Open source Redis is a single-threaded process to provide speed and simplicity. A single Redis process is bound by the CPU core that it is running on and available memory on the server. Redis Enterprise Software (RS) supports database clustering to allow customers to spread the load of a Redis process over multiple cores and the RAM of multiple servers. A database cluster is a set of Redis processes where each process manages a subset of the database keyspace.
Database replication helps ensure high availability. When replication is enabled, your dataset is replicated to a replica shard, which is constantly synchronized with the primary shard. If the primary shard fails, an automatic failover happens and the replica shard is promoted. That is, it becomes the new primary shard. When the old primary shard recovers, it becomes the replica shard of the new primary shard. This auto-failover mechanism guarantees that data is served with minimal interruption.
Rack-zone awareness is a Redis Enterprise feature that helps to ensure high-availability in the event of a rack or zone failure. When you enable rack-zone awareness in a Redis Enterprise Software (RS) cluster, you assign a rack-zone ID to each node. This ID is used to map the node to a physical rack or logical zone. The cluster can then ensure that master shards, corresponding slave shards, and associated endpoints are placed on nodes in different racks/zones.