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

ElastiCache Replication (Redis)

Amazon ElastiCache cache clusters are stand-alone, 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 clusters into a replication group where 1 to 5 read-only clusters contain replicate data of the group's single read/write primary cluster. In this scenario, if one cluster fails for any reason you do not lose all your data since it is replicated in one or more other clusters.

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