Dalli vs Redis-Store for Rails App

Question

I have been using Dalli until now for caching and today I came across Redis -Store.

I am wondering should I switch to redisstore. My app already uses redis for certain stuff so I have a redis server which is quite big(in terms of resources) and I also have another memcached server. So if I where to switch to redis-store it would mean that I can remove the memcached server(less server to maintain + less cost).

Has anyone done a comparison of these 2 solutions.

1. Performance
2. Is it a drop-in replacement(can I switch between these 2 anytime without code change)
3. Any other stuff I should know about.

Answer 1

Redis can be used as a cache or as a permanent store, but if you try to mix both, you can end up having "interesting issues".

When you have memcached, yo have a maximum amount of memory for the process, so when memcached gets full it will automatically remove the least recently used entries to make room for the new entries.

You can configure Redis to have that behaviour too, but you don't want to do that if you are using Redis for persistent storage, because in that case you would potentially lose keys that are meant to be persistent.

So if you are using persistent storage for Redis, you would need to have two different Redis processes: one for your persistant keys, one for caching. Of course you could always have only one process and set expiring times to every cache item, but no one would assure you you don't hit the memory limit before they expire and you lose data, so in practice you would need two processes. Besides, if you are setting a master/slave configuration for your persistent data and you store cache on the same server, you are basically wasting RAM, so separate processes are the way to go.

About performance, both redis and memcached are VERY performant, and on different tests they are on the same range when it comes to get/extract data, but memcached is better when you only need a cache.

Why is this so? First of all, since memcached only has one mission, which is storing key/values, it doesn't have any overhead when it comes to storing metadata. Redis on the other hand offers different data structures, so it stores more metadata which each key. One example of this: it's much "cheaper" to store data on a hash in Redis instead of using individual keys. You don't get any of this on memcached, since there is only one type of data. This means with the same amount of memory in your servers you can store more data on memcached than on redis. If you have a relatively small installation you don't really care, but the moment you start seeing growth, believe me you will want to keep those data under control.

So, as much as I like Redis, I prefer to have memcached for my caching needs and redis for my persistent storage/temporary storage/queue needs. I still use redis as a "cache" but not a temporary one with expiration, but as a lookup cache to save reading from a more expensive storage. For example, I keep a mapping between user IDs and nicknames on Redis. I never expire these mappings, so Redis is a perfect place for it.

In the case that you are dealing with a small amount of data, then it might make sense your idea of having a single technology for everything, but the moment you start growing over a few hundreds MB, I would say go with both of them.