Rack, a modular Ruby webserver interface

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

The exact details of this are described in the Rack specification, which all Rack applications should conform to.

Supported web servers

The included handlers connect all kinds of web servers to Rack:

  • Mongrel

  • EventedMongrel

  • SwiftipliedMongrel

  • WEBrick

  • FCGI

  • CGI

  • SCGI

  • LiteSpeed

  • Thin

These web servers include Rack handlers in their distributions:

  • Ebb

  • Fuzed

  • Glassfish v3

  • Phusion Passenger (which is mod_rack for Apache and for nginx)

  • Rainbows!

  • Unicorn

  • Zbatery

Any valid Rack app will run the same on all these handlers, without changing anything.

Supported web frameworks

These frameworks include Rack adapters in their distributions:

  • Camping

  • Coset

  • Halcyon

  • Mack

  • Maveric

  • Merb

  • Racktools::SimpleApplication

  • Ramaze

  • Ruby on Rails

  • Rum

  • Sinatra

  • Sin

  • Vintage

  • Waves

  • Wee

  • … and many others.

Current links to these projects can be found at wiki.ramaze.net/Home#other-frameworks

Available middleware

Between the server and the framework, Rack can be customized to your applications needs using middleware, for example:

  • Rack::URLMap, to route to multiple applications inside the same process.

  • Rack::CommonLogger, for creating Apache-style logfiles.

  • Rack::ShowException, for catching unhandled exceptions and presenting them in a nice and helpful way with clickable backtrace.

  • Rack::File, for serving static files.

  • …many others!

All these components use the same interface, which is described in detail in the Rack specification. These optional components can be used in any way you wish.


If you want to develop outside of existing frameworks, implement your own ones, or develop middleware, Rack provides many helpers to create Rack applications quickly and without doing the same web stuff all over:

  • Rack::Request, which also provides query string parsing and multipart handling.

  • Rack::Response, for convenient generation of HTTP replies and cookie handling.

  • Rack::MockRequest and Rack::MockResponse for efficient and quick testing of Rack application without real HTTP round-trips.


The plethora of useful middleware created the need for a project that collects fresh Rack middleware. rack-contrib includes a variety of add-on components for Rack and it is easy to contribute new modules.


rackup is a useful tool for running Rack applications, which uses the Rack::Builder DSL to configure middleware and build up applications easily.

rackup automatically figures out the environment it is run in, and runs your application as FastCGI, CGI, or standalone with Mongrel or WEBrick—all from the same configuration.

Quick start

Try the lobster!

Either with the embedded WEBrick starter:

ruby -Ilib lib/rack/lobster.rb

Or with rackup:

bin/rackup -Ilib example/lobster.ru

By default, the lobster is found at localhost:9292.

Installing with RubyGems

A Gem of Rack is available at gemcutter.org. You can install it with:

gem install rack

I also provide a local mirror of the gems (and development snapshots) at my site:

gem install rack --source http://chneukirchen.org/releases/gems/

Running the tests

Testing Rack requires the bacon testing framework:

gem install bacon

There are two rake-based test tasks:

rake test       tests all the fast tests (no Handlers or Adapters)
rake fulltest   runs all the tests

The fast testsuite has no dependencies outside of the core Ruby installation and bacon.

To run the test suite completely, you need:

* fcgi
* memcache-client
* mongrel
* thin

The full set of tests test FCGI access with lighttpd (on port 9203) so you will need lighttpd installed as well as the FCGI libraries and the fcgi gem:

Download and install lighttpd:


Installing the FCGI libraries:

curl -O http://www.fastcgi.com/dist/fcgi-2.4.0.tar.gz
tar xzvf fcgi-2.4.0.tar.gz
cd fcgi-2.4.0
./configure --prefix=/usr/local
sudo make install
cd ..

Installing the Ruby fcgi gem:

gem install fcgi

Furthermore, to test Memcache sessions, you need memcached (will be run on port 11211) and memcache-client installed.


  • March 3rd, 2007: First public release 0.1.

  • May 16th, 2007: Second public release 0.2.

    • HTTP Basic authentication.

    • Cookie Sessions.

    • Static file handler.

