Warden
Strategies?!
General Rack Authentication Framework
Rack
Ruby on Rails and Sinatra are both built on top of Rack
Provides a minimal interface between webservers supporting Ruby and Ruby frameworks.
The Russian Doll Pattern
class HelloAthega
  def call(env)
    [200, {"Content-Type" => "text/plain"}, ["Hello Athega!"]]
  end
end
Or, turtles all the way down.
The promise of multiple applications running in the same process, sub-applications and sub-sub-applications can be realized.
What?
Warden is a Rack-based middleware, designed to provide a mechanism for authentication in Ruby web applications.
require 'rack/flash'
require 'blog.rb'
require 'app.rb'

use Rack::Flash
use Blog
run App
Why?
How?
Warden sits in the Rack stack, after the session middleware.

Warden injects a lazy object into the Rack environment at env[‘warden’]
use Warden::Manager do |manager|
  manager.default_strategies :password
  manager.failure_app = App
end
Conceptually a strategy is where you put the logic for authenticating a request.
Warden::Strategies.add(:password) do
  def valid?
    params[:username] || params[:password]
  end

  def authenticate!
    u = User.authenticate(params[:username], params[:password])
    u.nil? ? fail!("Could not log in") : success!(u)
  end
end
Sharing is caring!
For example, you may want to throw up a quick test application in Sinatra or straight Rack. 

Using Warden you can share the same strategies from your main application and provide consistent, company wide authentication requirements.
config.ru