Serving Rails with Mongrel

What is Mongrel

Mongrel is an HTTP server for hosting Ruby web applications.

Why Mongrel

Ruby On Rails community experimented with many ways to host applications and wasn’t entirely happy with any. Until Mongrel came out and in a matter of weeks became a de-facto standard way to serve Rails.

Mongrel is stable, reasonably fast, very easy to configure, and can be used in both production and development environments.

Details

RubyWorks stack runs several (by default, four) Mongrel instances. They serve HTTP requests on ports 3002-3005, and execute a Rails application located in /usr/rails.

Mongrel configuration files are /etc/rails/mongrel_300[2-5].config. The only interesting settings in those files are:

environment – Rails environment to use; set to ‘production’ by default; for a non-production environment you’ll need to change it in all four mongrel_300[2-5].config files.

num_processors – maximum number of requests a single Mongrel instance can be processing at the same time; set to 5 by default. RubyWorks stack is configured to not send more than one request at a time to any given Mongrel, so, a low value here is generally alright.

Process management

Mongrels are started as runit services, by executing scripts /var/service/mongrel_300[2-5]/run. To control Mongrel processes, run the following commands:

Start: sudo monit -g mongrels start all
Stop: sudo monit -g mongrels stop all
Restart: sudo monit -g mongrels restart all

Troubleshooting

The Rails application logs go to /usr/rails/production.log (more precisely, to /usr/rails/[environment_name].log, if you changed the environment name in mongrel configuration files).

If a Mongrel process refuses to start, looking at the ps record of runsvdir process usually gets you to the root cause:

ps -ef | grep runsvdir

runsvdir is the main process of runit, and it shows in the ps record stderr output of services it controls.

If that doesn’t help, tell monit to stop Mongrel:

sudo monit stop mongrel_3002

and run its startup script from the console:

sudo /var/service/mongrel_3002/run

to see the actual stdout/stderr output.