Questions about connection pools


#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 S. 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

On Mar 15, 4:56 pm, Andrew S. removed_email_address@domain.invalid wrote:

default 5 connections/per mongrel, we’re getting 140 connections to

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?

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