Questions about connection pools

rubyonrails-talk
1

Hi all,

I currently am part of a team writing an app and I'm running into an issue with connection pools.

Quick infrastructure overview: Rails 2.2.2, Ruby 1.8.7p72, Apache/ Mongrel, mongrel running single threaded. Due to legacy schema, the mongrels are making connections to a couple of different DB.

We currently are running about 28 mongrels, and obviously with a default 5 connections/per mongrel, we're getting 140 connections to the db. This seems kind of wasteful, especially as we scale larger.

So the first thing I tried is setting the pool size to 1. I figured since only one request is being processed at a time, there's only a need for one db connection. However, this cause the app to start spewing 'could not obtain db connection' errors.

So I bumped the pool size up to 2, and things seem to be OK, however when I go into mysql and show processlist, it looks like there are 4 or 5 connections open.

Basically, I realize that there the connection pool is necessary to support multithreading, but is there any way to pare things down to one connection per mongrel like the old day, when we aren't using the multithreading?

Thanks,

Andrew

2

Andrew Selder wrote:

We currently are running about 28 mongrels, and obviously with a default 5 connections/per mongrel, we're getting 140 connections to the db. This seems kind of wasteful, especially as we scale larger.

Searching your post I don't see the word 'cache'. Explain?

3

You can set the connection pool size to 1, but if you do that you need to be careful not to leak connections. During the normal request/ response cycle this is all handled for you but at app load times you will have to be careful. Stuff like

class MyControllerClass < ActionController::Base   SOME_CONSTANT = People.find :all end

(contrived example but you get the point) will leak a connection because a connection will be used to execute the query but it won't be returned to the pool. You can clear out connections with ActiveRecord::Base.clear_active_connections!

Fred