Master Slave

Redis can increase availability and read throughput by using replication. lettuce provides dedicated Master/Slave support since 4.2 for topologies and ReadFrom-Settings.

Redis Master/Slave can be run standalone or together with Redis Sentinel, which provides automated failover and master promotion. Failover and master promotion is supported in lettuce already since version 3.1 for master connections.

Connections can be obtained from the MasterSlave connection provider by supplying the client, Codec, and one or multiple RedisURIs.

Redis Sentinel

Master/Slave using Redis Sentinel uses Redis Sentinel as registry and notification source for topology events. Details about the master and its slaves are obtained from Redis Sentinel. lettuce subscribes to Redis Sentinel events for notifications to all supplied Sentinels.

Standalone Master/Slave

Running a Standalone Master/Slave setup required one seed address to establish a Redis connection. Providing one RedisURI will discover other nodes which belong to the Master/Slave setup and use the discovered addresses for connections. The initial URI can point either to a master or a slave node.

Static Master/Slave with predefined node addresses

In some cases, topology discovery shouldn’t be enabled, or the discovered Redis addresses are not suited for connections. AWS ElastiCache falls into this category. lettuce allows to specify one or more Redis addresses as List and predefine the node topology. Master/Slave URIs will be treated in this case as static topology, and no additional hosts are discovered in such case. Redis Standalone Master/Slave will discover the roles of the supplied `RedisURI`s and issue commands to the appropriate node.

Topology discovery

Master-Slave topologies are either static or semi-static. Redis Standalone instances with attached slaves provide no failover/HA mechanism. Redis Sentinel managed instances are controlled by Redis Sentinel and allow failover (which include master promotion). The MasterSlave API supports both mechanisms. The topology is provided by a TopologyProvider:

• MasterSlaveTopologyProvider: Dynamic topology lookup using the INFO REPLICATION output. Slaves are listed as slaveN=…​ entries. The initial connection can either point to a master or a slave, and the topology provider will discover nodes. The connection needs to be re-established outside of lettuce in a case of a Master/Slave failover or topology changes.

• StaticMasterSlaveTopologyProvider: Topology is defined by the list of URIs and the ROLE output. MasterSlave uses only the supplied nodes and won’t discover additional nodes in the setup. The connection needs to be re-established outside of lettuce in case of a Master/Slave failover or topology changes.

• SentinelTopologyProvider: Dynamic topology lookup using the Redis Sentinel API. In particular, SENTINEL MASTER and SENTINEL SLAVES output. Master/Slave failover is handled by lettuce.

Topology Updates

• Standalone Master/Slave: Performs a one-time topology lookup which remains static afterward

• Redis Sentinel: Subscribes to all Sentinels and listens for Pub/Sub messages to trigger topology refreshing

Examples

Redis Standalone Master/Slave

RedisClient redisClient = RedisClient.create();

StatefulRedisMasterSlaveConnection<String, String> connection = MasterSlave.connect(redisClient, new Utf8StringCodec(),
        RedisURI.create("redis://localhost"));
connection.setReadFrom(ReadFrom.MASTER_PREFERRED);

System.out.println("Connected to Redis");

connection.close();
redisClient.shutdown();

Using Redis Sentinel

RedisClient redisClient = RedisClient.create();

StatefulRedisMasterSlaveConnection<String, String> connection = MasterSlave.connect(redisClient, new Utf8StringCodec(),
        RedisURI.create("redis-sentinel://localhost:26379,localhost:26380/0#mymaster"));
connection.setReadFrom(ReadFrom.MASTER_PREFERRED);

System.out.println("Connected to Redis");

connection.close();
redisClient.shutdown();

Using AWS ElastiCache Cluster

RedisClient redisClient = RedisClient.create();

List<RedisURI> nodes = Arrays.asList(RedisURI.create("redis://host1"),
        RedisURI.create("redis://host2"),
        RedisURI.create("redis://host3"));

StatefulRedisMasterSlaveConnection<String, String> connection = MasterSlave
        .connect(redisClient, new Utf8StringCodec(), nodes);
connection.setReadFrom(ReadFrom.MASTER_PREFERRED);

System.out.println("Connected to Redis");

connection.close();
redisClient.shutdown();