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.

Version support

| Version | Support |
|----------|------------------------------------|
| 3.2.x | Bug fixes and security patches. |
| 3.1.x | Security patches only. |
| 3.0.x | End of support. |
| 2.2.x | Security patches only. |
| <= 2.1.x | End of support. |

Rack 2.2.x is in security maintenance mode. Please upgrade to Rack 3.1+ as soon
as possible to ensure you are receiving the latest features and security patches.

Please see the [Security Policy] for more information.

Change log

See the [Changelog](CHANGELOG.md) for a detailed list of changes in each version of Rack.

Rack 3.2 (latest release)

This version of rack contains bug fixes and security patches.

Rack 3.1

This version of rack contains bug fixes and security patches.

Rack 3.0

This version of rack contains significant changes which are detailed in the
[Upgrade Guide](UPGRADE-GUIDE.md). It is recommended to upgrade to Rack 3 as soon
as possible to receive the latest features and security patches.

Rack 2.2

This version of Rack is receiving security patches only, and effort should be
made to move to Rack 3.

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.

Installation

Add the rack gem to your application bundle, or follow the instructions provided
by a [supported web framework](#supported-web-frameworks):

bash
# 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.

bash
$ gem install rack-session rackup

Usage

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

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

Run this using the rackup gem or another [supported web
server](#supported-web-servers).

bash
$ gem install rackup
$ rackup

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

Supported web servers

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.

Rackup

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.

Supported web frameworks

These frameworks and many others support the [Rack Specification]:

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

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::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.

Convenience interfaces

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.

Configuration

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

RACK_QUERY_PARSER_BYTESIZE_LIMIT

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
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.

The default limit can be overridden on a per-Rack::QueryParser basis using
the bytesize_limit keyword argument when creating the Rack::QueryParser.

RACK_QUERY_PARSER_PARAMS_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 with more than this many query parameters 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.

The default limit can be overridden on a per-Rack::QueryParser basis using
the params_limit keyword argument when creating the Rack::QueryParser.

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.

RACK_MULTIPART_BUFFERED_UPLOAD_BYTESIZE_LIMIT

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.

param_depth_limit

ruby
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.

multipart_file_limit

ruby
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)

multipart_total_part_limit

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.

Contributing

See [CONTRIBUTING.md](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.

See Also

rackup

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

rack-contrib

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

rack-session

Provides convenient session management for Rack.

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

Thanks

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.

License

Rack is released under the [MIT License](MIT-LICENSE).

[Rack Specification]: https://rack.github.io/rack/main/SPEC_rdoc.html
[Documentation]: https://rack.github.io/rack/
[Security Policy]: SECURITY.md

Collapse