Commit 646c958b authored by antirez's avatar antirez
Browse files

Modules: doc layout improved.

parent 745845df
......@@ -451,7 +451,7 @@ int RM_CreateCommand(RedisModuleCtx *ctx, const char *name, RedisModuleCmdFunc c
return REDISMODULE_OK;
}
/* Called by RM_Init() to setup the ctx->module structure.
/* Called by RM_Init() to setup the `ctx->module` structure.
*
* This is an internal function, Redis modules developers don't need
* to use it. */
......@@ -578,7 +578,7 @@ const char *RM_StringPtrLen(RedisModuleString *str, size_t *len) {
return str->ptr;
}
/* Convert the string into a long long integer, storing it at *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. */
......@@ -587,7 +587,7 @@ int RM_StringToLongLong(RedisModuleString *str, long long *ll) {
REDISMODULE_ERR;
}
/* Convert the string into a double, storing it at *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 RM_StringToDouble(RedisModuleString *str, double *d) {
......@@ -665,7 +665,7 @@ int RM_ReplyWithSimpleString(RedisModuleCtx *ctx, const char *msg) {
}
/* Reply with an array type of 'len' elements. However 'len' other calls
* to ReplyWith* style functions must follow in order to emit the elements
* 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
......@@ -1219,9 +1219,9 @@ int RM_ZsetAddFlagsFromCoreFlags(int 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).
* * The key was not opened for writing.
* * The key is of the wrong type.
* * 'score' double value is not a number (NaN).
*/
int RM_ZsetAdd(RedisModuleKey *key, double score, RedisModuleString *ele, int *flagsptr) {
int flags = 0;
......@@ -1274,8 +1274,8 @@ int RM_ZsetIncrby(RedisModuleKey *key, double score, RedisModuleString *ele, int
* 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 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
......@@ -1303,9 +1303,9 @@ int RM_ZsetRem(RedisModuleKey *key, RedisModuleString *ele, int *deleted) {
* '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.
* * There is no such element 'ele' in the sorted set.
* * The key is not a sorted set.
* * The key is an open empty key.
*/
int RM_ZsetScore(RedisModuleKey *key, RedisModuleString *ele, double *score) {
if (key->value == NULL) return REDISMODULE_ERR;
......@@ -1663,8 +1663,8 @@ int RM_ZsetRangePrev(RedisModuleKey *key) {
*
* 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.
* * The key was not open for writing.
* * The key was associated with a non Hash value.
*/
int RM_HashSet(RedisModuleKey *key, int flags, ...) {
va_list ap;
......
Redis Modules API reference manual
===
# Modules API reference
Redis modules make possible to extend Redis functionality using external
modules, implementing new Redis commands at a speed and with features
similar to what can be done inside the core itself.
## `RM_GetApi`
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.
int RM_GetApi(const char *funcname, void **targetPtrPtr);
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".
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 document is about an alpha version of Redis modules. API, functionalities
and other details may change in the future.
This function is not meant to be used by modules developer, it is only
used implicitly by including redismodule.h.
# Loading modules
## `RM_IsKeysPositionRequest`
In order to test the module you are developing, you can load the module
using the following `redis.conf` configuration directive:
int RM_IsKeysPositionRequest(RedisModuleCtx *ctx);
loadmodule /path/to/mymodule.so
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.
It is also possible to load a module at runtime using the following command:
## `RM_KeyAtPos`
MODULE LOAD /path/to/mymodule.so
void RM_KeyAtPos(RedisModuleCtx *ctx, int pos);
In order to list all loaded modules, use:
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:
MODULE LIST
if (`RedisModule_IsKeysPositionRequest(ctx))` {
`RedisModule_KeyAtPos(ctx`,1);
`RedisModule_KeyAtPos(ctx`,2);
}
Finally, you can unload (and later reload if you wish) a module using the
following command:
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.
MODULE UNLOAD mymodule
## `RM_CreateCommand`
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.
int RM_CreateCommand(RedisModuleCtx *ctx, const char *name, RedisModuleCmdFunc cmdfunc, const char *strflags, int firstkey, int lastkey, int keystep);
# The simplest module you can write
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.
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.
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.
