RedisX

A free, simple, and light-weight C/C++ Redis / Valkey client library.

API documentation
Project page on github.io

Author: Attila Kovacs

Last Updated: 13 December 2024

Introduction
Prerequisites
Building RedisX
Command-line interface (redisx-cli)
Linking your application against RedisX
Managing Redis server connections
Simple Redis queries
Accessing key / value data
Publish / subscribe (PUB/SUB) support
Atomic execution blocks and LUA scripts
Advanced queries and pipelining
Error handling
Debug support
Future plans

RedisX is a free, light-weight Redis client library for C/C++. As such, it should work with Redis forks / clones like Dragonfly or Valkey also. It supports both interactive and pipelined Redis queries, managing and processing subscriptions, atomic execution blocks, and LUA scripts loading. It can be used with multiple Redis servers simultaneously also. RedisX is free to use, in any way you like, without licensing restrictions.

While there are other C/C++ Redis clients available, this one is C99 compatible, and hence can be used on older platforms also. It is also small and fast, but still capable and versatile.

Rather than providing high-level support for every possible Redis command (which would probably be impossible given the pace new commands are being introduced all the time), it provides a basic framework for synchronous and asynchronous queries, with some higher-level functions, such as for managing key/value storage types (including hash tables) and PUB/SUB. Future releases may add further higher-level functionality based on demand for such features.

The RedisX library was created, and is maintained, by Attila Kovács at the Center for Astrophysics | Harvard & Smithsonian, and it is available through the Smithsonian/redisx repository on GitHub.

There are no official releases of RedisX yet. An initial 1.0.0 release is expected in late 2024 or early 2025. Before then the API may undergo slight changes and tweaks. Use the repository as is at your own risk for now.

A simple example

Below is a minimal example of a program snippet, which connects to a Redis server (without authentication) and runs a simple PING comand with a message, printing out the result on the console, while also following best practices of checking for errors, and handling them -- in this case by printing informative error messages:

  #include <redisx.h>

  // Initialize for Redis server at 'my-server.com'.
  Redis *redis = redisxInit("my-server.com");
  
  // Connect to redis
  if(redis != NULL && redisxConnect(redis, FALSE) == X_SUCCESS) {
    // Execute 'PING "Hello World!"' on the server 
    RESP *reply = redisxRequest(redis, "PING", "Hello World!", NULL, NULL, NULL);
    
    // Check that we got a response of the expected type (bulk string of any length)
    if(redisxCheckRESP(reply, RESP_BULK_STRING, 0) == X_SUCCESS)
      printf("%s\n", (char *) reply->value);
    else
      fprintf(stderr, "ERROR! bad response\n"); 
    
    // Clean up
    redisxDestroyRESP(reply);
    redisxDisconnect(redis);
  }
  else {
    perror("ERROR! could not connect");
  }
  
  // Free up the resources used by the 'redis' instance.
  redisxDestroy(redis);

You may find more details about each step further below. And, of course there are a lot more options and features, that allow you to customize your interactions with a Redis / Valkey server. But the basic principle, and the steps, follow the same pattern in general:

Initialize a Redis instance.
Configure the server properties: port, authentication, protocol, socket parameters etc. (not shown in above example).
Connect to the Redis / Valkey server.
Interact with the server: run queries interactively or in batch mode, process push notifications, publish or subscribe...
Disconnect from the server.

And at every step, you should check for and handle errors as appropriate.

Features overview

General Features

Feature	supported	comments
concurrent Redis instances	yes	You can manage and use multiple Redis servers simultaneously
connect over TCP	yes
connect over UDP	no
connect / disconnect hooks	yes	user-defined callbacks
socket customization	yes	(optional) user-defined timeout, buffer size and/or callback
custom socket error handling	yes	(optional) user-defined callback
RESP to JSON	yes	via `xchange` library
RESP to structured data	yes	via `xchange` library
debug error tracing	yes	via `xSetDebug()`
command-line client	yes	`redisx-cli`

Redis / Valkey Features

Redis / Valkey Feature	supported	comments
user authentication	yes	via `HELLO` if protocol is set, otherwise via `AUTH`
RESP3 / `HELLO` support	yes	(optional) if specific protocol is set
thread safe (MT-safe)	yes	synchronized + async calls with locking
push messages	yes	(optional) user-defined callback
attributes	yes	(optional) on demand
interactive queries	yes	dedicated (low-latency) client
pipelined (batch) processing	yes	dedicated (high-bandwidth) client / user-defined callback
PUB/SUB support	yes	dedicated client / user callbacks / subscription management
Sentinel support	yes	help me test it
cluster support	no	coming soon...
TLS support	no	coming soon...
resubscribe on reconnect	no

Prerequisites

The Smithsonian/xchange library is both a build and a runtime dependency of RedisX.

Additionally redisx-cli has the following dependencies on standard GNU/POSIX libraries:

POPT (popt-devel on RPM-based, or libpopt-dev on Debian based Linux).
BSD (libbsd-devel on RPM-based, or libbsd-dev on Debian based Linux).
readline (readline-devel on RPM based, or libreadline-dev on Debian based Linux).

Building RedisX

The RedisX library can be built either as a shared (libredisx.so[.1]) or as a static (libredisx.a) library, depending on what suits your needs best.

You can configure the build, either by editing config.mk or else by defining the relevant environment variables prior to invoking make. The following build variables can be configured:

CC: The C compiler to use (default: gcc).
CPPFLAGS: C preprocessor flags, such as externally defined compiler constants.
CFLAGS: Flags to pass onto the C compiler (default: -g -Os -Wall). Note, -Iinclude will be added automatically.
CSTANDARD: Optionally, specify the C standard to compile for, e.g. c99 to compile for the C99 standard. If defined then -std=$(CSTANDARD) is added to CFLAGS automatically.
WEXTRA: If set to 1, -Wextra is added to CFLAGS automatically.
LDFLAGS: Extra linker flags (default is not set). Note, -lm -lxchange will be added automatically.
CHECKEXTRA: Extra options to pass to cppcheck for the make check target
XCHANGE: If the Smithsonian/xchange library is not installed on your system (e.g. under /usr) set XCHANGE to where the distribution can be found. The build will expect to find xchange.h under $(XCHANGE)/include and libxchange.so / libxchange.a under $(XCHANGE)/lib or else in the default LD_LIBRARY_PATH.
STATICLINK: Set to 1 to prefer linking tools statically against libredisx.a. (It may still link dynamically if libredisx.so is also available.

