Redis Failover attempts to provides a full automatic master/slave failover solution for Ruby. Redis does not provide an automatic failover capability when configured for master/slave replication. When the master node dies, a new master must be manually brought online and assigned as the slave's new master. This manual switch-over is not desirable in high traffic sites where Redis is a critical part of the overall architecture. The existing standard Redis client for Ruby also only supports configuration for a single Redis server. When using master/slave replication, it is desirable to have all writes go to the master, and all reads go to one of the N configured slaves.

This gem attempts to address both the server and client problems. A redis failover server runs as a background daemon and monitors all of your configured master/slave nodes. When the server starts up, it automatically discovers who is the master and who are the slaves. Watchers are setup for each of the redis nodes. As soon as a node is detected as being offline, it will be moved to an "unreachable" state. If the node that went offline was the master, then one of the slaves will be promoted as the new master. All existing slaves will be automatically reconfigured to point to the new master for replication. All nodes marked as unreachable will be periodically checked to see if they have been brought back online. If so, the newly reachable nodes will be configured as slaves and brought back into the list of live servers. Note that detection of a node going down should be nearly instantaneous, since the mechanism used to keep tabs on a node is via a blocking Redis BLPOP call (no polling). This call fails nearly immediately when the node actually goes offline.

This gem provides a RedisFailover::Client wrapper that is master/slave aware. The client is configured with a single host/port pair that points to redis failover server. The client will automatically connect to the server to find out the current state of the world (i.e., who's the current master and who are the current slaves). The client also acts as a load balancer in that it will automatically dispatch Redis read operations to one of N slaves, and Redis write operations to the master. If it fails to communicate with any node, it will go back and ask the server for the current list of available servers, and then optionally retry the operation.