config.ru

require 'rack'
require 'pry'

class MyApp
  def self.call(env)
    request = Rack::Request.new(env)
    if request.path == '/'
      [200, {"Content-Type" => "text/html"}, ["<h1>You're at the root path!!!!</h1>"]]
    else
      [404, {"Content-Type" => "text/html"}, ["File Not Found"]]
    end
  end
end

class MySecurityApp
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    if request.params['password'] == 'secret'
      @app.call(env)
    else
      [401, {"Content-Type" => "text/html"}, ["Forbidden"]]
    end
  end
end

use MySecurityApp
run MyApp

middleware-config.ru

require 'rack'

class TopApp
  def initialize(app)
    puts "Initialing TopApp"
    puts "Passed app is currently #{app}"
    @app = app
  end

  def call(env)
    puts "Called TopApp"
    @app.call(env)
  end
end

class MiddleApp
  def initialize(app)
    puts "Initialing MiddleApp"
    puts "Passed app is currently #{app}"
    @app = app
  end

  def call(env)
    puts "Called MiddleApp"
    @app.call(env)
  end
end

class BottomApp
  def initialize(app)
    puts "Initialing BottomApp"
    puts "Passed app is currently #{app}"
    @app = app
  end

  def call(env)
    puts "Called BottomApp"
    @app.call(env)
  end
end

class MyApp
  def self.call(env)
    puts "In the main app"
    [200, {'Content-Type' => 'text/html'}, ["My Response"]]
  end
end

use TopApp
use MiddleApp
use BottomApp
run MyApp

sinatra-config.ru

require 'sinatra/base'

class MyMiddleware < Sinatra::Base
    get '/secret' do
      status, header, body = app.call(env)
      body.first.gsub!('ALL', 'SOME')
    end
end

class MyApp < Sinatra::Base
  get '/secret' do
    "ALL THE SECRETS"
  end
end

map '/admin' do
  use MyMiddleware
  run MyApp
end