#include "redismodule.h"
#include <stdlib.h>
The command function type is the following:
int HelloworldRand_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
RedisModule_ReplyWithLongLong(ctx,rand());
return REDISMODULE_OK;
}
int MyCommand_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc);
int RedisModule_OnLoad(RedisModuleCtx *ctx) {
if (RedisModule_Init(ctx,"helloworld",1,REDISMODULE_APIVER_1)
== REDISMODULE_ERR) return REDISMODULE_ERR;
And is supposed to always return `REDISMODULE_OK`.
if (RedisModule_CreateCommand(ctx,"helloworld.rand",
HelloworldRand_RedisCommand) == REDISMODULE_ERR)
return REDISMODULE_ERR;
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:
return REDISMODULE_OK;
}
* **"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.
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.
## `RM_SetModuleAttribs`
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.
void RM_SetModuleAttribs(RedisModuleCtx *ctx, const char *name, int ver, int apiver);
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.
Called by `RM_Init()` to setup the `ctx->module` structure.
# Module initialization
This is an internal function, Redis modules developers don't need
to use it.
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:
## `RM_AutoMemory`
int RedisModule_Init(RedisModuleCtx *ctx, const char *modulename,
int module_version, int api_version);
void RM_AutoMemory(RedisModuleCtx *ctx);
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.
Enable automatic memory management. See API.md for more information.
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.
The function must be called as the first function of a command implementation
that wants to use automatic memory.
Before the `Init` function is called, no other API function can be called,
otherwise the module will segfault and the Redis instance will crash.
## `RM_CreateString`
The second function called, `RedisModule_CreateCommand`, is used in order
to register commands into the Redis core. The following is the prototype:
RedisModuleString *RM_CreateString(RedisModuleCtx *ctx, const char *ptr, size_t len);
int RedisModule_CreateCommand(RedisModuleCtx *ctx, const char *cmdname,
RedisModuleCmdFunc cmdfunc);
Create a new module string object. The returned string must be freed
with `RedisModule_FreeString()`, unless automatic memory is enabled.
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.
The string is created by copying the `len` bytes starting
at `ptr`. No reference is retained to the passed buffer.
To create a new command, the above function needs the context, the command
name, and the function pointer of the function implementing the command,
which must have the following prototype:
## `RM_CreateStringFromLongLong`
RedisModuleString *RM_CreateStringFromLongLong(RedisModuleCtx *ctx, long long ll);
int mycommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc);
Like `RedisModule_CreatString()`, but creates a string starting from a long long
integer instead of taking a buffer and its length.
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.
The returned string must be released with `RedisModule_FreeString()` or by
enabling automatic memory management.
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.
## `RM_FreeString`
Zooming into the example command implementation, we can find another call:
void RM_FreeString(RedisModuleCtx *ctx, RedisModuleString *str);
int RedisModule_ReplyWithLongLong(RedisModuleCtx *ctx, long long integer);
Free a module string object obtained with one of the Redis modules API calls
that return new string objects.
This function returns an integer to the client that invoked the command,
exactly like other Redis commands do, like for example `INCR` or `SCARD`.
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.
# Setup and dependencies of a Redis module
## `RM_StringPtrLen`
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.
const char *RM_StringPtrLen(RedisModuleString *str, size_t *len);
The module will be able to load into different versions of Redis.
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.
# Working with RedisModuleString objects
## `RM_StringToLongLong`
The command argument vector `argv` passed to module commands, and the
return value of other module APIs functions, are of type `RedisModuleString`.
int RM_StringToLongLong(RedisModuleString *str, long long *ll);
Usually you directly pass module strings to other API calls, however sometimes
you may need to directly access the string object.
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.
There are a few functions in order to work with string objects:
## `RM_StringToDouble`
const char *RedisModule_StringPtrLen(RedisModuleString *string, size_t *len);
int RM_StringToDouble(RedisModuleString *str, double *d);
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.
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.
However, if you want, you can create new string objects using the following
API:
## `RM_WrongArity`
RedisModuleString *RedisModule_CreateString(RedisModuleCtx *ctx, const char *ptr, size_t len);
int RM_WrongArity(RedisModuleCtx *ctx);
The string returned by the above command must be freed using a corresponding
call to `RedisModule_FreeString()`:
Send an error about the number of arguments given to the command,
citing the command name in the error message.
void RedisModule_FreeString(RedisModuleString *str);
Example:
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.
if (argc != 3) return `RedisModule_WrongArity(ctx)`;
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.
