Mount Cuba App in Rails 4

Create the "Hello World" Cuba Rack app.

# lib/cuba_app.rb
require 'cuba'

class CubaApp < Cuba; end

CubaApp.define do
  on get do
    on root do
      res.write 'Saludos desde Cuba'
    end
  end
end

Mount the Cuba app at a defined location.

# config/routes.rb
Rails.application.routes.draw do
  mount CubaApp, at: '/cuba'
end

Load Ruby files in the lib folder.

# config/initializers/lib.rb
Dir["#{Rails.root}/lib/*.rb"].each { |file| load file }

Start your Rails application and visit http://lvh.me:3000/cuba

For more information about Cuba, visit http://cuba.is or call your local travel agent.

Cite:
http://guides.rubyonrails.org/routing.html#routing-to-rack-applications
https://github.com/soveran/cuba#composition