Redis and Memcache or just Redis?

Assuming that migrating from memcached to redis for the caching you already do is easy enough, I'd go with redis only to keep things simple.

In redis persistence is optional, so you can use it much like memcached if that is what you want. You may even find that making your cache persistent is useful to avoid lots of cache misses after a restart. Expiry is available also - the algorithm is a bit different from memcached, but not enough to matter for most purposes - see http://redis.io/commands/expire for details.

I'm the author of redis-store, there is no need to use directly Redis commands, just use the :expires_in option like this:

ActionController::Base.cache_store = :redis_store, :expires_in => 5.minutes

The advantage of using Redis is fastness, and with my gem, is that you already have stores for Rack::Cache, Rails.cache or I18n.

I've seen a few large rails sites that use both Memcached and Redis. Memcached is used for ephemeral things that are nice to keep hot in memory but can be lost/regenerated if needed, and Redis for persistent storage. Both are used to take a load off the main DB for reading/write heavy operations.

More details:

Memcached: used for page/fragment/response caching and it's ok to hit the memory limit on Memcached because it will LRU (least recently used) to expire the old stuff, and frequently keep accessed keys hot in memory. It's important that anything in Memcached could be recreated from the DB if needed (it's not your only copy). But you can keep dumping things into it, and Memcached will figure which are used most frequently and keep those hot in memory. You don't have to worry about removing things from Memcached.

redis: you use this for data that you would not want to lose, and is small enough to fit in memory. This usually includes resque/sidekiq jobs, counters for rate limiting, split test results, or anything that you wouldn't want to lose/recreate. You don't want to exceed the memory limit here, so you have to be a little more careful about what you store and clean up later.

Redis starts to suffer performance problems once it exceeds its memory limit (correct me if I'm wrong). It's possible to solve this by configuring Redis to act like Memcached and LRU expire stuff, so it never reaches its memory limit. But you would not want to do this with everything you are keeping in Redis, like resque jobs. So instead of people often keep the default, Rails.cache set to use Memcached (using the dalli gem). And then they keep a separate $redis = ... global variable to do redis operations.

# in config/application.rbconfig.cache_store = :dalli_store  # memcached# in config/initializers/redis.rb$redis = $redis = Redis.connect(url: ENV['REDIS_URL'])

There might be an easy way to do this all in Redis - perhaps by having two separate Redis instances, one with an LRU hard memory limit, similar to Memcache, and another for persistent storage? I haven't seen this used, but I'm guessing it would be doable.