## `RM_ReplyWithLongLong`
## Creating strings from numbers or parsing strings as numbers
int RM_ReplyWithLongLong(RedisModuleCtx *ctx, long long ll);
Creating a new string from an integer is a very common operation, so there
is a function to do this:
Send an integer reply to the client, with the specified long long value.
The function always returns `REDISMODULE_OK`.
RedisModuleString *mystr = RedisModule_CreateStringFromLongLong(ctx,10);
## `RM_ReplyWithError`
Similarly in order to parse a string as a number:
int RM_ReplyWithError(RedisModuleCtx *ctx, const char *err);
long long myval;
if (RedisModule_StringToLongLong(ctx,argv[1],&myval) == REDISMODULE_OK) {
/* Do something with 'myval' */
}
Reply with the error 'err'.
## 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,"INCR","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 slaves and AOF. It is ignored from the point of view of arguments parsing.
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,"INCR","sc",argv[1],"10");
if (RedisModule_CallReplyType(reply) == REDISMODULE_REPLY_INTEGER) {
long long myval = RedisModule_CallReplyInteger(reply);
/* Do something with myval. */
}
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:
Valid reply types are:
`RM_ReplyWithError(ctx`,"ERR Wrong Type");
* `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.
and not just:
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:
`RM_ReplyWithError(ctx`,"Wrong Type");
size_t reply_len = RedisModule_CallReplyLength(reply);
The function always returns `REDISMODULE_OK`.
In order to obtain the value of an integer reply, the following function is used, as already shown in the example above:
## `RM_ReplyWithSimpleString`
long long reply_integer_val = RedisModule_CallReplyInteger(reply);
int RM_ReplyWithSimpleString(RedisModuleCtx *ctx, const char *msg);
Called with a reply object of the wrong type, the above function always
returns `LLONG_MIN`.
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.
Sub elements of array replies are accessed this way:
The function always returns `REDISMODULE_OK`.
RedisModuleCallReply *subreply;
subreply = RedisModule_CallReplyArrayElement(reply,idx);
## `RM_ReplyWithArray`
The above function returns NULL if you try to access out of range elements.
int RM_ReplyWithArray(RedisModuleCtx *ctx, long len);
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):
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.
size_t len;
char *ptr = RedisModule_CallReplyStringPtr(reply,&len);
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).
If the reply type is not a string or an error, NULL is returned.
The function always returns `REDISMODULE_OK`.
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.
## `RM_ReplySetArrayLength`
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:
void RM_ReplySetArrayLength(RedisModuleCtx *ctx, long len);
RedisModuleString *mystr = RedisModule_CreateStringFromCallReply(myreply);
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.
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).
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.
# Releasing call reply objects
For example in order to output an array like [1,[10,20,30]] we
could write:
Reply objects must be freed using `RedisModule_FreeCallRelpy`. 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.
`RedisModule_ReplyWithArray(ctx`,`REDISMODULE_POSTPONED_ARRAY_LEN`);
`RedisModule_ReplyWithLongLong(ctx`,1);
`RedisModule_ReplyWithArray(ctx`,`REDISMODULE_POSTPONED_ARRAY_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
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).
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.
## Returning values from Redis commands
## `RM_ReplyWithStringBuffer`
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 elemented. 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).
int RM_ReplyWithStringBuffer(RedisModuleCtx *ctx, const char *buf, size_t len);
All the functions to send a reply to the client are called
`RedisModule_ReplyWith<something>`.
Reply with a bulk string, taking in input a C buffer pointer and length.
To return an error, use:
The function always returns `REDISMODULE_OK`.
RedisModule_ReplyWithError(RedisModuleCtx *ctx, const char *err);
## `RM_ReplyWithString`
There is a predefined error string for key of wrong type errors:
int RM_ReplyWithString(RedisModuleCtx *ctx, RedisModuleString *str);
REDISMODULE_ERRORMSG_WRONGTYPE
Reply with a bulk string, taking in input a RedisModuleString object.
Example usage:
The function always returns `REDISMODULE_OK`.
RedisModule_ReplyWithError(ctx,"ERR invalid arguments");
## `RM_ReplyWithNull`
We already saw how to reply with a long long in the examples above:
int RM_ReplyWithNull(RedisModuleCtx *ctx);
RedisModule_ReplyWithLongLong(ctx,12345);
Reply to the client with a NULL. In the RESP protocol a NULL is encoded
as the string "$-1\r\n".