After configuring, you can simply run make, which will build the shared (lib/libredisx.so[.1]) and static (lib/libredisx.a) libraries, local HTML documentation (provided doxygen is available), and performs static analysis via the check target. Or, you may build just the components you are interested in, by specifying the desired make target(s). (You can use make help to get a summary of the available make targets).

After building the library you can install the above components to the desired locations on your system. For a system-wide install you may simply run:

  $ sudo make install

Or, to install in some other locations, you may set a prefix and/or DESTDIR. For example, to install under /opt instead, you can:

  $ sudo make prefix="/opt" install

Or, to stage the installation (to /usr) under a 'build root':

  $ make DESTDIR="/tmp/stage" install

Command-line interface (`redisx-cli`)

The RedisX library provides its own command-line tool, called redisx-cli. It works very similar to redis-cli, except that our client has somewhat fewer bells and whistles.

 $ redisx-cli ping "Hello World"

will print:

 "Hello world!"

provided it successfully connected to the Redis / Valkey server on localhost. (Otherwise it will print an error and a trace). It can also be used in interactive mode if no Redis command arguments are supplied. And, you can run redisx-cli --help to see what options are available, and you can also consult the redis-cli documentation for the same general description and usage (so far as our implementation supports it).

Linking your application against RedisX

Provided you have installed the shared (libredisx.so and libxchange.so) or static (libredisx.a and libxchange.a) libraries in a location that is in your LD_LIBRARY_PATH (e.g. in /usr/lib or /usr/local/lib) you can simply link your program using the -lredisx -lxchange flags. Your Makefile may look like:

myprog: ...
	$(CC) -o $@ $^ $(LDFLAGS) -lredisx -lxchange

(Or, you might simply add -lredisx -lxchange to LDFLAGS and use a more standard recipe.) And, in if you installed the RedisX and/or xchange libraries elsewhere, you can simply add their location(s) to LD_LIBRARY_PATH prior to linking.

Managing Redis server connections

Initializing
Configuring
Connecting
Disconnecting

The library maintains up to three separate connections (channels) for each separate Redis server instance used: (1) an interactive client for sequential round-trip transactions, (2) a pipeline client for bulk queries and asynchronous background processing, and (3) a subscription client for PUB/SUB requests and notifications. The interactive client is always connected, the pipeline client is connected only if explicitly requested at the time of establishing the server connection, while the subscription client is connected only as needed.

Initializing

The first step is to create a Redis object, with the server name or IP address.

  // Configure the redis server to connect to "redis.mydomain.com".
  Redis *redis = redisxInit("redis.mydomain.com");
  if (redis == NULL) {
    // Abort: something did not got to plan...
    return;
  }
  
  // (optional) configure a non-standard port number to use
  redisxSetPort(redis, 7089);

Sentinel

Alternatively, instead of redisxInit() above you may initialize the client for a high-availability configuration using with a set of Redis Sentinel servers, using redisxInitSentinel(), e.g.:

  // An array defining N sentinel servers and ports to use.
  // A port number 0 or negative will use the default Redis port of 6379.
  RedisServer sentinels[N] = { { "server1", 0 }, { "server2", 7024 } ... };
  
  // Configure a Redis client instance for the Sentinel servers and "my-service" service name
  Redis *redis = redisxInitSentinel(sentinels, N, "my-service");
  if (redis == NULL) {
    // Abort: something did not got to plan...
    return;
  }

  // (optional) set a sentinel discovery timeout in ms...
  redisxSetSentinelTimeout(redis, 30);

After successful initialization, you may proceed with the configuration the same way as for the regular standalone server connection.

One thing to keep in mind about Sentinel is that once the connection to the master is established, it works just like a regular server connection, including the possibility of that connection being broken. It is up to the application to initiate reconnection and recovery as appropriate in case of errors. (See more in on Reconnecting further below).

Configuring

Before connecting to the Redis server, you may configure the database authentication (if any):

  Redis *redis = ...
  
  // (optional) Configure the database user (since Redis 6.0, using ACL)
  redisxSetUser(redis, "johndoe"); 
  
  // (optional) Configure the database password...
  redisxSetPassword(redis, mySecretPasswordString);

You can also set the RESP protocol to use (provided your server is compatible with Redis 6 or later):

  // (optional) Use RESP3 (provided the server supports it)
  redisxSetProtocol(redis, REDISX_RESP3);

The above call will use the HELLO command (since Redis 6) upon connecting. If you do not set the protocol, HELLO will not be used, and RESP2 will be assumed -- which is best for older servers (Redis <6). (Note, that you can always check the actual protocol used after connecting, using redisxGetProtocol()). Note, that after connecting, you may retrieve the set of server properties sent in response to HELLO using redisxGetHelloData().

Optionally, you can select the database index to use now (or later, after connecting), if not the default (index 0):

  // (optional) Select the database index 2
  redisxSelectDB(redis, 2);

Note, that you can switch the database index any time, with the caveat that it's not possible to change it for the subscription client when there are active subscriptions.

Socket-level configuration

You might also tweak the socket options used for clients, if you find the socket defaults sub-optimal for your application:

   // (optional) Set 1000 ms socket read/write timeout for future connections.
   redisxSetSocketTimeout(redis, 1000);

   // (optional) Set the TCP send/rcv buffer sizes to use if not default values.
   redisxSetTcpBuf(redis, 65536);

If you want, you can perform further customization of the client sockets via a user-defined callback function, e.g.:

  int my_socket_config(int sock, enum redisx_channel channel) {
     // Set up the socket any way you like...
     ...
     
     return X_SUCCESS;
  }

which you can then apply to your Redis instance as:

  redisxSetSocketConfigurator(my_socket_config);

Connection & disconnection hooks

The user of the RedisX library might want to know when connections to the server are established, or when clients get disconnected (either as intended or as a result of errors), and may want to perform some configuration or clean-up accordingly. For this reason, the library provides support for connection 'hooks' -- that is custom functions that are called in the even of connecting to or disconnecting from a Redis server.

