This is originally from tech.rufy.com/entry/15, but half the time I can't access his site so I'm storing it here. He usually has this code in the index actions of his site.

It checks for an existing process and if it doesn't find one, spawns a new thread to handle maintenance tasks.

pid_file = '/tmp/running.pid'
begin
  File.stat pid_file
rescue
  f = File.new pid_file, 'w'
  f < < fork {
    loop {
      # do lots of maintenance
      sleep 15*60
    }
  }
  f.close
end