    • Improved Rack::Request.

    • Improved Rack::Response.

    • Added Rack::ShowStatus, for better default error messages.

    • Bug fixes in the Camping adapter.

    • Removed Rails adapter, was too alpha.

  • February 26th, 2008: Third public release 0.3.

    • LiteSpeed handler, by Adrian Madrid.

    • SCGI handler, by Jeremy Evans.

    • Pool sessions, by blink.

    • OpenID authentication, by blink.

    • :Port and :File options for opening FastCGI sockets, by blink.

    • Last-Modified HTTP header for Rack::File, by blink.

    • Rack::Builder#use now accepts blocks, by Corey Jewett. (See example/protectedlobster.ru)

    • HTTP status 201 can contain a Content-Type and a body now.

    • Many bugfixes, especially related to Cookie handling.

  • August 21st, 2008: Fourth public release 0.4.

    • New middleware, Rack::Deflater, by Christoffer Sawicki.

    • OpenID authentication now needs ruby-openid 2.

    • New Memcache sessions, by blink.

    • Explicit EventedMongrel handler, by Joshua Peek <[email protected]>

    • Rack::Reloader is not loaded in rackup development mode.

    • rackup can daemonize with -D.

    • Many bugfixes, especially for pool sessions, URLMap, thread safety and tempfile handling.

    • Improved tests.

    • Rack moved to Git.

  • January 6th, 2009: Fifth public release 0.9.

    • Rack is now managed by the Rack Core Team.

    • Rack::Lint is stricter and follows the HTTP RFCs more closely.

    • Added ConditionalGet middleware.

    • Added ContentLength middleware.

    • Added Deflater middleware.

    • Added Head middleware.

    • Added MethodOverride middleware.

    • Rack::Mime now provides popular MIME-types and their extension.

    • Mongrel Header now streams.

    • Added Thin handler.

    • Official support for swiftiplied Mongrel.

    • Secure cookies.

    • Made HeaderHash case-preserving.

    • Many bugfixes and small improvements.

  • January 9th, 2009: Sixth public release 0.9.1.

    • Fix directory traversal exploits in Rack::File and Rack::Directory.

  • April 25th, 2009: Seventh public release 1.0.0.

    • SPEC change: Rack::VERSION has been pushed to [1,0].

    • SPEC change: header values must be Strings now, split on “n”.

    • SPEC change: Content-Length can be missing, in this case chunked transfer encoding is used.

    • SPEC change: rack.input must be rewindable and support reading into a buffer, wrap with Rack::RewindableInput if it isn't.

    • SPEC change: rack.session is now specified.

    • SPEC change: Bodies can now additionally respond to #to_path with a filename to be served.

    • NOTE: String bodies break in 1.9, use an Array consisting of a single String instead.

    • New middleware Rack::Lock.

    • New middleware Rack::ContentType.

    • Rack::Reloader has been rewritten.

    • Major update to Rack::Auth::OpenID.

    • Support for nested parameter parsing in Rack::Response.

    • Support for redirects in Rack::Response.

    • HttpOnly cookie support in Rack::Response.

    • The Rakefile has been rewritten.

    • Many bugfixes and small improvements.

  • October 18th, 2009: Eighth public release 1.0.1.

    • Bump remainder of rack.versions.

    • Support the pure Ruby FCGI implementation.

    • Fix for form names containing “=”: split first then unescape components

    • Fixes the handling of the filename parameter with semicolons in names.

    • Add anchor to nested params parsing regexp to prevent stack overflows

    • Use more compatible gzip write api instead of “<<”.

    • Make sure that Reloader doesn't break when executed via ruby -e

    • Make sure WEBrick respects the :Host option

    • Many Ruby 1.9 fixes.

  • January 3rd, 2010: Ninth public release 1.1.0.

    • Moved Auth::OpenID to rack-contrib.

    • SPEC change that relaxes Lint slightly to allow subclasses of the required types

    • SPEC change to document rack.input binary mode in greator detail

    • SPEC define optional rack.logger specification

    • File servers support X-Cascade header

    • Imported Config middleware

    • Imported ETag middleware

    • Imported Runtime middleware

    • Imported Sendfile middleware

    • New Logger and NullLogger middlewares

    • Added mime type for .ogv and .manifest.

