hello_rack.rb

#! /usr/bin/env ruby
#  encoding: utf-8

require 'rack'

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

  def call(env)
    code, headers, body = @app.call(env)
    body << 'this is my middleware'
    [code, headers, body]
  end
end

class MyApp
  def call(env)
    [200, {'Content-Type' => 'text/plain'}, ['hello world']]
  end
end

app = Rack::Builder.app do 
  map '/' do
    run MyApp.new
  end
  map '/middleware' do
    use MyMiddleware
    run MyApp.new
  end
end

run app