is executed and a key holding this module type is found. Currently this is not yet implemented so the function ca be left empty.
  • mem_usage is called when the MEMORY command asks for the total memory consumed by a specific key, and is used in order to get the amount of bytes used by the module value.
  • free is called when a key with the module native type is deleted via DEL or in any other mean, in order to let the module reclaim the memory associated with such a value.

Ok, but why modules types require a 9 characters name?

Oh, I understand you need to understand this, so here is a very specific explanation.

When Redis persists to RDB files, modules specific data types require to be persisted as well. Now RDB files are sequences of key-value pairs like the following:

[1 byte type] [key] [a type specific value]

The 1 byte type identifies strings, lists, sets, and so forth. In the case of modules data, it is set to a special value of module data, but of course this is not enough, we need the information needed to link a specific value with a specific module type that is able to load and handle it.

So when we save a type specific value about a module, we prefix it with a 64 bit integer. 64 bits is large enough to store the informations needed in order to lookup the module that can handle that specific type, but is short enough that we can prefix each module value we store inside the RDB without making the final RDB file too big. At the same time, this solution of prefixing the value with a 64 bit signature does not require to do strange things like defining in the RDB header a list of modules specific types. Everything is pretty simple.

So, what you can store in 64 bits in order to identify a given module in a reliable way? Well if you build a character set of 64 symbols, you can easily store 9 characters of 6 bits, and you are left with 10 bits, that are used in order to store the encoding version of the type, so that the same type can evolve in the future and provide a different and more efficient or updated serialization format for RDB files.

So the 64 bit prefix stored before each module value is like the following:

6|6|6|6|6|6|6|6|6|10

The first 9 elements are 6-bits characters, the final 10 bits is the encoding version.

When the RDB file is loaded back, it reads the 64 bit value, masks the final 10 bits, and searches for a matching module in the modules types cache. When a matching one is found, the method to load the RDB file value is called with the 10 bits encoding version as argument, so that the module knows what version of the data layout to load, if it can support multiple versions.

Now the interesting thing about all this is that, if instead the module type cannot be resolved, since there is no loaded module having this signature, we can convert back the 64 bit value into a 9 characters name, and print an error to the user that includes the module type name! So that she or he immediately realizes what’s wrong.

Setting and getting keys

After registering our new data type in the RedisModule_OnLoad() function, we also need to be able to set Redis keys having as value our native type.

This normally happens in the context of commands that write data to a key. The native types API allow to set and get keys to module native data types, and to test if a given key is already associated to a value of a specific data type.

The API uses the normal modules RedisModule_OpenKey() low level key access interface in order to deal with this. This is an example of setting a native type private data structure to a Redis key:

RedisModuleKey *key = RedisModule_OpenKey(ctx,keyname,REDISMODULE_WRITE);
struct some_private_struct *data = createMyDataStructure();
RedisModule_ModuleTypeSetValue(key,MyType,data);

The function RedisModule_ModuleTypeSetValue() is used with a key handle open for writing, and gets three arguments: the key handle, the reference to the native type, as obtained during the type registration, and finally a void* pointer that contains the private data implementing the module native type.

Note that Redis has no clues at all about what your data contains. It will just call the callbacks you provided during the method registration in order to perform operations on the type.

Similarly we can retrieve the private data from a key using this function:

struct some_private_struct *data;
data = RedisModule_ModuleTypeGetValue(key);

We can also test for a key to have our native type as value:

if (RedisModule_ModuleTypeGetType(key) == MyType) {
    /* ... do something ... */
}

However for the calls to do the right thing, we need to check if the key is empty, if it contains a value of the right kind, and so forth. So the idiomatic code to implement a command writing to our native type is along these lines:

RedisModuleKey *key = RedisModule_OpenKey(ctx,argv[1],
    REDISMODULE_READ|REDISMODULE_WRITE);
int type = RedisModule_KeyType(key);
if (type != REDISMODULE_KEYTYPE_EMPTY &&
    RedisModule_ModuleTypeGetType(key) != MyType)
{
    return RedisModule_ReplyWithError(ctx,REDISMODULE_ERRORMSG_WRONGTYPE);
}

Then if we successfully verified the key is not of the wrong type, and we are going to write to it, we usually want to create a new data structure if the key is empty, or retrieve the reference to the value associated to the key if there is already one:

/* Create an empty value object if the key is currently empty. */
struct some_private_struct *data;
if (type == REDISMODULE_KEYTYPE_EMPTY) {
    data = createMyDataStructure();
    RedisModule_ModuleTypeSetValue(key,MyTyke,data);
} else {
    data = RedisModule_ModuleTypeGetValue(key);
}
/* Do something with 'data'... */

Free method

As already mentioned, when Redis needs to free a key holding a native type value, it needs help from the module in order to release the memory. This is the reason why we pass a free callback during the type registration:

typedef void (*RedisModuleTypeFreeFunc)(void *value);

A trivial implementation of the free method can be something like this, assuming our data structure is composed of a single allocation:

void MyTypeFreeCallback(void *value) {
    RedisModule_Free(value);
}

However a more real world one will call some function that performs a more complex memory reclaiming, by casting the void pointer to some structure and freeing all the resources composing the value.

