What is the difference between Redis replication and sharding?

• Sharding, also known as partitioning, is splitting the data up by key. Sharding is useful to increase performance, reducing the hit and memory load on any one resource.
• While replication, also known as mirroring, is to copy all data. Replication is useful for getting a high availability of reads. If you read from multiple replicas, you will also reduce the hit rate on all resources, but the memory requirement for all resources remains the same.

Any key-value store (of which Redis is only one example) supports sharding, though certain cross-key functions will no longer work. Redis supports replication out of the box.