I recently had the need to separate some logic from my main web application, after thinking about it for a while (months actually) I finally got an illumination: what about rack ?
After a quick test aside of my project I confirmed that building a rack middleware is really easy ( even easier that I thought ), this article is a quick tutorial to build a simple middleware.

What is rack ?

Rack is a library used in the Ruby to standardize the interactions between the different frameworks we have and the web server themselves, thanks to Rack we can now build part of an application in sinatra, another in rails, etc...
Rack is a really nice piece of software but I doubt many out there really used it directly, I knew it for quite a long time now but never really felt the need to dig in it.

You can learn more about Rack rack

Our middleware

The middleware will simply serve an url with sinatra, I consider this simple enough to get anyone interested started, based on this you can do nearly anything.
The middleware will also accept parameters since I had to do some search to found out how to do it.

require 'rack'
require 'sinatra/base'

class AboutApp < Sinatra::Base
  def initialize(app, opts)
    @name = opts.delete(:name)
  end

  get '/about' do
    "Hello, my name is #{@str} !"
  end
end

And here is a config.ru file showing how to use it:

require 'about_app'

# this class will act as our main application
class MyApp < Sinatra::Base
  get '/test' do
    "Test it yourself !"
  end
end


use AboutApp, :name => "Julien"
run MyApp

And that is really all you need, now you can run your application with any rack compliant web server which basically means you can run it on every web server supporting ruby ;)

For example:

$ unicorn