Daemons provides an easy way to wrap existing ruby scripts (for example a self-written server) to be run as a daemon and to be controlled by simple start/stop/restart commands.

If you want, you can also use daemons to run blocks of ruby code in a daemon process and to control these processes from the main application.

Besides this basic functionality, daemons offers many advanced features like exception backtracing and logging (in case your ruby script crashes) and monitoring and automatic restarting of your processes if they crash.

You can use Daemons in four different ways:

Layout: suppose you have your self-written server myserver.rb:

# this is myserver.rb
# it does nothing really useful at the moment

loop do
  sleep(5)
end

To use myserver.rb in a production environment, you need to be able to run myserver.rb in the background (this means detach it from the console, fork it in the background, release all directories and file descriptors).

Just create myserver_control.rb like this:

# this is myserver_control.rb
require 'daemons'

Daemons.run('myserver.rb')

And use it like this from the console:

$ ruby myserver_control.rb start
    (myserver.rb is now running in the background)
$ ruby myserver_control.rb restart
    (...)
$ ruby myserver_control.rb stop

For testing purposes you can even run myserver.rb without forking in the background:

$ ruby myserver_control.rb run

An additional nice feature of Daemons is that you can pass additional arguments to the script that should be daemonized by seperating them by two hyphens:

$ ruby myserver_control.rb start -- --file=anyfile --a_switch another_argument

Layout: suppose you have some code you want to run in the background and control that background process from a script:

# this is your code
# it does nothing really useful at the moment

loop do
  sleep(5)
end

To run this code as a daemon create myproc_control.rb like this and include your code:

# this is myproc_control.rb
require 'daemons'

Daemons.run_proc('myproc.rb') do
  loop do
    sleep(5)
  end
end

And use it like this from the console:

$ ruby myproc_control.rb start
    (myproc.rb is now running in the background)
$ ruby myproc_control.rb restart
    (...)
$ ruby myproc_control.rb stop

For testing purposes you can even run myproc.rb without forking in the background:

$ ruby myproc_control.rb run

Layout: you have an application my_app.rb that wants to run a bunch of server tasks as daemon processes.

# this is my_app.rb
require 'daemons'

task1 = Daemons.call(:multiple => true) do
  # first server task

  loop do
    conn = accept_conn()
    serve(conn)
  end
end

task2 = Daemons.call do
  # second server task

  loop do
    something_different()
  end
end

# the parent process continues to run

# we can even control our tasks, for example stop them
task1.stop
task2.stop

exit

Layout: you have an application my_daemon.rb that wants to run as a daemon (but without the ability to be controlled by daemons via start/stop commands)

# this is my_daemons.rb
require 'daemons'

# Initialize the app while we're not a daemon
init()

# Become a daemon
Daemons.daemonize

# The server loop
loop do
  conn = accept_conn()
  serve(conn)
end

For further documentation, refer to the module documentation of Daemons.

When daemonizing a process using a wrapper script, as examples 1 and 2 above, the status can be shown using

$ ruby myproc_control.rb status

By default this will display whether or not the daemon is running and, if it is, its PID.

A custom message can be shown with

def custom_show_status(app)
  # Display the default status information
  app.default_show_status

  puts
  puts "PS information"
  system("ps -p #{app.pid.pid.to_s}")

  puts
  puts "Size of log files"
  system("du -hs /path/to/logs")
end

Daemons.run('myserver.rb', { show_status_callback: :custom_show_status })

Documentation can be found at http://www.rubydoc.info/gems/daemons.

Written 2005-2021 by Thomas Uehlinger, 2014-2016 by Aaron Stone.