What it is: In-memory data store (Redis/Valkey, Memcached). The resilience story is two features people constantly confuse.
Exam-testable facts:
- Redis replication group: 1 primary + up to 5 read replicas. With automatic failover, the replica with the least replication lag is promoted to primary on failure; the primary/reader DNS endpoints follow automatically.
- Multi-AZ, failover, and replicas move together: Multi-AZ needs automatic failover, which needs a replica.
- Multi-AZ ≠ cross-Region. Multi-AZ survives node/AZ failure within one Region. Cross-Region DR is Global Datastore: async replication (typically sub-second lag), secondaries serve local reads — but regional failover is manual (promote the secondary yourself).
- Cluster mode disabled (single shard): full Redis command support, vertical scaling only. Cluster mode enabled (up to 500 shards): horizontal write scaling; multi-key commands need all keys in one hash slot (use
{hash tags}). - Exam trap: “automatic cross-Region failover for ElastiCache” does not exist.
Sources: https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/redis-ug.pdf
Added during the 2026-10-03 review. All prose is original; facts verified against the AWS documentation links in Sources above.