Modules API reference


void *RedisModule_Alloc(size_t bytes);

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().


void *RedisModule_Calloc(size_t nmemb, size_t size);

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.


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

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


void RedisModule_Free(void *ptr);

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.


char *RedisModule_Strdup(const char *str);

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


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

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.


int RedisModule_GetApi(const char *funcname, void **targetPtrPtr);

Lookup the requested module API and store the function pointer into the target pointer. The function returns REDISMODULE_ERR if there is no such named API, otherwise REDISMODULE_OK.

This function is not meant to be used by modules developer, it is only used implicitly by including redismodule.h.


int RedisModule_IsKeysPositionRequest(RedisModuleCtx *ctx);

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.


void RedisModule_KeyAtPos(RedisModuleCtx *ctx, int pos);

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, like in the following example:

if (RedisModule_IsKeysPositionRequest(ctx)) {

Note: in the example below the get keys API would not be needed since keys are at fixed positions. This interface is only used for commands with a more complex structure.


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

Register a new command in the Redis server, that will be handled by calling the function pointer ‘func’ 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 compoesd 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.
  • “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 propoagate the command on monitor. 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.


void RedisModule_SetModuleAttribs(RedisModuleCtx *ctx, const char *name, int ver, int apiver);

Called by RM_Init() to setup the ctx->module structure.

This is an internal function, Redis modules developers don’t need to use it.


long long RedisModule_Milliseconds(void);

Return the current UNIX time in milliseconds.


void RedisModule_AutoMemory(RedisModuleCtx *ctx);

Enable automatic memory management. See for more information.

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


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

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.


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

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().


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

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.


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

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.


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

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.


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

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.


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

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.


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

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.


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

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.


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

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.


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

Append the specified buffere 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 returend and the operation is not performed.


int RedisModule_WrongArity(RedisModuleCtx *ctx);

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


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


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

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


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

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.


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

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.


int RedisModule_ReplyWithArray(RedisModuleCtx *ctx, long len);

Reply with an array type of ‘len’ elements. However ‘len’ other calls to ReplyWith* style functions must follow in order to emit the elements of the array.

When producing arrays with a number of element that is not known beforehand the function can be called with the special count REDISMODULE_POSTPONED_ARRAY_LEN, and the actual number of elements can be later set with RedisModule_ReplySetArrayLength() (which will set the latest “open” count if there are multiple ones).

The function always returns REDISMODULE_OK.


void RedisModule_ReplySetArrayLength(RedisModuleCtx *ctx, long len);

When RedisModule_ReplyWithArray() is used with the argument REDISMODULE_POSTPONED_ARRAY_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_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 interator or other ways of creating the output so that is not easy to calculate in advance the number of elements.


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

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

The function always returns REDISMODULE_OK.


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

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

The function always returns REDISMODULE_OK.


int RedisModule_ReplyWithNull(RedisModuleCtx *ctx);

Reply to the client with a NULL. In the RESP protocol a NULL is encoded as the string “$-1\r\n”.

The function always returns REDISMODULE_OK.


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

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.

The function always returns REDISMODULE_OK.


int RedisModule_ReplyWithDouble(RedisModuleCtx *ctx, double d);

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.

The function always returns REDISMODULE_OK.


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

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.

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


int RedisModule_ReplicateVerbatim(RedisModuleCtx *ctx);

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.


unsigned long long RedisModule_GetClientId(RedisModuleCtx *ctx);

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.


int RedisModule_GetSelectedDb(RedisModuleCtx *ctx);

Return the currently selected DB.


int RedisModule_SelectDb(RedisModuleCtx *ctx, int newid);

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.


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

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 repesenting the key, that must be closed with RM_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.


void RedisModule_CloseKey(RedisModuleKey *key);

Close a key handle.


int RedisModule_KeyType(RedisModuleKey *key);

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


size_t RedisModule_ValueLength(RedisModuleKey *key);

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.


int RedisModule_DeleteKey(RedisModuleKey *key);

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.


mstime_t RedisModule_GetExpire(RedisModuleKey *key);

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.


int RedisModule_SetExpire(RedisModuleKey *key, mstime_t expire);

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.


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

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.


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

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:


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 RM_StringTruncate() is called, to continue with the DMA access, RM_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 RM_StringTruncate() call should be used if there is to enlarge the string, and later call StringDMA() again to get the pointer.


int RedisModule_StringTruncate(RedisModuleKey *key, size_t newlen);

If the string 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, RM_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.


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

Push an element into a list, on head or tail depending on ‘where’ argumnet. If the key pointer is about an empty key opened for writing, the key is created. On error (key opened for read-only operations or of the wrong type) REDISMODULE_ERR is returned, otherwise REDISMODULE_OK is returned.


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

Pop an element from the list, and returns it as a module string object that the user should be free with RM_FreeString() or by enabling automatic memory. ‘where’ specifies if the element should be popped from head or tail. The command returns NULL if: 1) The list is empty. 2) The key was not open for writing. 3) The key is not a list.


int RedisModule_ZsetAddFlagsToCoreFlags(int flags);

Conversion from/to public flags of the Modules API and our private flags, so that we have everything decoupled.


int RedisModule_ZsetAddFlagsFromCoreFlags(int flags);

See previous function comment.


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

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.

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).


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

This function works exactly like RM_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 resuts 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.


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

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.


