広島岡山交流会01 でのサンプルコード http://hirosimaokayamarb.doorkeeper.jp/events/8993

app.rb

require 'action_dispatch'

header = {'Content-Type' => 'text/html'}

routes = ActionDispatch::Routing::RouteSet.new
routes.draw do
  get 'hello', to: -> env { [200, header, ['hello']] }
  get 'goodbye', to: -> env { [200, header, ['goobye']] }
  root to: -> env { [200, header, ['Hiroshima Okayama rb']]}
end

Rack::Handler.default.run routes, Port: 3000