The simplest foreground Rails service framework you'll ever see

service.rb

# vim: ft=ruby
#
# Usage:
#
# Save as ./script/service
# 
# Create your ./script/job containing:
#
#   load './script/service'
#   Service.new do
#     log 'Starting marvellous service'
#     on_graceful_exit {log 'Bye bye!'}
#     loop {puts 'something'; sleep 10}
#   end
#
# chmod +x ./script/job
# ./script/job
#
# Run it under daemontools, runit, upstart, God, Bluepill, launchd, whatever.
# Read the source and have fun. Tests and kudos are welcome :-p
#
#  - vjt  Thu Mar 10 14:06:17 CET 2011
#

require 'pathname'

class Service

  def initialize(name = nil, &service)
    @name = name || File.basename($0)

    check_pidfile
    write_pidfile
    setup_signals

    boot! service

  rescue StandardError => e
    @horrible_proc.call if @horrible_proc rescue nil
    fail! "Uncaught exception #{e.inspect}\n  #{e.backtrace.join("\n  ")}"
  end

  protected
    def log(*message)
      puts [Time.now, "[#$$]", *message].join(' ')
    end

    def fail!(message)
      log message
      exit 1
    end

    def graceful(reason)
      @graceful_proc.call if @graceful_proc
      log "Exiting via #{reason}"
      exit 0
    end

    def on_graceful_exit(&block)
      @graceful_proc = block
    end

    def on_horrible_exit(&block)
      @horrible_proc = block
    end

  private
    def check_pidfile
      @pidfile = Pathname("tmp/pids/#@name.pid")
      return unless @pidfile.exist?

      pid = @pidfile.read.to_i

      begin
        if Process.kill 0, pid
          fail! "#@name is already running as PID #{pid}"
        end

      rescue Errno::ESRCH, Errno::EPERM => e
        # Process does not exist, or is owned by another user:
        # either way, it's a stale pifile: remove it and go on
        log "Removing stale pidfile [#{pid}]"
        @pidfile.unlink

      rescue Errno
        fail! "#@pidfile already exist: is #@name already running as PID #{pid}?"

      end
    end

    def write_pidfile
      @pidfile.open('w+') {|f| f.write($$)}
    end

    def setup_signals
      trap('EXIT') { @pidfile.unlink } # Reminds me of Erlang times...
      trap('TERM') { graceful 'SIGTERM' }
      trap('INT')  { graceful 'SIGINT'  }
    end

    def boot!(service)
      log 'Loading application...'
      require 'config/environment'

      instance_eval &service
    end

end