# config.ru run Proc.new {|env| [200, {"Content-Type" => "text/html"}, "Hello Rack!"]} Under the hood, rackup converts your config script to an instance of Rack::Builder. What is Rack::Builder ? Rack::Builder implements a small DSL to iteratively construct Rack applications. - Rack API Docs Rack::Builder is the thing that glues various Rack middlewares and applications together and convert them in