Benefits of connection pooling with Redis and Unicorn

Question

Background: I have a Ruby/Rails + Nginx/Unicorn web app with connections to multiple Redis DBs (i.e. I am not using Redis.current and am instead using global variables for my different connections). I understanding that I need to create a new connection in the after_fork block when a new Unicorn worker is created, as explained here and here.

My question is about the need for connection pooling. According to this SO thread, "In Unicorn each process establishes its own connection pool, so you if your db pool setting is 5 and you have 5 Unicorn workers then you can have up to 25 connections. However, since each unicorn worker can handle only one connection at a time, then unless your app uses threading internally each worker will only actually use one db connection... Having a pool size greater than 1 means each Unicorn worker has access to connections it can't use, but it won't actually open the connections, so that doesn't matter."

Since I am NOT using Sidekiq, do I even need to use connection pools for my Redis connections? Is there any benefit of a connection pool with a pool size of 1? Or should I simply use variables with single connections -- e.g. Redis.new(url: ENV["MY_CACHE"])?

Answer 1

Connection pool is only used when ActiveRecord talks to the SQL databases defined in your databases.yml config file. It is not related to Redis at all and the SO answer that you cite is actually not relevant for Redis.

So, unless you wanted to use some custom connection pool solution for Redis, you don't have to deal with it at all, as there is no pool for Redis in Rails by default. I guess the custom pool might be suitable if you had multiple threads in your applications which is not your case.

Update: Does building a connection pool make sense in your scenario? I doubt it. Connection pool is a way to reuse open connections (typically among multiple threads / requests). But you say that you:

1. use unicorn, the workers of which are separate, independent processes, not threads,
2. open a stable connection (or two) during after_fork, a connection which is then open all the time the unicorn worker lives
3. do not use threads in your application anywhere (I'd check if this is true again - it's not only Sidekiq but it might be any gem that tends to do things in the background).

In such scenario, pooling connection to Redis makes no sense to me as there seems to be no code that would benefit from reusing the connection - it is open all the time anyway.