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

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.


void RedisModule_ZsetRangeStop(RedisModuleKey *key);

Stop a sorted set iteration.


int RedisModule_ZsetRangeEndReached(RedisModuleKey *key);

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


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

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.


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

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


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

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.


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

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


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

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.


int RedisModule_ZsetRangeNext(RedisModuleKey *key);

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.


int RedisModule_ZsetRangePrev(RedisModuleKey *key);

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.


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

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).

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


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:


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.

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:


Return value:

The number of fields updated (that may be less than the number of fields specified because of the XX or NX options).

In the following case the return value is always zero:

  • The key was not open for writing.
  • The key was associated with a non Hash value.


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

Get fields from an hash value. This function is called using a variable number of arguments, alternating a field name (as a StringRedisModule pointer) with a pointer to a StringRedisModule pointer, that is set to the value of the field if the field exist, or NULL if the field did 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;

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

REDISMODULE_HASH_CFIELD: 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 esists or not and expects an integer pointer as the second element of each pair.


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


 int exists;

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.


void RedisModule_FreeCallReply_Rec(RedisModuleCallReply *reply, int freenested);

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


void RedisModule_FreeCallReply(RedisModuleCallReply *reply);

Wrapper for the recursive free reply function. This is needed in order to have the first level function to return on nested replies, but only if called by the module API.


int RedisModule_CallReplyType(RedisModuleCallReply *reply);

Return the reply type.


size_t RedisModule_CallReplyLength(RedisModuleCallReply *reply);

Return the reply type length, where applicable.


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

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.


long long RedisModule_CallReplyInteger(RedisModuleCallReply *reply);

Return the long long of an integer reply.


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

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


RedisModuleString *RedisModule_CreateStringFromCallReply(RedisModuleCallReply *reply);

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


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

Exported API to call any Redis command from modules. On success a RedisModuleCallReply object is returned, otherwise NULL is returned and errno is set to the following values:

EINVAL: command non existing, wrong arity, wrong format specifier. EPERM: operation in Cluster instance with key in non local slot.


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

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


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

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 the file.

  • 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,    }
  • 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.

The digest* and **mem_usage methods should currently be omitted since they are not yet implemented inside the Redis modules core.

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 gobal 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(...);


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

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.


moduleType *RedisModule_ModuleTypeGetType(RedisModuleKey *key);

Assuming RedisModule_KeyType() returned REDISMODULE_KEYTYPE_MODULE on the key, returns the moduel 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.


void *RedisModule_ModuleTypeGetValue(RedisModuleKey *key);

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_ModuleTypeSet().

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


void RedisModule_SaveUnsigned(RedisModuleIO *io, uint64_t value);

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.


uint64_t RedisModule_LoadUnsigned(RedisModuleIO *io);

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.


void RedisModule_SaveSigned(RedisModuleIO *io, int64_t value);

Like RedisModule_SaveUnsigned() but for signed 64 bit values.


int64_t RedisModule_LoadSigned(RedisModuleIO *io);

Like RedisModule_LoadUnsigned() but for signed 64 bit values.


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

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.


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

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


RedisModuleString *RedisModule_LoadString(RedisModuleIO *io);

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.


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

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 termianted, it is loaded exactly as it was stored inisde the RDB file.


void RedisModule_SaveDouble(RedisModuleIO *io, double value);

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().


double RedisModule_LoadDouble(RedisModuleIO *io);

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


void RedisModule_SaveFloat(RedisModuleIO *io, float value);

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().


float RedisModule_LoadFloat(RedisModuleIO *io);

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


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

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 {

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 {

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 {


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

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


void RedisModule_DigestEndSequence(RedisModuleDigest *md);

See the doucmnetation for RedisModule_DigestAddElement().


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

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.


void RedisModule_LogRaw(RedisModule *module, const char *levelstr, const char *fmt, va_list ap);

This is the low level function implementing both:



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

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”
  • “verbose”
  • “notice”
  • “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 limti is not specified but is guaranteed to be more than a few lines of text.


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

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.


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

Block a client in the context of a blocking command, returning an handle which will be used, later, in order to block 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.

reply_timeout:   called when the timeout is reached in order to send an
                 error to the client.

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


int RedisModule_UnblockClient(RedisModuleBlockedClient *bc, void *privdata);

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: this function can be called from threads spawned by the module.


int RedisModule_AbortBlock(RedisModuleBlockedClient *bc);

Abort a blocked client blocking operation: the client will be unblocked without firing the reply callback.


int RedisModule_IsBlockedReplyRequest(RedisModuleCtx *ctx);

Return non-zero if a module command was called in order to fill the reply for a blocked client.


int RedisModule_IsBlockedTimeoutRequest(RedisModuleCtx *ctx);

Return non-zero if a module command was called in order to fill the reply for a blocked client that timed out.


void *RedisModule_GetBlockedClientPrivateData(RedisModuleCtx *ctx);

Get the privata data set by RedisModule_UnblockClient()


RedisModuleCtx *RedisModule_GetThreadSafeContext(RedisModuleBlockedClient *bc);

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:

... make your call here ...

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.

TODO: thread safe contexts do not inherit the blocked client selected database.


void RedisModule_FreeThreadSafeContext(RedisModuleCtx *ctx);

Release a thread safe context.


void RedisModule_ThreadSafeContextLock(RedisModuleCtx *ctx);

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.


void RedisModule_ThreadSafeContextUnlock(RedisModuleCtx *ctx);

Release the server lock after a thread safe API call was executed.