Start or ensure that Delayed Job runs when an application/server restarts

Question

We have to use delayed_job (or some other background-job processor) to run jobs in the background, but we're not allowed to change the boot scripts/boot-levels on the server. This means that the daemon is not guaranteed to remain available if the provider restarts the server (since the daemon would have been started by a capistrano recipe that is only run once per deployment).

Currently, the best way I can think of to ensure the delayed_job daemon is always running, is to add an initializer to our Rails application that checks if the daemon is running. If it's not running, then the initializer starts the daemon, otherwise, it just leaves it be.

The question, therefore, is how do we detect that the Delayed_Job daemon is running from inside a script? (We should be able to start up a daemon fairly easily, bit I don't know how to detect if one is already active).

Anyone have any ideas?

Regards, Bernie

Based on the answer below, this is what I came up with. Just put it in config/initializers and you're all set:

#config/initializers/delayed_job.rb

DELAYED_JOB_PID_PATH = "#{Rails.root}/tmp/pids/delayed_job.pid"

def start_delayed_job
  Thread.new do 
    `ruby script/delayed_job start`
  end
end

def process_is_dead?
  begin
    pid = File.read(DELAYED_JOB_PID_PATH).strip
    Process.kill(0, pid.to_i)
    false
  rescue
    true
  end
end

if !File.exist?(DELAYED_JOB_PID_PATH) && process_is_dead?
  start_delayed_job
end

Answer 1

Check for the existence of the daemons PID file (File.exist? ...). If it's there then assume it's running else start it up.