Rack provides a modular and adaptable interface for developing web applications in Ruby. By wrapping HTTP requests and responses it unifies the API for web servers, web frameworks, and software in between (the so-called middleware) into a single method call.

Rack is used by many Ruby web frameworks and libraries, such as Ruby On Rails and Sinatra. It is available as a Ruby Gem.

Rack has already inspired a JavaScript framework(jackjs) and a Perl one (Plack), a Common Lisp one(Clack), and has resulted in the Ruby developer quasi-standard of “rack-compliant”