A shard (API/CLI: node group) is a collection of 1 to 6 Redis nodes. A Redis (cluster mode disabled) cluster will never have more than one shard. Redis (cluster mode enabled) clusters can have from 1 to 15 shards. The cluster's data is partitioned across the cluster's shards. If there is more than one node in a shard, the shard implements replication with one node being the read/write primary node and the other nodes read-only replica nodes.
When you create a Redis (cluster mode enabled) cluster using the ElastiCache console, you specify the number of shards in the cluster and the number of nodes in the shards. For more information, see Creating a Redis (cluster mode enabled) Cluster (Console). If you use the ElastiCache API or AWS CLI to create a cluster (called replication group in the API/CLI), you can configure the number of nodes in a shard (API/CLI: node group) independently. For more information, see:
Each node in a shard has the same compute, storage and memory specifications. The ElastiCache API lets you control shard-wide attributes, such as the number of nodes, security settings, and system maintenance windows.
Redis shard configurations