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 bridge between web servers, web frameworks, and web application into a single method call.

The exact details of this are described in the Rack Specification, which all Rack applications should conform to. Browse the Documentation for more information.

Rack 2.2.x is in security maintenance mode, and all support will end in May 2027. Please upgrade to Rack 3.2 as soon as possible to ensure you are receiving the latest bug fixes and security patches.

Please see the Security Policy for more information.

See the Changelog for a detailed list of changes in each version of Rack.

This version of rack is supported with bug fixes and security patches.

This version of rack is supported with security patches only.

This version of rack is no longer supported. It contains significant changes which are detailed in the Upgrade Guide.

This version of Rack is receiving security patches only, and effort should be made to move to Rack 3.2.

Starting in Ruby 3.4 the base64 dependency will no longer be a default gem, and may cause a warning or error about base64 being missing. To correct this, add base64 as a dependency to your project.

Add the rack gem to your application bundle, or follow the instructions provided by a supported web framework:

# Install it generally:
$ gem install rack

# or, add it to your current application gemfile:
$ bundle add rack

If you need features from Rack::Session or bin/rackup please add those gems separately.

$ gem install rack-session rackup

Create a file called config.ru with the following contents:

run do |env|
  [200, {}, ["Hello World"]]
end

Run this using the rackup gem or another supported web server.

$ gem install rackup
$ rackup

# In another shell:
$ curl http://localhost:9292
Hello World

Rack is supported by a wide range of servers, including:

• Agoo
• Falcon
• Iodine
• NGINX Unit
• Phusion Passenger (which is mod_rack for Apache and for nginx)
• Pitchfork
• Puma
• Thin
• Unicorn
• uWSGI
• Lamby (for AWS Lambda)

You will need to consult the server documentation to find out what features and limitations they may have. In general, any valid Rack app will run the same on all these servers, without changing anything.

Rack provides a separate gem, rackup which is a generic interface for running a Rack application on supported servers, which include WEBRick, Puma, Falcon and others.

These frameworks and many others support the Rack Specification:

• Camping
• Hanami
• Ramaze
• Padrino
• Roda
• Ruby on Rails
• Rum
• Sinatra
• Utopia
• WABuR

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

• 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 a content-length header based on body size.
• Rack::ContentType for setting a default content-type header for responses.
• Rack::Deflater for compressing responses with gzip.
• Rack::ETag for setting etag header on bodies that can be buffered.
• Rack::Events for providing easy hooks when a request is received and when the response is sent.
• Rack::Head for returning an empty body for HEAD requests.
• Rack::Lint for checking conformance to the Rack Specification.
• Rack::Lock for serializing requests using a mutex.
• 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 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::Files for serving files under a given directory, without 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.

Rack exposes several configuration parameters to control various features of the implementation.

This environment variable sets the default for the maximum query string bytesize that Rack::QueryParser will attempt to parse. Attempts to use a query string or application/x-www-form-urlencoded request body that exceeds this number of bytes will result in a Rack::QueryParser::QueryLimitError exception. If this enviroment variable is provided, it must be an integer, or Rack::QueryParser will raise an exception. If the value is negative, the bytesize limit is disabled.

The default limit can be overridden on a per-Rack::QueryParser basis using the bytesize_limit keyword argument when creating the Rack::QueryParser. Passing nil for this keyword argument disables the bytesize limit.

This environment variable sets the default for the maximum number of query parameters that Rack::QueryParser will attempt to parse. Attempts to use a query string or application/x-www-form-urlencoded request body with more parameters than this will result in a Rack::QueryParser::QueryLimitError exception. If this enviroment variable is provided, it must be an integer, or Rack::QueryParser will raise an exception. If the value is negative, the params limit is disabled.

The default limit can be overridden on a per-Rack::QueryParser basis using the params_limit keyword argument when creating the Rack::QueryParser. Passing nil for this keyword argument disables the params limit.

This is implemented by counting the number of parameter separators in the query string, before attempting parsing, so if the same parameter key is used multiple times in the query, each counts as a separate parameter for this check.

This environment variable sets the maximum amount of memory Rack will use to buffer multipart parameters when parsing a request body. This considers the size of the multipart mime headers and the body part for multipart parameters that are buffered in memory and do not use tempfiles. This defaults to 16MB if not provided.

Rack::Utils.param_depth_limit = 32 # default

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

?a[b][c]=d

but this query string would not be allowed:

?a[b][c][d]=e

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

Rack::Utils.multipart_file_limit = 128 # default

The maximum number of parts with a filename a request can contain. Accepting too many parts 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_FILE_LIMIT environment variable.

(This is also aliased as multipart_part_limit and RACK_MULTIPART_PART_LIMIT for compatibility)

The maximum total number of parts a request can contain of any type, including both file and non-file form fields.

The default is 4096, which means that a single request can't contain more than 4096 parts.

Set to 0 for no limit.

Can also be set via the RACK_MULTIPART_TOTAL_PART_LIMIT environment variable.

See CONTRIBUTING.md for specific details about how to make a contribution to Rack.

Please post bugs, suggestions and patches to GitHub Issues.

Please check our Security Policy for responsible disclosure and security bug reporting process. 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.

A useful tool for running Rack applications from the command line, including Rackup::Server (previously Rack::Server) for scripting servers.

• https://github.com/rack/rackup

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.

• https://github.com/rack/rack-contrib

Provides convenient session management for Rack.

• https://github.com/rack/rack-session

The Rack Core Team, consisting of

• Aaron Patterson tenderlove
• Samuel Williams ioquatix
• Jeremy Evans jeremyevans
• Eileen Uchitelle eileencodes
• Matthew Draper matthewd
• Rafael França rafaelfranca

and the Rack Alumni

• Ryan Tomayko rtomayko
• Scytrin dai Kinthra scytrin
• Leah Neukirchen leahneukirchen
• James Tucker raggi
• Josh Peek josh
• José Valim josevalim
• Michael Fellinger manveru
• Santiago Pastorino spastorino
• Konstantin Haase rkh

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.

Rack is released under the MIT License.