Redis Sentinel

Redis replication is a very simple to use and configure master-slave replication that allows slave Redis servers to be exact copies of master servers. The following are some very important facts about Redis replication:

• Redis uses asynchronous replication. Starting with Redis 2.8, however, slaves will periodically acknowledge the amount of data processed from the replication stream.
• A master can have multiple slaves.
• Slaves are able to accept connections from other slaves. Aside from connecting a number of slaves to the same master, slaves can also be connected to other slaves in a graph like structure.
• Redis replication is nonblocking on the master side. This means that the master will continue to handle queries when one or more slaves perform the initial synchronization.

Redis Sentinel (Active-Passive) is only one Redis instance, the master, serving requests at all time. Data is replicated from the master to the rest of the Redis instances, the slaves. One of the slaves would replace the master when the master is down. Therefore HA essentially needs to take care of two major issues, data availability and service availability. Redis replication provides data availability, service availability and monitors Redis instances. For example, consider a group of three Redis instances, A, B, and C, with A as the initial master. The following figure illustrate how Redis Sentinel monitors Redis instances.

When the master is down, Redis Sentinel automatically elects a new master and failover to the the new master as shown in the following figure.