RDB load and save methods

The RDB saving and loading callbacks need to create (and load back) a representation of the data type on disk. Redis offers an high level API that can automatically store inside the RDB file the following types:

  • Unsigned 64 bit integers.
  • Signed 64 bit integers.
  • Doubles.
  • Strings.

It is up to the module to find a viable representation using the above base types. However note that while the integer and double values are stored and loaded in an architecture and endianness agnostic way, if you use the raw string saving API to, for example, save a structure on disk, you have to care those details yourself.

This is the list of functions performing RDB saving and loading:

void RedisModule_SaveUnsigned(RedisModuleIO *io, uint64_t value);
uint64_t RedisModule_LoadUnsigned(RedisModuleIO *io);
void RedisModule_SaveSigned(RedisModuleIO *io, int64_t value);
int64_t RedisModule_LoadSigned(RedisModuleIO *io);
void RedisModule_SaveString(RedisModuleIO *io, RedisModuleString *s);
void RedisModule_SaveStringBuffer(RedisModuleIO *io, const char *str, size_t len);
RedisModuleString *RedisModule_LoadString(RedisModuleIO *io);
char *RedisModule_LoadStringBuffer(RedisModuleIO *io, size_t *lenptr);
void RedisModule_SaveDouble(RedisModuleIO *io, double value);
double RedisModule_LoadDouble(RedisModuleIO *io);

The functions don’t require any error checking from the module, that can always assume calls succeed.

As an example, imagine I’ve a native type that implements an array of double values, with the following structure:

struct double_array {
    size_t count;
    double *values;
};

My rdb_save method may look like the following:

void DoubleArrayRDBSave(RedisModuleIO *io, void *ptr) {
    struct dobule_array *da = ptr;
    RedisModule_SaveUnsigned(io,da->count);
    for (size_t j = 0; j < da->count; j++)
        RedisModule_SaveDouble(io,da->values[j]);
}

What we did was to store the number of elements followed by each double value. So when later we’ll have to load the structure in the rdb_load method we’ll do something like this:

void *DoubleArrayRDBLoad(RedisModuleIO *io, int encver) {
    if (encver != DOUBLE_ARRAY_ENC_VER) {
        /* We should actually log an error here, or try to implement
           the ability to load older versions of our data structure. */
        return NULL;
    }

    struct double_array *da;
    da = RedisModule_Alloc(sizeof(*da));
    da->count = RedisModule_LoadUnsigned(io);
    da->values = RedisModule_Alloc(da->count * sizeof(double));
    for (size_t j = 0; j < da->count; j++)
        da->values[j] = RedisModule_LoadDouble(io);
    return da;
}

The load callback just reconstruct back the data structure from the data we stored in the RDB file.

Note that while there is no error handling on the API that writes and reads from disk, still the load callback can return NULL on errors in case what it reads does not look correct. Redis will just panic in that case.

AOF rewriting

void RedisModule_EmitAOF(RedisModuleIO *io, const char *cmdname, const char *fmt, ...);

Handling multiple encodings

WORK IN PROGRESS

Allocating memory

Modules data types should try to use RedisModule_Alloc() functions family in order to allocate, reallocate and release heap memory used to implement the native data structures (see the other Redis Modules documentation for detailed information).

This is not just useful in order for Redis to be able to account for the memory used by the module, but there are also more advantages:

  • Redis uses the jemalloc allocator, that often prevents fragmentation problems that could be caused by using the libc allocator.
  • When loading strings from the RDB file, the native types API is able to return strings allocated directly with RedisModule_Alloc(), so that the module can directly link this memory into the data structure representation, avoiding an useless copy of the data.

Even if you are using external libraries implementing your data structures, the allocation functions provided by the module API is exactly compatible with malloc(), realloc(), free() and strdup(), so converting the libraries in order to use these functions should be trivial.

In case you have an external library that uses libc malloc(), and you want to avoid replacing manually all the calls with the Redis Modules API calls, an approach could be to use simple macros in order to replace the libc calls with the Redis API calls. Something like this could work:

#define malloc RedisModule_Alloc
#define realloc RedisModule_Realloc
#define free RedisModule_Free
#define strdup RedisModule_Strdup

However take in mind that mixing libc calls with Redis API calls will result into troubles and crashes, so if you replace calls using macros, you need to make sure that all the calls are correctly replaced, and that the code with the substituted calls will never, for example, attempt to call RedisModule_Free() with a pointer allocated using libc malloc().