Here is an example of a connection hook, which simply prints a message about the connection to the console.

  void my_connect_hook(Redis *redis) {
     printf("Connected to Redis server: %s\n", redis->id);
  }

And, it can be added to a Redis instance, between the redisxInit() and the redisxConnect() calls.

  Redis *redis = ...
  
  redisxAddConnectHook(redis, my_connect_hook);

You may add multiple callbacks. All of them will be called (in the same order as added) when connection is established. You may also remove specific connection callbacks via redisxRemoveConnectHook() if you now longer want a particular function to be called any more in the even.

The same goes for disconnect hooks, using redisxAddDisconnectHook() / redisxRemoveDisconnectHook() instead.

Connecting

Once configured, you can connect to the server as:

   Redis *redis = ...

   // Connect to Redis, including a 2nd dedicated client for pipelined requests
   int status = redisxConnect(redis, TRUE);
   if (status != X_SUCCESS) {
      // Abort: we could not connect for some reason...
      ...
      // Clean up...
      redisxDestroy(redis);
      ...
   }

The above will establish both an interactive connection and a pipelined connection client, for processing both synchronous and asynchronous requests (and responses). For Sentinel configurations, it will return with X_SUCCESS only after having located and connected to the master server, and confirmed that it is indeed the master.

Disconnecting

When you are done with a specific Redis server, you should disconnect from it:

  Redis *redis = ...
  
  redisxDisconnect(redis);

And then to free up all resources used by the Redis instance, you might also call

  // Destroy the Redis instance and free up resources
  redisxDestroy(redis);
  
  // Set the destroyed Redis instance pointer to NULL, as best practice.
  redis = NULL;

Reconnecting

Reconnections to the Redis servers are never automatic, and there is no automatic failover for RedisX clients (there are good reasons for that). It is up to you to decide when to reconnect and how exactly. For example, the application may reconnect to the same or different server (including Sentinel), and perform a set of necessary recovery steps, to continue where things were left off on the previous connection, such as:

reload LUA scripts
reinstate subscriptions
re-submit any request for which no replies have been received before the connection was broken.
re-publish any notifications on PUB/SUB, which may not have been delivered.

Simple Redis queries

Interactive transactions
Bundled Attributes
Push notifications
RESP data type

Redis queries are sent as strings, according the the specification of the Redis protocol. All responses sent back by the server using the RESP protocol. Specifically, Redis uses version 2 of the RESP protocol (a.k.a. RESP2) by default, with optional support for the newer RESP3 introduced in Redis version 6.0. The RedisX library provides support for both RESP2 and RESP3.

Interactive transactions

The simplest way for running a few Redis queries is to do it in interactive mode:

  Redis *redis = ...
  RESP *resp;     // This will be the pointer we receive to the Redis response
  int status;     // execution status to be populated.

  // Send "HGET my_table my_key" request
  resp = redisxRequest(redis, "HGET", "my_table", "my_key", NULL, &status);
  
  // Check return status...
  if (status != X_SUCCESS) {
    // Oops something went wrong...
    ...
  }
  ...

The redisxRequest() sends a command with up to three arguments. If the command takes fewer than 3 parameters, then the remaining ones must be set to NULL. This function thus offers a simple interface for running most basic sequential queries. In cases where 3 parameters are not sufficient, you may use redisxArrayRequest() instead, e.g.:

  ...
  char *cmd[] = { "HGET", "my_table", "my_key" };  // command and parameters as an array...

  // Send "HGET my_table my_key" request with an array of 3 components...
  resp = redisxArrayRequest(redis, cmd, NULL, 3, &status);
  ...

The 3rd argument in the list is an optional int[] array defining the individual string lengths of the parameters (if need be, or else readily available). Here, we used NULL instead, which will use strlen() on each supplied string-terminated parameter to determine its length automatically. Specifying the length may be necessary if the individual parameters are not 0-terminated strings.

In interactive mode, each request is sent to the Redis server, and the response is collected before the call returns with that response (or NULL if there was an error).

Bundled Attributes

Redis 6 introduced the possibility of sending optional attributes along with responses, using the RESP3 protocol. These attributes are not included in the responses sent to users, in accordance with RESP3 protocol. Rather, they are made available to users on demand, when needed, after the response to a request is received. You may retrieve the attributes to interactive requests after the redisxRequest() or redisxArrayRequest() queries, using redisxGetAttributes(), e.g.:

  ...
  
  // Some interactive request
  RESP *reply = redisxRequest(redis, ...);
  
  // Get the attributes (if any) that were bundled with the response...
  RESP *attributes = redisxGetAttributes(redis);
  
  ...

Push notifications

Redis 6 introduced out-of-band push notifications along with RESP3. It allows the server to send messages to any connected client that are not in response to a query. For example, Redis 6 allows CLIENT TRACKING to use such push notifications (e.g. INVALIDATE foo), to notify connected clients when a watched variable has been updated from somewhere else.

RedisX allows you to specify a custom callback RedisPushProcessor function to handle such push notifications, e.g.:

  void my_push_processor(RESP *message, void *ptr) {
    char *owner = (char *) ptr;  // Additional argument we need, in this case a string.
    printf("[%s] Got push message: type %c, n = %d.\n", owner, message->type, message->n);
  }

Then you can activate the processing of push notifications with redisxSetPushProcessor(). You can specify the optional additional data that you want to pass along to the push processor function -- just make sure that the data has a sufficient scope / lifetime such that it is valid at all times while push messages are being processed. E.g.

  static owner = "my process"; // The long life data we want to pass to my_push_processor...
  
  // Use my_push_processor and pass along the owner as a parameter
  redisxSetPushProcessor(redis, my_push_processor, owner);

There are some things to look out for in your RedisPushProcessor implementation:

The call should not block (except perhaps for a quick mutex lock) and should return quickly. If blocking calls, or extensive processing is required, you should place a copy of the PUSH notification onto a queue and let an asynchronous thread take it from there.
The call should not attempt to alter or destroy the push message. If needed it can copy parts or the whole.
You should not attempt to lock or release clients from the call. If you need access to a client (e.g. to submit a new Redis request), it's best to put a copy of the RESP notification onto a queue and let an asynchronous thread deal with it.

