how to write a robust never-ending process?

I've asked a different question about directory watching, which was answered, but the other half of the question is how to best create a never ending process, in ruby, to do this. Here are the requirements:

• run forever
• be monitorable (i.e. know if it is up or down)
• have some sort of way of restarting it and ensuring it is up (God?)
• start / stop using Capistrano (would be nice!)

We've looked at BackgroundRb, but that seems a bit outdated and to be honest unreliable! We've looked at DelayedJob, but that seems geared for one off jobs (because a never-ending job seems to block any other job from getting done as jobs are done sequentially).

We are running a bunc开发者_如何学编程h of Ubuntu servers that form our environment.

Any ideas?

I have an event machine loop tailing some nginx log files and putting them into MongoDB. The "log eater" scripts are running with ruby daemons. http://daemons.rubyforge.org/

I have found it to be much more reliable than god. This also monitors and restarts your script if it dies. If you want notification if the runner dies, you can use monit to do that.

Here is my runner script for daemons:

#!/usr/bin/env ruby
require 'rubygems'
require 'bundler' 
Bundler.require(:default)
Bundler.setup(:default)

options = {
  :app_name   => "log_eater",
  :dir_mode   => :system,
  :multiple   => true,
  :backtrace  => true,
  :monitor    => true
}

Daemons.run(File.join(File.dirname(__FILE__), 'log_eater.rb'), options)

This has been running for many months with no leak or no problem. God had problems with leaks and dying. Capistrano can restart this by restarting your startup script.

Here is an excerpt from mine for gentoo linux

start() {
ebegin "Starting log-eater"
    cd /ruby/STABLE/quickanalytics
   `scripts/log_eater_runner.rb start -- /usr/logs/nginx.log`
eend $? "Failed to start log-eater" 
}

-- after the start command is for any args you want passed to your script.

I'd probably have a look at daemon-kit. Not sure if it meets all your requirements though:

https://github.com/kennethkalmer/daemon-kit

In your case, I'd use Resque. It seems to satisfy your requirements. I believe it comes with sample scripts for capistrano for controlling the workers. Monitoring the workers with god is slightly more complicated but it does come with a web console so that you can see what your workers are up to. There are a ton of plugins for it too to suite whatever needs you might have.

https://github.com/defunkt/resque

See Paul Dix's book Service-Oriented Design with Ruby and Rails. Also look at Sinatra.