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:
The function always returns `REDISMODULE_OK`.
RedisModule_ReplyWithSimpleString(ctx,"OK");
## `RM_ReplyWithCallReply`
It's possible to reply with "bulk strings" that are binary safe, using
two different functions:
int RM_ReplyWithCallReply(RedisModuleCtx *ctx, RedisModuleCallReply *reply);
int RedisModule_ReplyWithStringBuffer(RedisModuleCtx *ctx, const char *buf, size_t len);
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.
int RedisModule_ReplyWithString(RedisModuleCtx *ctx, RedisModuleString *str);
The function always returns `REDISMODULE_OK`.
The first function gets a C pointer and length. The second a RedisMoudleString
object. Use one or the other depending on the source type you have at hand.
## `RM_ReplyWithDouble`
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:
int RM_ReplyWithDouble(RedisModuleCtx *ctx, double d);
RedisModule_ReplyWithArray(ctx,2);
RedisModule_ReplyWithStringBuffer(ctx,"age",3);
RedisModule_ReplyWithLongLong(ctx,22);
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.
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.
The function always returns `REDISMODULE_OK`.
## Returning arrays with dynamic length
## `RM_Replicate`
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()`:
int RM_Replicate(RedisModuleCtx *ctx, const char *cmdname, const char *fmt, ...);
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_ARRAY_LEN);
Replicate the specified command and arguments to slaves and AOF, as effect
of execution of the calling command implementation.
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
se use the following call:
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.
RedisModule_ReplySetArrayLength(ctx, number_of_items);
Modules should try to use one interface or the other.
In the case of the FACTOR command, this translates to some code similar
to this:
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.
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_ARRAY_LEN);
number_of_factors = 0;
while(still_factors) {
RedisModule_ReplyWithLongLong(ctx, some_factor);
number_of_factors++;
}
RedisModule_ReplySetArrayLength(ctx, number_of_factors);
Please refer to `RedisModule_Call()` for more information.
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.
The command returns `REDISMODULE_ERR` if the format specifiers are invalid
or the command name does not belong to a known command.
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()`:
## `RM_ReplicateVerbatim`
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_ARRAY_LEN);
... generate 100 elements ...
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_ARRAY_LEN);
... generate 10 elements ...
RedisModule_ReplySetArrayLength(ctx, 10);
RedisModule_ReplySetArrayLength(ctx, 100);
int RM_ReplicateVerbatim(RedisModuleCtx *ctx);
This creates a 100 items array having as last element a 10 items array.
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.
# Arity and type checks
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.
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:
The function always returns `REDISMODULE_OK`.
if (argc != 2) return RedisModule_WrongArity(ctx);
## `RM_GetClientId`
Checking for the wrong type involves opening the key and checking the type:
unsigned long long RM_GetClientId(RedisModuleCtx *ctx);
RedisModuleKey *key = RedisModule_OpenKey(ctx,argv[1],
REDISMODULE_READ|REDISMODULE_WRITE);
Return the ID of the current client calling the currently active module
command. The returned ID has a few guarantees:
int keytype = RedisModule_KeyType(key);
if (keytype != REDISMODULE_KEYTYPE_STRING &&
keytype != REDISMODULE_KEYTYPE_EMPTY)
{
RedisModule_CloseKey(key);
return RedisModule_ReplyWithError(ctx,REDISMODULE_ERRORMSG_WRONGTYPE);
}
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.
Note that you often want to proceed with a command both if the key
is of the expected type, or if it's empty.
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.
## Low level access to keys
## `RM_GetSelectedDb`
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.
int RM_GetSelectedDb(RedisModuleCtx *ctx);
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.
Return the currently selected DB.
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:
## `RM_SelectDb`
* 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.
int RM_SelectDb(RedisModuleCtx *ctx, int newid);
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:
Change the currently selected DB. Returns an error if the id
is out of range.
RedisModuleKey *key;
key = RedisModule_OpenKey(ctx,argv[1],REDISMODULE_READ);
Note that the client will retain the currently selected DB even after
the Redis command implemented by the module calling this function
returns.
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.
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.
You can open non exisitng 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.
## `RM_OpenKey`
Once you are done using a key, you can close it with:
void *RM_OpenKey(RedisModuleCtx *ctx, robj *keyname, int mode);
RedisModule_CloseKey(key);
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.
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.
The return value is the handle repesenting the key, that must be
closed with `RM_CloseKey()`.