RESP data type

All responses coming from the Redis server are represented by a dynamically allocated RESP type (defined in redisx.h) structure.

typedef struct RESP {
  char type;                    // RESP type: RESP_ARRAY, RESP_INT ...
  int n;                        // Either the integer value of a RESP_INT response, or the 
                                // dimension of the value field.
  void *value;                  // Pointer to text (char *) content or to an array of components 
                                // (RESP **)
} RESP;

whose contents are:

RESP `type`	Redis ID	`n`	`value` cast in C
`RESP_ARRAY`	`*`	number of `RESP *` pointers	`(RESP **)`
`RESP_INT`	`:`	integer return value	`(void)`
`RESP_SIMPLE_STRING`	`+`	string length	`(char *)`
`RESP_ERROR`	`-`	total string length	`(char *)`
`RESP_BULK_STRING`	`$`	string length or -1 if `NULL`	`(char *)`
`RESP3_NULL`	`_`	0	`(void)`
`RESP3_BOOLEAN`	`#`	1 if true, 0 if false	`(void)`
`RESP3_DOUBLE`	`,`	unused	`(double *)`
`RESP3_BIG_NUMBER`	`(`	string representation length	`(char *)`
`RESP3_BLOB_ERROR`	`!`	total string length	`(char *)`
`RESP3_VERBATIM_TEXT`	`=`	text length (incl. type)	`(char *)`
`RESP3_SET`	`~`	number of `RESP *` pointers	`(RESP *)`
`RESP3_MAP`	`%`	number of key / value pairs	`(RedisMapEntry *)`
`RESP3_ATTRIBUTE`	`\|`	number of key / value pairs	`(RedisMapEntry *)`
`RESP3_PUSH`	`>`	number of `RESP *` pointers	`(RESP **)`

Each RESP has a type (e.g. RESP_SIMPLE_STRING), an integer value n, and a value pointer to further data. If the type is RESP_INT, then n represents the actual return value (and the value pointer is not used). For string type values n is the number of characters in the string value (not including termination), while for RESP_ARRAY types the value is a pointer to an embedded RESP array and n is the number of elements in that.

To help with deciding what cast to use for a given value field of the RESP data structure, we provide the convenience methods redisxIsScalarType(), redisxIsStringType(), redisxIsArrayType(), and redisxIsMapType() functions.

You can check that two RESP data structures are equivalent with redisxIsEqualRESP(RESP *a, RESP *b).

You may also check the integrity of a RESP using redisxCheckRESP(). Since RESP data is dynamically allocated, the user is responsible for discarding them once they are no longer needed, e.g. by calling redisxDestroyRESP(). The two steps may be combined to automatically discard invalid or unexpected RESP data in a single step by calling redisxCheckDestroyRESP().

  RESP *r = ...
  
  // Let's say we expect 'r' to contain of an embedded RESP array of 3 elements... 
  int status = redisxCheckDestroyRESP(r, RESP_ARRAY, 3);
  if (status != X_SUCCESS) {
     // Oops, 'r' was either NULL, or does not contain a RESP array with 3 elements...
     ...
  }
  else {
     // Process the expected response...
     ...
     redisxDestroyRESP(r);
  }

Before destroying a RESP structure, the caller may want to dereference values within it if they are to be used as is (without making copies), e.g.:

  RESP *r = ...
  char *stringValue = NULL;   // to be extracted from 'r'
  
  // Let's say we expect 'r' to contain of a simple string response (of whatever length)
  int status = redisxCheckDestroyRESP(r, RESP_SIMPLE_STRING, 0);
  if (status != X_SUCCESS) {
    // Oops, 'r' was either NULL, or it was not a simple string type
    ...
  }
  else {
    // Set 'stringValue' and dereference the value field in the RESP so it's not 
    // destroyed with the RESP itself.
    stringValue = (char *) r->value;
    r->value = NULL;
     
    redisxDestroyRESP(r);     // 'stringValue' is still a valid pointer after! 
  }

Note, that you can convert a RESP to an XField, and/or to JSON representation using the redisxRESP2XField() and redisxRESP2JSON() functions, e.g.:

 Redis redis = ...
 
 // Obtain a copy of the response received from HELLO upon connecting...
 RESP *resp = redisxGetHelloData(redis);
 
 // Print the response from HELLO to the standard output in JSON format
 char *json = redisxRESP2JSON("hello_response", resp);
 if(json != NULL) {
   printf("%s", json);
   free(json);
 }
   
 ...
 
 // Clean up
 redisxDestroyRESP(resp);

All RESP can be represented in JSON format. This is trivial for map entries, which have strings as their keywords -- which is the case for all RESP sent by Redis. And, it is also possible for map entries with non-string keys, albeit via a more tedious (and less standard) JSON representation, stored under the .non-string-keys keyword.

Accessing key / value data

Getting and setting keyed values
Listing and scanning

Getting and setting keyed values

Key/value pairs are the bread and butter of Redis. They come in two varieties: (1) there are top-level key-value pairs, and (2) there are key-value pairs organized into hash tables, where the table name is a top-level key, but the fields in the table are not. The RedisX library offers a unified approach for dealing with key/value pairs, whether they are top level or hash-tables. Simply, a table name NULL is used to refer to top-level keys.

Retrieving individual keyed values is simple:

  Redis *redis = ...;
  int len; // Variable in which we return the length of the value or an error code 
  
  // Get the "property" field from the "system:subsystem" hash table
  char *value = redisxGetStringValue(redis, "system:subsystem", "property", &len);
  if (len < 0) {
    // Oops something went wrong.
    ...
  }
  
  ...
  
  // Discard the value once it's no longer needed.
  if(value) free(value);

The same goes for top-level keyed values, using NULL for the hash table name:

  // Get value for top-level key (not stored in hash table!)
  char *value = redisxGetStringValue(redis, NULL, "my-key", &len);

Alternatively, you can get the value as an undigested RESP, using redisxGetValue() instead, which allows you to check and inspect the response in more detail (e.g. to check for potential errors, or unexpected response types).

