app.rb

require 'sinatra'

# To see how it work run:
# thin -d -p 9494 start
# With commented exit! method thin will not even create pidfile and it won't be able to stop app by thin stop.
# With uncommented exit! method pid file will be created in tmp/pids/thin.pid
get "/at_exit_example" do
  fork do
    # it isn't improtant what to do in fork, just wait
    sleep(5)
    puts "Fork ended"

    # comment/uncomment this
    #exit!
  end
  [200, "Fork example"]
end

config.ru

require 'app'

run Sinatra::Application

Gemfile

source "http://rubygems.org"

gem "sinatra"
gem "thin"