    • Don't squeeze PATH_INFO slashes

    • Use Content-Type to determine POST params parsing

    • Update Rack::Utils::HTTP_STATUS_CODES hash

    • Add status code lookup utility

    • Response should call #to_i on the status

    • Add Request#user_agent

    • Request#host knows about forwared host

    • Return an empty string for Request#host if HTTP_HOST and SERVER_NAME are both missing

    • Allow MockRequest to accept hash params

    • Optimizations to HeaderHash

    • Refactored rackup into Rack::Server

    • Added Utils.build_nested_query to complement Utils.parse_nested_query

    • Added Utils::Multipart.build_multipart to complement Utils::Multipart.parse_multipart

    • Extracted set and delete cookie helpers into Utils so they can be used outside Response

    • Extract parse_query and parse_multipart in Request so subclasses can change their behavior

    • Enforce binary encoding in RewindableInput

    • Set correct external_encoding for handlers that don't use RewindableInput

  • June 13th, 2010: Tenth public release 1.2.0.

    • Removed Camping adapter: Camping 2.0 supports Rack as-is

    • Removed parsing of quoted values

    • Add Request.trace? and Request.options?

    • Add mime-type for .webm and .htc


    • Various multipart fixes

    • Switch test suite to bacon

  • June 15th, 2010: Eleventh public release 1.2.1.

    • Make CGI handler rewindable

    • Rename spec/ to test/ to not conflict with SPEC on lesser operating systems


Please post bugs, suggestions and patches to the bug tracker at <github.com/rack/rack/issues>.

Mailing list archives are available at <groups.google.com/group/rack-devel>.

Git repository (send Git patches to the mailing list):

You are also welcome to join the #rack channel on irc.freenode.net.


The Rack Core Team, consisting of

  • Christian Neukirchen (chneukirchen)

  • James Tucker (raggi)

  • Josh Peek (josh)

  • Michael Fellinger (manveru)

  • Ryan Tomayko (rtomayko)

  • Scytrin dai Kinthra (scytrin)

would like to thank:

  • Adrian Madrid, for the LiteSpeed handler.

  • Christoffer Sawicki, for the first Rails adapter and Rack::Deflater.

  • Tim Fletcher, for the HTTP authentication code.

  • Luc Heinrich for the Cookie sessions, the static file handler and bugfixes.

  • Armin Ronacher, for the logo and racktools.

  • Alex Beregszaszi, Alexander Kahn, Anil Wadghule, Aredridel, Ben Alpert, Dan Kubb, Daniel Roethlisberger, Matt Todd, Tom Robinson, Phil Hagelberg, S. Brent Faulkner, Bosko Milekic, Daniel Rodríguez Troitiño, Genki Takiuchi, Geoffrey Grosenbach, Julien Sanchez, Kamal Fariz Mahyuddin, Masayoshi Takahashi, Patrick Aljordm, Mig, Kazuhiro Nishiyama, Jon Bardin, Konstantin Haase, Larry Siden, Matias Korhonen, Sam Ruby, Simon Chiang, Tim Connor, Timur Batyrshin, and Zach Brock for bug fixing and other improvements.

  • Eric Wong, Hongli Lai, Jeremy Kemper for their continuous support and API improvements.

  • Yehuda Katz and Carl Lerche for refactoring rackup.

  • Brian Candler, for Rack::ContentType.

  • Graham Batty, for improved handler loading.

  • Stephen Bannasch, for bug reports and documentation.

  • Gary Wright, for proposing a better Rack::Response interface.

  • Jonathan Buch, for improvements regarding Rack::Response.

  • Armin Röhrl, for tracking down bugs in the Cookie generator.

  • Alexander Kellett for testing the Gem and reviewing the announcement.

  • Marcus Rückert, for help with configuring and debugging lighttpd.

  • The WSGI team for the well-done and documented work they've done and Rack builds up on.

  • All bug reporters and patch contributers not mentioned above.

Copyright (C) 2007, 2008, 2009, 2010 Christian Neukirchen <purl.org/net/chneukirchen>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.




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