Setting values is straightforward also:

  Redis *redis = ...;
  
  // Set the "property" field in the "system:subsystem" hash table to -2.5
  // using the interactive client connection, without requiring confirmation. 
  int status = redisxSetValue(redis, "system:subsystem", "property", "-2.5", FALSE);
  if (status != X_SUCCESS) {
    // Oops something went wrong.
    ...
  }

It's worth noting here, that values in Redis are always represented as strings, hence non-string data, such as floating-point values, must be converted to strings first. Additionally, the redisxSetValue() function works with 0-terminated string values only, but Redis allows storing unterminated byte sequences of known length also. If you find that you need to store an unterminated string (such as a binary sequence) as a value, you may just use the lower-level redisxArrayRequest() instead to process a Redis SET or HSET command with explicit byte-length specifications.

In the above example we have set the value using the interactive client to Redis, which means that the call will return only after confirmation or result is received back from the server. As such, a subsequent redisxGetValue() of the same table/key will be guaranteed to return the updated value always.

However, we could have set the new value asynchronously over the pipeline connection (by using TRUE as the last argument). In that case, the call will return as soon as the request was sent to Redis (but not confirmed, nor possibly transmitted yet!). As such, a subsequent redisxGetValue() on the same key/value field may race the request in transit, and may return the previous value on occasion. So, it's important to remember that while pipelining can make setting multiple Redis fields very efficient, we have to be careful about retrieving the same values afterwards from the same program thread. (Arguably, however, there should never be a need to query values we set ourselves, since we readily know what they are.)

Finally, if you want to set values for multiple fields in a Redis hash table atomically, you may use redisxMultiSet(), which provides a high-level interface to the Redis HMSET command.

Listing and Scanning

The functions redisxGetKeys() and redisxGetTable() allow to return the set of Redis keywords or all key/value pairs in a table atomically. However, these commands can be computationally expensive for large tables and/or many top-level keywords, which means that the Redis server may block for undesirably long times while the result is computed.

This is where scanning offers a less selfish (hence much preferred) alternative. Rather than returning all the keys or key/value pairs contained in a table atomically at once, it allows to do it bit by bit with byte-sized individual transactions that are guaranteed to not block the Redis server long, so it may remain responsive to other queries also. For the caller the result is the same (notwithstanding the atomicity), except that the result is computed via a series of quick Redis queries rather than with one potentially very expensive query.

For example, to retrieve all top-level Redis keys, sorted alphabetically, using the scanning approach, you may write something like:

  Redis *redis = ...
  
  int nMatches;  // We'll return the number of matching Redis keys here...
  int status;    // We'll return the error status here...
  
  //  Return all Redis top-level keywords starting with "system:"
  char **keys = redisxScanKeys(redis, "system:*", &nMatches, &status);
  if (status != X_SUCCESS) {
    // Oops something went wrong...
    ...
  }
  
  // Use 'keys' as appropriate, possibly dereferencing values we want to
  // retain in other persistent data structures...
  ...
  
  // Once done using the 'keys' array, we should destroy it
  redisxDestroyKeys(keys, nMatches);

Or, to retrieve the values from a hash table for a set of keywords that match a glob pattern:

  ...
  
  // Scan all key/value pairs in hash table "system:subsystem"
  RedisEntry *entries = redisxScanTable(redis, "system:subsystem", "*", &nMatches, &status);
  if (status != X_SUCCESS) {
    // Oops something went wrong.
    ... 
  }
  
  // Use 'entries' as appropriate, possibly dereferencing values we want to
  // retain in other persistent data structures...
  ...
  
  // Once done using the 'keys' array, we should destroy it
  redisxDestroyEntries(entries, nMatches);

Finally, you may use redisxSetScanCount() to tune just how many results should individual scan queries should return (but only if you are really itching to tweak it). Please refer to the Redis documentation on the behavior of the SCAN and HSCAN commands to learn more.

Publish / subscribe (PUB/SUB) support

Broadcasting messages
Subscriptions

Broadcasting messages

It is simple to send messages to subscribers of a given channel:

  Redis *redis = ...

  // publish a message to the "hello_channel" subscribers.
  int status = redisxPublish(redis, "hello_channel", "Hello world!", 0);

The last argument is an optional string length, if readily available, or if sending a byte sequence that is not null-terminated. If zero is used for the length, as in the example above, it will automatically determine the length of the 0-terminated string message using strlen().

Alternatively, you may use the redisxPublishAsync() instead if you want to publish on a subscription client to which you have already have exclusive access (e.g. after an appropriate redisxLockConnected() call).

Subscriptions

Subscriptions work conceptually similarly to pipelined requests. To process incoming messages you need to first specify one or more RedisSubscriberCall functions, which will process PUB/SUB notifications automatically, in the background, as soon as they are received. Each RedisSubscriberCall can pre-filter the channels for which it receives notifications, by defining a channel stem. This way, the given processor function won't even be invoked if a notification on a completely different channel arrives. Still, each RedisSubscriberCall implementation should further check the notifying channel name as appropriate to ensure that it is in fact qualified to deal with a given message.

Here is an example RedisSubscriberCall implementation to process messages:

  void my_event_processor(const char *pattern, const char *channel, const char *msg, long len) {
    // We'll print the message onto the console
    printf("Incoming message on channel %s: %s\n", channel, msg == NULL ? "<null>" : msg);
  }

There are some basic rules (best practices) for message processing. They should be fast, and never block for extended periods. If extensive processing is required, or may need to wait extensively for some resource or mutex locking, then its best that the processing function simply places the incoming message onto a queue, and let a separate background thread do the heavy lifting without holding up the subscription processing of other callback routines, or without losing responsiveness to other incoming messages.

Also, it is important that the call should never attempt to modify or call free() on the supplied string arguments, since that would interfere with other subscriber calls.

Once the function is defined, you can activate it via:

  Redis *redis = ...
  
  int status = redisxAddSubscriber(redis, "event:", my_event_processor);
  if (status != X_SUCCESS) {
    // Oops, something went wrong...
    ...
  }

We should also start subscribing to specific channels and/or channel patterns.

  Redis *redis = ...
  
