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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 can connect these web servers to Rack:

These web servers include Rack handlers in their distributions:

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

Supported web frameworks

These frameworks and many others support the Rack API:

Available middleware shipped with Rack

Between the server and the framework, Rack can be customized to your applications needs using middleware. Rack itself ships with the following middleware:

  • Rack::Chunked, for streaming responses using chunked encoding.

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

  • Rack::ConditionalGet, for returning not modified responses when the response has not changed.

  • Rack::Config, for modifying the environment before processing the request.

  • Rack::ContentLength, for setting content-length header based on body size.

  • Rack::ContentType, for setting default content-type header for responses.

  • Rack::Deflater, for compressing responses with gzip.

  • Rack::ETag, for setting ETag header on string bodies.

  • Rack::Events, for providing easy hooks when a request is received and when the response is sent.

  • Rack::Files, for serving static files.

  • Rack::Head, for returning an empty body for HEAD requests.

  • Rack::Lint, for checking conformance to the Rack API.

  • Rack::Lock, for serializing requests using a mutex.

  • Rack::Logger, for setting a logger to handle logging errors.

  • Rack::MethodOverride, for modifying the request method based on a submitted parameter.

  • Rack::Recursive, for including data from other paths in the application, and for performing internal redirects.

  • Rack::Reloader, for reloading files if they have been modified.

  • Rack::Runtime, for including a response header with the time taken to process the request.

  • Rack::Sendfile, for working with web servers that can use optimized file serving for file system paths.

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

  • Rack::ShowStatus, for using nice error pages for empty client error responses.

  • Rack::Static, for more configurable serving of static files.

  • Rack::TempfileReaper, for removing temporary files creating during a request.

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.

  • Rack::Cascade, for trying additional Rack applications if an application returns a not found or method not supported response.

  • Rack::Directory, for serving files under a given directory, with directory indexes.

  • Rack::MediaType, for parsing content-type headers.

  • Rack::Mime, for determining content-type based on file extension.

  • Rack::RewindableInput, for making any IO object rewindable, using a temporary file buffer.

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


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 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/

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

Installing with RubyGems

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

gem install rack


You should require the library:

require 'rack'

Rack uses autoload to automatically load other files Rack ships with on demand, so you should not need require paths under rack. If you require paths under rack without requiring rack itself, things may not work correctly.


Several parameters can be modified on Rack::Utils to configure Rack behaviour.


Rack::Utils.param_depth_limit = 3


The maximum amount of nesting allowed in parameters. For example, if set to 3, this query string would be allowed:


but this query string would not be allowed:


Limiting the depth prevents a possible stack overflow when parsing parameters.

Defaults to 32.


The maximum number of parts a request can contain. Accepting too many part can lead to the server running out of file handles.

The default is 128, which means that a single request can't upload more than 128 files at once.

Set to 0 for no limit.

Can also be set via the RACK_MULTIPART_PART_LIMIT environment variable.


No longer has an effect, deprecated.






Please post bugs, suggestions and patches to the bug tracker at issues.

Please post security related bugs and suggestions to the core team at <!forum/rack-core> or [email protected] This list is not public. Due to wide usage of the library, it is strongly preferred that we manage timing in order to provide viable patches at the time of disclosure. Your assistance in this matter is greatly appreciated.

Mailing list archives are available at <!forum/rack-devel>.

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

You are also welcome to join the #rack channel on


The Rack Core Team, consisting of

and the Rack Alumni

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 contributors not mentioned above.



Official Rack repositories


Rack Bug Tracking


rack-devel mailing list



Rack is released under the MIT License.