if the bottle neck is the number of connections, is it better to scale redis by sharding or by replication?

Question

Background

last week we had a major outage, we identified the bottle neck to be redis number of connections:

Our quick fix was to scale redis from cache.m4.4xlarge to chache.m4.10xlarge (vertical scaling)

However this scaling is quite expensive:

we would like to scale horizontally: Unfortunately, the application code we are using now only allows us to either create a redis cluster with multiple shards, or using a single shard with multiple replicas (see discussion here).

Question

considering that our bottle neck is the connection limit, would it be getter to scale by clustering and increasing the shard number, or by having a single shard and replicating it?

Answer 1

Redis Cluster could be considered as an OOTB solution for the Scalability requirements. There is another interesting open source library known as Redis Shard for sharding implementations on Redis. Please find the GitHub repository here.

Answer 2

Just for the sake of completeness: Redis documentation https://redislabs.com/ebook/part-3-next-steps/chapter-10-scaling-redis/ suggest a general rule of thumb - add replicas for reads, add shards for writes and memory capacity. And it does make sense.

The replica can have a complete copy of its shard. The more shards the smaller they are. Also writing to one shard does not limit other ones.

If the only concern is reading, then replicas are the simplest solution.

If you need both, just combine.