  // We subscribe to all channels that beging with "event:"...
  int status = redisxSubscribe(redis, "event:*");
  if (status != X_SUCCESS) {
    // Oops, something went wrong...
    ...
  }

The redisxSubscribe() function will translate to either a Redis PSUBSCRIBE or SUBSCRIBE command, depending on whether the pattern argument contains globbing patterns or not (respectively).

Now, we are capturing and processing all messages published to channels whose name begins with "event:", using our custom my_event_processor function.

To end the subscription, we trace back the same steps by calling redisxUnsubscribe() to stop receiving further messages to the subscription channel or pattern, and by removing the my_event_procesor subscriber function as appropriate (provided no other subscription needs it) via redisxRemoveSubscriber().

Atomic execution blocks and LUA scripts

Execution blocks
LUA script loading and execution
Custom Redis functions

Sometimes you may want to execute a series of Redis command atomically, such that nothing else may alter the database while the set of commands execute, so that related values are always in a coherent state. For example, you want to set or query a collection of related variables so they change together and are reported together. You have two choices. (1) you can execute the Redis commands in an execution block, or else (2) load a LUA script onto the Redis server and call it with some parameters (possibly many times over).

Execution blocks

Execution blocks offer a fairly simple way of bunching together a set of Redis commands that need to be executed atomically. Such an execution block in RedisX may look something like:

  Redis *redis = ...;
  RESP *result;
  int status;
  
  // Obtain a lock on the client on which to execute the block.
  // e.g. the interactive client channel.
  RedisClient *cl = redisxGetLockedConnectedClient(redis, REDISX_INTERACTIVE_CHANNEL);
  if (cl == NULL) {
    // Abort: we don't have exclusive access to the client
    return;
  }

  // -------------------------------------------------------------------------
  // Start an atomic execution block
  redisxStartBlockAsync(cl);
  
  // Send a number of Async requests
  status = redisxSendRequestAsync(cl, ...);
  if(status < 0) {
    // Oops, something went wrong, we should abort and return
    redisxAbortBlockAsync(cl);
    return status;
  }
  ...

  // Execute the block of commands above atomically, and get the resulting RESP
  result = redisxExecBlockAsync(cl, &status);
  // -------------------------------------------------------------------------
  
  // Release exlusive access to the client
  redisxUnlockClient(cl);
  
  if(status != X_SUCCESS) {
    // Oops, the block execition failed...
    return status;
  }
  
  // Inspect the RESP, etc...
  ...

If at any point things don't go according to plan in the middle of the block, you can call redisAbortBlockAsync() to abort and discard all prior commands submitted in the execution block already. It is important to remember that every time you call redisxStartBlockAsync(), you must call either redisxExecBlockAsync() to execute it or else redisxAbortBlockAsync() to discard it. Failure to do so, will effectively end you up with a hung Redis client.

LUA script loading and execution

LUA scripting offers a more capable version of executing complex routines on the Redis server. LUA is a scripting language akin to python, and allows you to add extra logic, string manipulation etc. to your Redis queries. Best of all, once you upload the script to the server, it can reduce network traffic significantly by not having to repeatedly submit the same set of Redis commands every single time. LUA scripts also get executed very efficiently on the server, and produce only the result you want/need without returning unnecessary intermediates.

Assuming you have prepared your LUA script appropriately, you can upload it to the Redis server as:

  Redis *redis = ...
  char *script = ...         // The LUA script as a 0-terminated string.
  char *scriptSHA1 = NULL;   // We'll store the SHA1 sum of the script here

  // Load the script onto the Redis server
  int status = redixLoadScript(redis, script, &scriptSHA1);
  if(status != X_SUCCESS) {
    // Oops, something went wrong...
    ...
  }

Redis will refer to the script by its SHA1 sum, so it's important keep a record of it. You'll call the script with its SHA1 sum, a set of Redis keys the script may use, and a set of other parameters it might need.

  Redis *redis = ...
  
  char *keyArgs[] = { "my-redis-key-argument", NULL }; // NULL-terminated array of keyword arguments
  char *params[] = { "some-string", "-1.11", NULL };   // NULL-terminated array of extra parameters
  
  int status;     // We will return error status in this variable
  
  // Execute the script, with the specified keyword arguments and parameters
  RESP *reply = redisxRunScript(redis, scriptSHA1, keyArgs, params, &status);

  if(status != X_SUCCESS) {
     // Oops, failed to run script...
     ...
  }

  // Check and inspect the reply
  ...

Or, you can use redisxRunScriptAsync() instead to send the request to run the script, and then collect the response later, as appropriate.

One thing to keep in mind about LUA scripts is that they are not fully persistent. They will be lost each time the Redis server is restarted.

Custom Redis functions

Functions, introduced in Redis 7, offer another evolutionary step over the LUA scripting described above. Unlike scripts, functions are persistent and they can be called by name rather than a cryptic SHA1 sum. Otherwise, they offer more or less the same functionality as scripts. RedisX does not currently have a built-in high-level support for managing and calling user-defined functions, but it is a feature that may be added in the not-too-distant future. Stay tuned.

Advanced queries and pipelining

Asynchronous client processing
Bundled Attributes
Pipelined transactions

Asynchronous client processing

Sometimes you might want to micro manage how requests are sent and responses to them are received. RedisX provides a set of asynchronous client functions that do that. (You've seen these already further above in the Pipelined transaction section.) These functions should be called with the specific client's mutex locked, to ensure that other threads do not interfere with your sequence of requests and responses. E.g.:

  // Obtain the appropriate client with an exclusive lock on it.
  RedisClient *cl = redisxGetLockedConnectedClient(...);
  if (cl == NULL) {
    // Abort: no such client is connected
    return;
  }
  
  // Now send commands, and receive responses as you like using the redisx...Async() calls
  ...
  
  // When done, release the lock
  redisxUnlockClient(cl);

While you have the exclusive lock you may send any number of requests, e.g. via redisxSendRequestAsync() and/or redixSendArrayRequestAsync(). Then collect replies either with redisxReadReplyAsync() or else redisxIgnoreReplyAsync(). For example, the basic anatomy of sending a single request and then receiving a response, while we have exclusive access to the client, might look something like this:

  ...
  // Send a command to Redis
  int status = redisxSendRequestAsync(cl, ...);
  
  if(status == X_SUCCESS) {
    // Read the response
    RESP *reply = redisxReadReplyAsync(cl, &status);
    if(status != X_SUCCESS) {
      // Ooops, not the reply what we expected...
      ...
    }
    
    // check and process the response
    if(redisxCheckRESP(reply, ...) != X_SUCCESS) {
      // Ooops, not the reply what we expected...
      ...
    } 
    else {
      // Process the response
      ...
    }
    
    // Destroy the reply
    redisxDestroyRESP(reply);
  }
  ...

