To do a "zero-downtime deploy" or "graceful reload" by sending the USR2 signal, you can add a new case to the Unicorn init script in /etc/init.d/unicorn

  upgrade)
        log_daemon_msg "Upgrading $DESC" $NAME || true
        if start-stop-daemon --stop --signal USR2 --quiet --oknodo --pidfile $PID; then
          sleep 3
          if start-stop-daemon --stop --signal QUIT --quiet --oknodo --pidfile $PID.oldbin; then
              log_end_msg 0 || true
          else
              log_end_msg 1 || true
          fi
        else
          log_end_msg 1 || true
        fi
        ;;

This sends the signal and then reaps the old processes when you run service unicorn upgrade

Give it and try and let us know how it goes!

Thanks @asb, I'll try this.

I see that this script is autonomous ; it sends the USR2 signal, sleeps (3 seconds should be enough for most of web requests to finish), then sends a QUIT signal to the old master.

There another approach where Unicorn is responsible for killing an eventual old master when forking workers.

before_fork do |server, worker|
  ##
  # When sent a USR2, Unicorn will suffix its pidfile with .oldbin and
  # immediately start loading up a new version of itself (loaded with a new
  # version of our app). When this new Unicorn is completely loaded
  # it will begin spawning workers. The first worker spawned will check to
  # see if an .oldbin pidfile exists. If so, this means we've just booted up
  # a new Unicorn and need to tell the old one that it can now die. To do so
  # we send it a QUIT.
  #
  # Using this method we get 0 downtime deploys.

  old_pid = "#{server.config[:pid]}.oldbin"
  if File.exists?(old_pid) && server.pid != old_pid
    begin
      Process.kill("QUIT", File.read(old_pid).to_i)
    rescue Errno::ENOENT, Errno::ESRCH
      # someone else did our job for us
    end
  end
end

This snippet is considered common use by a lot of Unicorn users. It is even in the examples shipped with Unicorn source code.

I guess it might be possible to have an init script that starts by sending the USR2 signal, sleeps (maybe a bit longer), then checks if the reloading has properly occurred.
Right now, I guess it fails (returning a 1 code) if the old PID is missing (removed by Unicorn itself within the sleep time.

What do you think?