## Getting the key type
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.
In order to obtain the value of a key, use the `RedisModule_KeyType()` function:
## `RM_CloseKey`
int keytype = RedisModule_KeyType(key);
void RM_CloseKey(RedisModuleKey *key);
It returns one of the following values:
Close a key handle.
REDISMODULE_KEYTYPE_EMPTY
REDISMODULE_KEYTYPE_STRING
REDISMODULE_KEYTYPE_LIST
REDISMODULE_KEYTYPE_HASH
REDISMODULE_KEYTYPE_SET
REDISMODULE_KEYTYPE_ZSET
## `RM_KeyType`
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.
int RM_KeyType(RedisModuleKey *key);
## Creating new keys
Return the type of the key. If the key pointer is NULL then
`REDISMODULE_KEYTYPE_EMPTY` is returned.
To create a new key, open it for writing and then write to it using one
of the key writing functions. Example:
## `RM_ValueLength`
RedisModuleKey *key;
key = RedisModule_OpenKey(ctx,argv[1],REDISMODULE_READ);
if (RedisModule_KeyType(key) == REDISMODULE_KEYTYPE_EMPTY) {
RedisModule_StringSet(key,argv[2]);
}
size_t RM_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.
## `RM_DeleteKey`
int RM_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.
## `RM_GetExpire`
mstime_t RM_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.
## `RM_SetExpire`
int RM_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.
## `RM_StringSet`
int RM_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.
## `RM_StringDMA`
char *RM_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:
`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 `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.
## `RM_StringTruncate`
int RM_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.
## `RM_ListPush`
int RM_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.
## `RM_ListPop`
RedisModuleString *RM_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.
## `RM_ZsetAddFlagsToCoreFlags`
int RM_ZsetAddFlagsToCoreFlags(int flags);
Conversion from/to public flags of the Modules API and our private flags,
so that we have everything decoupled.
## `RM_ZsetAddFlagsFromCoreFlags`
int RM_ZsetAddFlagsFromCoreFlags(int flags);
See previous function comment.
## `RM_ZsetAdd`
int RM_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).
## `RM_ZsetIncrby`
int RM_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.
## `RM_ZsetRem`
int RM_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.
## `RM_ZsetScore`
int RM_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.
## `RM_ZsetRangeStop`
void RM_ZsetRangeStop(RedisModuleKey *key);
Stop a sorted set iteration.
## `RM_ZsetRangeEndReached`
int RM_ZsetRangeEndReached(RedisModuleKey *key);
Return the "End of range" flag value to signal the end of the iteration.
## `RM_ZsetFirstInScoreRange`
int RM_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.
## `RM_ZsetLastInScoreRange`
int RM_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.
## `RM_ZsetFirstInLexRange`
int RM_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.
## `RM_ZsetLastInLexRange`
int RM_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.
## Deleting keys
## `RM_ZsetRangeCurrentElement`
Just use:
RedisModuleString *RM_ZsetRangeCurrentElement(RedisModuleKey *key, double *score);
RedisModule_DeleteKey(key);
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.
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).
## `RM_ZsetRangeNext`
## Managing key expires (TTLs)
int RM_ZsetRangeNext(RedisModuleKey *key);
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.
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.
One function is used in order to query the current expire of an open key:
## `RM_ZsetRangePrev`
mstime_t RedisModule_GetExpire(RedisModuleKey *key);
int RM_ZsetRangePrev(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`).
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.
In order to change the expire of a key the following function is used instead:
## `RM_HashSet`
int RedisModule_SetExpire(RedisModuleKey *key, mstime_t expire);
int RM_HashSet(RedisModuleKey *key, int flags, ...);
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).
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.
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.
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).
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.
Example to set the hash argv[1] to the value argv[2]:
## Obtaining the length of values
`RedisModule_HashSet(key`,`REDISMODULE_HASH_NONE`,argv[1],argv[2],NULL);
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).
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`:
size_t len = RedisModule_ValueLength(key);
`RedisModule_HashSet(key`,`REDISMODULE_HASH_NONE`,argv[1],
`REDISMODULE_HASH_DELETE`,NULL);
If the key does not exist, 0 is returned by the function:
The behavior of the command changes with the specified flags, that can be
set to `REDISMODULE_HASH_NONE` if no special behavior is needed.
## String type API
`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.
