Starting or restarting Unicorn with Capistrano 3.x

Question

I\'m trying to start or restart Unicorn when I do cap production deploy with Capistrano 3.0.1. I have some examples that I got working with Capistrano 2.x using so

Answer 1

Can't say anything specific about capistrano 3(i use 2), but i think this may help: How to run shell commands on server in Capistrano v3?. Also i can share some unicorn-related experience, hope this helps.

I assume you want 24/7 graceful restart approach.

Let's consult unicorn documentation for this matter. For graceful restart(without downtime) you can use two strategies:

1. kill -HUP unicorn_master_pid It requires your app to have 'preload_app' directive disabled, increasing starting time of every one of unicorn workers. If you can live with that - go on, it's your call.

2. kill -USR2 unicorn_master_pid kill -QUIT unicorn_master_pid

More sophisticated approach, when you're already dealing with performance concerns. Basically it will reexecute unicorn master process, then you should kill it's predecessor. Theoretically you can deal with usr2-sleep-quit approach. Another(and the right one, i may say) way is to use unicorn before_fork hook, it will be executed, when new master process will be spawned and will try to for new children for itself. You can put something like this in config/unicorn.rb:

# Where to drop a pidfile
pid project_home + '/tmp/pids/unicorn.pid'

before_fork do |server, worker|
  server.logger.info("worker=#{worker.nr} spawning in #{Dir.pwd}")

  # graceful shutdown.
  old_pid_file = project_home + '/tmp/pids/unicorn.pid.oldbin'
  if File.exists?(old_pid_file) && server.pid != old_pid_file
    begin
      old_pid = File.read(old_pid_file).to_i
      server.logger.info("sending QUIT to #{old_pid}")
      # we're killing old unicorn master right there
      Process.kill("QUIT", old_pid)
    rescue Errno::ENOENT, Errno::ESRCH
      # someone else did our job for us
    end
  end
end

It's more or less safe to kill old unicorn when the new one is ready to fork workers. You won't get any downtime that way and old unicorn will wait for it's workers to finish.

And one more thing - you may want to put it under runit or init supervision. That way your capistrano tasks will be as simple as sv reload unicorn, restart unicorn or /etc/init.d/unicorn restart. This is good thing.