Amazon ElastiCache
User Guide (API Version 2015-02-02)

ElastiCache Replication (Redis)

Stand-alone Amazon ElastiCache cache clusters are in-memory entities without any redundant data protection services. In this scenario, if your cluster fails for any reason, you lose all the cluster's data. However, if you're running the Redis engine, you can group 2 to 6 nodes into a replication group where 1 to 5 read-only nodes contain replicate data of the group's single read/write primary node. In this scenario, if one node fails for any reason you do not lose all your data since it is replicated in one or more other nodes.

If the replication group has Multi-AZ enabled and the primary node fails, the replication group fails over to a read replica. Because the data is updated on the replica nodes asynchronously, there may be some data loss due to latency in updating the replica nodes. For more information, see Mitigating Failures when Running Redis.