Setting a new string value, like the Redis `SET` command does, is performed
using:
Unless NX is specified, the command overwrites the old field value with
the new one.
int RedisModule_StringSet(RedisModuleKey *key, RedisModuleString *str);
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:
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.
`RedisModule_HashSet(key`,`REDISMODULE_HASH_CFIELDS`,"foo",
`REDISMODULE_HASH_DELETE`,NULL);
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.
Return value:
size_t len, j;
char *myptr = RedisModule_StringDMA(key,REDISMODULE_WRITE,&len);
for (j = 0; j < len; j++) myptr[j] = 'A';
The number of fields updated (that may be less than the number of fields
specified because of the XX or NX options).
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.
In the following case the return value is always zero:
DMA pointers are only valid if no other operations are performed with the key
before using the pointer, after the DMA call.
- The key was not open for writing.
- The key was associated with a non Hash value.
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:
## `RM_HashGet`
RedisModule_StringTruncate(mykey,1024);
int RM_HashGet(RedisModuleKey *key, int flags, ...);
The function truncates, or enlarges the string as needed, padding it with
zero bytes if the previos 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.
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.
Note that every time `StringTruncate()` is called, we need to re-obtain
the DMA pointer again, since the old may be invalid.
This is an example usage:
## List type API
RedisModuleString *first, *second;
`RedisModule_HashGet(mykey`,`REDISMODULE_HASH_NONE`,argv[1],&first,
argv[2],&second,NULL);
It's possible to push and pop values from list values:
As with `RedisModule_HashSet()` the behavior of the command can be specified
passing flags different than `REDISMODULE_HASH_NONE`:
int RedisModule_ListPush(RedisModuleKey *key, int where, RedisModuleString *ele);
RedisModuleString *RedisModule_ListPop(RedisModuleKey *key, int where);
`REDISMODULE_HASH_CFIELD`: field names as null terminated C strings.
In both the APIs the `where` argument specifies if to push or pop from tail
or head, using the following macros:
`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.
REDISMODULE_LIST_HEAD
REDISMODULE_LIST_TAIL
Example of `REDISMODULE_HASH_CFIELD`:
Elements returned by `RedisModule_ListPop()` are like strings craeted with
`RedisModule_CreateString()`, they must be released with
`RedisModule_FreeString()` or by enabling automatic memory management.
RedisModuleString *username, *hashedpass;
`RedisModule_HashGet(mykey`,"username",&username,"hp",&hashedpass, NULL);
## Set type API
Example of `REDISMODULE_HASH_EXISTS`:
Work in progress.
int exists;
`RedisModule_HashGet(mykey`,argv[1],&exists,NULL);
## Sorted set type API
The function returns `REDISMODULE_OK` on success and `REDISMODULE_ERR` if
the key is not an hash value.
Documentation missing, please refer to the top comments inside `module.c`
for the following functions:
Memory management:
* `RedisModule_ZsetAdd`
* `RedisModule_ZsetIncrby`
* `RedisModule_ZsetScore`
* `RedisModule_ZsetRem`
The returned RedisModuleString objects should be released with
`RedisModule_FreeString()`, or by enabling automatic memory management.
And for the sorted set iterator:
## `RM_FreeCallReply_Rec`
* `RedisModule_ZsetRangeStop`
* `RedisModule_ZsetFirstInScoreRange`
* `RedisModule_ZsetLastInScoreRange`
* `RedisModule_ZsetFirstInLexRange`
* `RedisModule_ZsetLastInLexRange`
* `RedisModule_ZsetRangeCurrentElement`
* `RedisModule_ZsetRangeNext`
* `RedisModule_ZsetRangePrev`
* `RedisModule_ZsetRangeEndReached`
void RM_FreeCallReply_Rec(RedisModuleCallReply *reply, int freenested);
## Hash type API
Free a Call reply and all the nested replies it contains if it's an
array.
Documentation missing, please refer to the top comments inside `module.c`
for the following functions:
## `RM_FreeCallReply`
* `RedisModule_HashSet`
* `RedisModule_HashGet`
void RM_FreeCallReply(RedisModuleCallReply *reply);
## Iterating aggregated values
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.
Work in progress.
## `RM_CallReplyType`
# Replicating commands
int RM_CallReplyType(RedisModuleCallReply *reply);
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.
Return the reply type.
