A Redis (cluster mode disabled) replication group is a collection of cache clusters, where one of the cache clusters is a read/write primary and the others are read-only replicas. Writes to the primary are asynchronously propagated to the replicas.
A Redis (cluster mode enabled) replication group is a collection of 1 to 15 node groups (shards). Each node group (shard) has one read/write primary node and up to 5 read-only replica nodes. Writes to the primary are asynchronously propagated to the replicas. Redis (cluster mode enabled) replication groups partition the data across node groups (shards).
When a Redis (cluster mode disabled) replication group has been successfully created, you can add one or more read replicas to it, up to a total of 5 read replicas. You cannot alter a Redis (cluster mode enabled) replication group after it has been created.
This operation is valid for Redis only.