For the best performance, you may want to leave the processing of the replies until after you unlock the client. I.e., you only block other threads from accessing the client while you send off the requests and collect the corresponding responses. You can then analyze the responses at your leisure outside of the mutexed section.

In some cases you may be OK with just firing off some Redis commands, without necessarily caring about responses. Rather than ignoring the replies with redisxIgnoreReplyAsync() you might call redisxSkipReplyAsync() instead before redisxSendRequestAsync() to instruct Redis to not even bother about sending a response to your request (it saves time and network bandwidth!):

 // We don't want to receive a response to our next command... 
 int status = redisxSkipReplyAsync(cl);
 
 if (status == X_SUCCESS) {
   // Now send the request...
   status = redisxSendRequest(cl, ...);
 }
 
 if (status != X_SUCCESS) {
    // Ooops, the request did not go through...
    ...
 }

Of course you can build up arbitrarily complex set of queries and deal with a set of responses in different ways. Do what works best for your application.

Bundled Attributes

As of Redis 6, the server might send ancillary data along with replies, if the RESP3 protocol is used. These are collected together with the expected responses. However, these optional attributes are not returned to the user automatically. Instead, the user may retrieve attributes directly after getting a response from redisxReadReplyAsync() using redisxGetAttributesAsync(). And, attributes that were received previously can be discarded with redisxClearAttributesAsync(). For example,

  RedisClient *cl = ...	 // The client we use for our transactions
  
  if(redisxLockEnabled(cl) == X_SUCCESS) {
    ...
    
    // Clear any prior attributes we may have previously received for the client...
    redisxClearAttributesAsync(cl);
    
    // Read a response for a request we sent earlier...
    RESP *reply = redisxReadReplyAsync(cl);
    
    // Retrieve the attributes (if any) that were sent with the response.
    RESP *attributes = redisxGetAttributesAsync(cl);
    
    ...
    
    redisxUnlockClient(cl);
  }

Pipelined transactions

Depending on round-trip times over the network, interactive queries may be suitable for running up to a few thousand queries per second. For higher throughput (up to ~1 million Redis transactions per second) you may need to access the Redis database in pipelined mode. RedisX provides a dedicated pipeline client/channel to the Redis server (provided the option to enable it has been used when redixConnect() was called).

In pipeline mode, requests are sent to the Redis server over the pipeline client in quick succession, without waiting for responses to return for each one individually. Responses are processed in the background by a designated callback function (or else discarded if no callback function has been set). This is what a callback function looks like:

  // Your own function to process responses to pipelined requests...
  void my_resp_processor(RESP *r) {
    // Do what you need to do with the asynchronous responses
    // that come from Redis to bulk requests.
    ...
  }

It is important to note that the processing function should not call free on the RESP pointer argument, but it may dereference and use parts of it as appropriate (just remember to set the bits referenced elsewhere to NULL so they do not get destroyed when the pipeline listener destroys the RESP after your function is done processing it). Before sending the pipelined requests, the user first needs to specify the function to process the responses, e.g.:

  Redis *redis = ...

  redisxSetPipelineConsumer(redis, my_resp_processor);

Request are sent via the redisxSendRequestAsync() and redisxSendArrayRequestAsync() functions. Note again, the Async naming, which indicates the asynchronous nature of this calls -- and which indicates that these functions should be called with the appropriate mutex locked to prevent concurrency issues, and to maintain a predictable order (very important!) for processing the responses.

  Redis *redis = ...

  // We'll use a dedicated pipeline connection for asynchronous requests
  // This way, interactive requests can still be sent independently on the interactive
  // channel independently, if need be.
  RedisClient *pipe = redisxGetLockedConnectedClient(redis, REDISX_PIPELINE_CHANNEL);

  // Check that the client is valid...
  if (pipe == NULL) {
     // Abort: we do not appear to have an active pipeline connection...
     return;
  }

  // -------------------------------------------------------------------------
  // Submit a whole bunch of asynchronous requests, e.g. from a loop...
  for (...) {
    int status = redisxSendRequestAsync(pipe, ...);
    if (status != X_SUCCESS) {
      // Oops, that did not go through...
      ...
    }
    else {
      // We probably want to keep a record of what was requested and in what order
      // so our processing function can make sense of the reponses as they arrive
      // (in the same order...)
      ...
    }
  }
  // -------------------------------------------------------------------------
  
  // Release the exclusive lock on the pipeline channel, so
  // other threads may use it now that we sent off our requests...
  redisxUnlockClient(pipe);

It is important to remember that on the pipeline client you should never try to process responses directly from the same function from which commands are being sent. That's what the interactive connection is for. Pipeline responses are always processed by a background thread (or, if you don't specify your callback function they will be discarded). The only thing your callback function can count on is that the same number of responses will be received as the number of asynchronous requests that were sent out, and that the responses arrive in the same order as the order in which the requests were sent.

It is up to you and your callback function to keep track of what responses are expected and in what order. Some best practices to help deal with pipeline responses are summarized here:

Use redisxSkipReplyAsync() prior to sending pipeline requests for which you do not need a response. (This way your callback does not have to deal with unnecessary responses at all.
For requests that return a value, keep a record (in a FIFO) of the expected types and your data that depends on the content of the responses. For example, for pipelined HGET commands, your FIFO should have a record that specifies that a bulk string response is expected, and a pointer to data which is used to store the returned value -- so that you pipeline response processing callback function can check that the response is the expected type (and size) and knows to assign/process the response appropriately to your application data.
You may insert Redis PING/ECHO commands to section your responses, or to provide directives to your pipeline response processor function. You can tag them uniquely so that the echoed responses can be parsed and interpreted by your callback function. For example, you may send a PING/ECHO commands to Redis with the tag "@my_resp_processor: START sequence A", or something else meaningful that you can uniquely distinguish from all other responses that you might receive.