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:
## `RM_CallReplyLength`
reply = RedisModule_Call(ctx,"INCR","!sc",argv[1],"10");
size_t RM_CallReplyLength(RedisModuleCallReply *reply);
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".
Return the reply type length, where applicable.
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:
## `RM_CallReplyArrayElement`
RedisModule_ReplicateVerbatim(ctx);
RedisModuleCallReply *RM_CallReplyArrayElement(RedisModuleCallReply *reply, size_t idx);
When you use the above API, you should not use any other replication function
since they are not guaranteed to mix well.
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.
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 / slaves stream. Example:
## `RM_CallReplyInteger`
RedisModule_Replicate(ctx,"INCRBY","cl","foo",my_increment);
long long RM_CallReplyInteger(RedisModuleCallReply *reply);
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.
Return the long long of an integer reply.
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.
## `RM_CallReplyStringPtr`
# Automatic memory management
const char *RM_CallReplyStringPtr(RedisModuleCallReply *reply, size_t *len);
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.
Return the pointer and length of a string or error reply.
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).
## `RM_CreateStringFromCallReply`
When automatic memory management is enabled:
RedisModuleString *RM_CreateStringFromCallReply(RedisModuleCallReply *reply);
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.
Return a new string object from a call reply of type string, error or
integer. Otherwise (wrong reply type) return NULL.
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.
## `RM_Call`
In order to enable automatic memory management, just call the following
function at the start of the command implementation:
RedisModuleCallReply *RM_Call(RedisModuleCtx *ctx, const char *cmdname, const char *fmt, ...);
RedisModule_AutoMemory(ctx);
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:
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.
EINVAL: command non existing, wrong arity, wrong format specifier.
EPERM: operation in Cluster instance with key in non local slot.
# Writing commands compatible with Redis Cluster
## `RM_CallReplyProto`
Documentation missing, please check the following functions inside `module.c`:
const char *RM_CallReplyProto(RedisModuleCallReply *reply, size_t *len);
RedisModule_IsKeysPositionRequest(ctx);
RedisModule_KeyAtPos(ctx,pos);
Return a pointer, and a length, to the protocol returned by the command
that returned the reply object.
Redis Modules API reference manual
===
Redis modules make possible to extend Redis functionality using external
modules, implementing new Redis commands at a speed and 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) {
if (RedisModule_Init(ctx,"helloworld",1,REDISMODULE_APIVER_1)
== REDISMODULE_ERR) return REDISMODULE_ERR;
if (RedisModule_CreateCommand(ctx,"helloworld.rand",
HelloworldRand_RedisCommand) == 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 *cmdname,
RedisModuleCmdFunc cmdfunc);
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
name, and the function pointer of the function implementing the command,
which 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`.
# 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.
# 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,"INCR","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 slaves and AOF. It is ignored from the point of view of arguments parsing.
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,"INCR","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_FreeCallRelpy`. 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 elemented. 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 RedisMoudleString
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_ARRAY_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
se 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_ARRAY_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_ARRAY_LEN);
... generate 100 elements ...
RedisModule_ReplyWithArray(ctx, REDISMODULE_POSTPONED_ARRAY_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 exisitng 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_READ);
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,REDISMODULE_WRITE,&len);
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 previos 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 craeted 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,"INCR","!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 / slaves 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.
# Writing commands compatible with Redis Cluster
Documentation missing, please check the following functions inside `module.c`:
RedisModule_IsKeysPositionRequest(ctx);
RedisModule_KeyAtPos(ctx,pos);
......@@ -6,6 +6,11 @@ def markdown(s)
s = s.gsub(/\*\/$/,"")
s = s.gsub(/^ \* {0,1}/,"")
s = s.gsub(/^\/\* /,"")
if s[0] != ' '
s = s.gsub(/RM_[A-z()]+/){|x| "`#{x}`"}
s = s.gsub(/RedisModule_[A-z()]+/){|x| "`#{x}`"}
s = s.gsub(/REDISMODULE_[A-z]+/){|x| "`#{x}`"}
end
s.chop! while s[-1] == "\n" || s[-1] == " "
return s
end
......@@ -15,7 +20,7 @@ end
def docufy(src,i)
m = /RM_[A-z0-9]+/.match(src[i])
proto = src[i].sub("{","").strip+";\n"
puts "## #{m[0]}\n\n"
puts "## `#{m[0]}`\n\n"
puts " #{proto}\n"
comment = ""
while true
......
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