Example of an asynchronous rack based app on heroku

config.ru

# config.ru
#
# The Heroku architecture is based on Thin, which is based on EventMachine.  This means
# you can access the EventMachine API and make use of Thin's asynchronous extension to
# the rack API.
#
# This is a simple asynchronous rack backed app that will hold a request for 4 seconds 
# and respond with "Hello".
#
# In theory an app structured like this should be able to handle a huge number of
# simulatenous requests,  however, the heroku routing mesh seems to throttle your 
# dyno back to 2 or 3 requests at a time.
#
# You could potentially use the EventMachine API as a free alternative to scheduling 
# a job on a worker, although it's probably not a good idea.
#
# This can be run like any other heroku app.  Throw this file into config.ru and 
# "git push heroku master"
run lambda { |env|
  EM.add_timer(4) do
    # Complete the response asychronously
    env['async.callback'].call([200, {'Content-Type' => 'text/plain'}, "hello\r\n"])
  end

  # This informs Thin that the request will be completed asynchronously
  throw :async
}