Eredis is a C client library built over Hiredis. It is lightweight, high performance, reentrant and thread-safe.
It aims to provide features for real production environment and keep it simple.

For write commands (SET, HSET, EXPIRE, ...):

• Async commands via asynchronous events (libev)
• Async commands mirroring across multiple redis servers (shared-nothing)

For any commands:

• Thread-safe high speed pool of blocking connections: efficient persistent connections
• Automatic and seamless fail-over and reconnect

Integration:

• Embedded Hiredis ensures the full Redis protocol support
• Reply structure is the Hiredis efficient redisReply
• Eredis allows you to switch from a single host mode to a an efficient shared-nothing/fail-over mode by just activating a new host.

The integrated master-slave mechanism in Redis works ok for a small number of nodes or workload, but it could kill a master node bandwidth in high load. The many-to-many mechanism permits a more efficient and reliable workload by obviously needing exactly the same bandwidth on each node.

git clone ...
cd eredis
cmake .
make
# optional
make doc
make install

Each function is described in the C code (doxygen).
The raw list is available in the unique header file: eredis.h.

#include <eredis.h>

eredis_t *e = eredis_new();

/* Set timeout - default 5000ms */
eredis_timeout( e, 200 );

/* Set max readers - default 10 */
eredis_r_max( e, 50 );

/* Set retry for reader - default 1 */
eredis_r_retry( e, 1 );

The first host provided becomes the "prefered" one.
The readers will always reconnect to this one in case of a down/up event.

/* manually */
eredis_host_add( e, "/var/run/eredis.sock", 0 );
eredis_host_add( e, "host2", 6379 );
eredis_host_add( e, "host3", 6379 );

/* via configuration file - one line per host:port */
eredis_host_file( e, "my-hosts.conf" );

Mandatory for using async writes or auto-reconnection to the "prefered" host.

/* in its own thread - blocking call */
eredis_run( e );

/* in a eredis managed thread - non-blocking call */
eredis_run_thr( e );

eredis_reader_t *r;
eredis_reply_t *reply; /* aka redisReply from hiredis */

/* get a reader */
r = eredis_r( e );

/* Add one request (pipelining) */
eredis_r_append_cmd( reader, "GET key1");
/* Add one request and process - got key1 reply */
reply = eredis_r_cmd( reader, "GET key2");
/* key2 reply - not mandatory to get all replies */
reply = eredis_r_reply( reader );
...

/* release a reader */
r = eredis_r_release( e );

eredis_w_cmd( e, "SET key1 10" );

/* Exit the event loop (from any thread) */
eredis_shutdown( e );

/* Stop the event loop and threadif needed,
   close connections and release memory */
eredis_free( e );

When a Redis server goes down, Eredis async loop will detect it and retry to reconnect every second HOST_DISCONNECTED_RETRIES (10) times (= 10 seconds).
After this, it will retry once every HOST_FAILED_RETRY_AFTER (20) seconds.

To avoid data loss, if all specified Redis server are down, Eredis will keep in memory the last unsent QUEUE_MAX_UNSHIFT (10000) commands.

If a Redis server goes down and up, it could be needed to resynchronize with an active node. The master-slave mechanism is perfect for that.
In redis.conf, add 'slave-read-only no'.
After the Redis server start, make a "redis-cli SLAVEOF hostX".
Once 'redis-cli info Replication' pops a 'master_sync_in_progress:0', it's done.
make a "redis-cli SLAVEOF no one" and it comes back to a 'master' status.
This process can easily be scripted.

Eredis is used on nearly 500 servers at Eulerian Technologies.
It is written and maintained by Guillaume Fougnies (guillaume at eulerian dot com).
Redis and Hiredis are great!
We hope you will find Eredis great.
It is released under the BSD license.