RedisX optimizes the pipeline client for high throughput (bandwidth), whereas the interactive and subscription clients are optimized for low-latency, at the socket level.

Error handling

The principal error handling of RedisX is an extension of that of xchange, with further error codes defined in redisx.h. The RedisX functions that return an error status (either directly, or into the integer designated by a pointer argument), can be inspected by redisxErrorDescription(), e.g.:

  Redis *redis ...
  int status = redisxSetValue(...);
  if (status != X_SUCCESS) {
    // Ooops, something went wrong...
    fprintf(stderr, "WARNING! set value: %s", redisErrorDescription(status));
    ...
  }

In addition you can define your own handler function to deal with transmission (send/receive) errors, by defining your own RedisErrorHandler function, such as:

  void my_error_handler(Redis *redis, enum redisx_channel channel, const char *op) {
    fprintf(stderr, "ERROR! %s: Redis at %s, channel %d\n", op, redis->id, channel);
  }

Then activate it as:

  Redis *redis = ...
  
  redisxSetSocketErrorHandler(redis, my_error_handler);

After that, every time there is an error with sending or receiving packets over the network to any of the Redis clients used, your handler will report it the way you want it.

Socket-level errors

There are multiple ways the user may get informed when errors happen at the socket read / write level:

RedisX will propagate and report errors from failed socket reads and writes, both via return values to its functions (via X_NO_SERVICE error code or NULL returns, plus errno), and
RedisX will call the configured socket error handler callback function (see above) as soon as the error is encountered.
RedisX will call all disconnect hooks configured if the client disconnects (either as intended or unexpectedly as a result of a persistent socket-level error).

It is up to you to decide which of the following method(s) you wish to rely on to detect broken connections and act as appropriate. To help with your decisions, below is a step-by-step outline of how RedisX handles errors originating from the socket level of clients, indicating also the points at which users are notified by each of the above mentioned methods.

The socket error handler (if configured) callback function is called is immediately after a read or write error is detected on a client's socket. The error handler callback function is called while the affected client is still locked and nominally in a 'functioning' state. That means you are free to use any Async call on the affected client as appropriate, but the error handler should not attempt to release the exclusive lock on the client or call synchronized functions on the Redis instance or the affected client. The background processing of replies (on the pipeline and/or subscription clients) is still active at this stage.
If the error is caused by a timeout (errno being EAGAIN or EWOULDBLOCK), nothing changes at this stage. However, if the error is persistent, the client will be disabled and reset, and subsequent read or write calls will fail immediately. Any disconnection hooks will be called also as the client is disconnected. The background processing of server replies (on the pipeline and subscriptions channels) will stop soon after the disconnection is initiated.
The RedisX call returns either X_NO_SERVICE, or X_TIMEDOUT, or else NULL. The application should check return values (and/or errno) as appropriate.

Debug support

You can enable verbose output of the RedisX library with xSetVerbose(boolean). When enabled, it will produce status messages to stderrso you can follow what's going on. In addition (or alternatively), you can enable debug messages with xSetDebug(boolean). When enabled, all errors encountered by the library (such as invalid arguments passed) will be printed to stderr, including call traces, so you can walk back to see where the error may have originated from. (You can also enable debug messages by default by defining the DEBUG constant for the compiler, e.g. by adding -DDEBUG to CFLAGS prior to calling make).

In addition, you can use redisxDebugTraffic(boolean) to debug low-level traffic to/from the Redis server, which prints excerpts of all incoming and outgoing messages from/to Redis to stderr.

For helping to debug your application, the xchange library provides two macros: xvprintf() and xdprintf(), for printing verbose and debug messages to stderr. Both work just like printf(), but they are conditional on verbosity being enabled via xSetVerbose(boolean) and xSetDebug(boolean), respectively. Applications using RedisX may use these macros to produce their own verbose and/or debugging outputs conditional on the same global settings.

Future plans

Some obvious ways the library could evolve and grow in the not too distant future:

Automated regression testing and coverage tracking.
Keep track of subscription patterns, and automatically resubscribe to them on reconnecting.
TLS support.
Redis cluster support.
Add high-level support for managing and calling custom Redis functions.
Add more high-level Redis commands, e.g. for lists, streams, etc.

If you have an idea for a must have feature, please let me (Attila) know. Pull requests, for new features or fixes to existing ones, are especially welcome!

Name		Name	Last commit message	Last commit date
Latest commit History 183 Commits
.github		.github
.settings		.settings
examples		examples
include		include
man/man1		man/man1
resources		resources
src		src
test		test
.cproject		.cproject
.gitignore		.gitignore
.project		.project
CHANGELOG.md		CHANGELOG.md
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
CONTRIBUTING.md		CONTRIBUTING.md
Doxyfile		Doxyfile
LICENSE		LICENSE
Makefile		Makefile
README.md		README.md
build.mk		build.mk
config.mk		config.mk
examples.mk		examples.mk
requirements.apt		requirements.apt
requirements.dnf		requirements.dnf
test.mk		test.mk
tools.mk		tools.mk

License

Smithsonian/redisx

Folders and files

Latest commit

History

Repository files navigation

RedisX

Table of Contents

Introduction

A simple example

Features overview

General Features

Redis / Valkey Features

Related links

Prerequisites

Building RedisX

Command-line interface (redisx-cli)

Linking your application against RedisX

Managing Redis server connections

Initializing

Sentinel

Configuring

Socket-level configuration

Connection & disconnection hooks

Connecting

Disconnecting

Reconnecting

Simple Redis queries

Interactive transactions

Bundled Attributes

Push notifications

RESP data type

Accessing key / value data

Getting and setting keyed values

Listing and Scanning

Publish / subscribe (PUB/SUB) support

Broadcasting messages

Subscriptions

Atomic execution blocks and LUA scripts

Execution blocks

LUA script loading and execution

Custom Redis functions

Advanced queries and pipelining

Asynchronous client processing

Bundled Attributes

Pipelined transactions

Error handling

Socket-level errors

Debug support

Future plans

About

Topics

Resources

License

Code of conduct

Stars

Watchers

Forks

Releases

Languages

Command-line interface (`redisx-cli`)