How to horizontally scale Elacticache Redis cluster?

Hello.

I think the following documents will be helpful for how to use them and compare them.
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/Replication.Redis-RedisCluster.html

If cluster mode is disabled, the number of shards cannot be increased, so if the load is based on writes, the instance type must be increased.
In case of read load, you can increase the number of nodes in the shard to up to 5, which can handle it.
However, if the load load cannot be handled even by increasing the number of nodes, the instance You need to make the type larger.
Also, if the primary node goes down due to an AZ failure, recovery will be done by promoting a replica node in the same shard since there is no replacement primary node.
However, during this recovery process, the primary node is in a 0 state, so writing is not possible during that time.
Therefore, by enabling cluster mode, if the primary node goes down due to an AZ failure, the primary node in another shard is alive, so you can ensure that there is at least one primary node.
This is the advantage of cluster mode.
If you are concerned about the write process stopping even for a moment, we recommend enabling cluster mode, creating multiple shards, and placing the primary nodes in different AZs.