This specification aims to formalize the Rack protocol. You can (and should) use Rack::Lint to enforce it.

When you develop middleware, be sure to add a Lint before and after to catch all mistakes.

Rack applications

A Rack application is a Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns a non-frozen Array of exactly three values: The status, the headers, and the body.

The Environment

The environment must be an unfrozen instance of Hash that includes CGI-like headers. The Rack application is free to modify the environment.

The environment is required to include these variables (adopted from PEP 333), except when they’d be empty, but see below.


The HTTP request method, such as “GET” or “POST”. This cannot ever be an empty string, and so is always required.


The initial portion of the request URL’s “path” that corresponds to the application object, so that the application knows its virtual “location”. This may be an empty string, if the application corresponds to the “root” of the server.


The remainder of the request URL’s “path”, designating the virtual “location” of the request’s target within the application. This may be an empty string, if the request URL targets the application root and does not have a trailing slash. This value may be percent-encoded when originating from a URL.


The portion of the request URL that follows the ?, if any. May be empty, but is always required!


When combined with SCRIPT_NAME and PATH_INFO, these variables can be used to complete the URL. Note, however, that HTTP_HOST, if present, should be used in preference to SERVER_NAME for reconstructing the request URL. SERVER_NAME can never be an empty string, and so is always required.


An optional Integer which is the port the server is running on. Should be specified if the server is running on a non-standard port.


A string representing the HTTP version used for the request.

HTTP_ Variables

Variables corresponding to the client-supplied HTTP request headers (i.e., variables whose names begin with HTTP_). The presence or absence of these variables should correspond with the presence or absence of the appropriate HTTP header in the request. See RFC3875 section 4.1.18 for specific behavior.

In addition to this, the Rack environment must include these Rack-specific variables:


http or https, depending on the request URL.


See below, the input stream.


See below, the error stream.


See below, if present and true, indicates that the server supports partial hijacking.


See below, if present, an object responding to call that is used to perform a full hijack.

Additional environment specifications have approved to standardized middleware APIs. None of these are required to be implemented by the server.


A hash-like interface for storing request session data. The store must implement: store(key, value) (aliased as []=); fetch(key, default = nil) (aliased as []); delete(key); clear; to_hash (returning unfrozen Hash instance);


A common object interface for logging messages. The object must implement:

info(message, &block)
debug(message, &block)
warn(message, &block)
error(message, &block)
fatal(message, &block)

An Integer hint to the multipart parser as to what chunk size to use for reads and writes.


An object responding to #call with two arguments, the filename and content_type given for the multipart form field, and returning an IO-like object that responds to #<< and optionally #rewind. This factory will be used to instantiate the tempfile for each multipart form file upload field, rather than the default class of Tempfile.

The server or the application can store their own data in the environment, too. The keys must contain at least one dot, and should be prefixed uniquely. The prefix rack. is reserved for use with the Rack core distribution and other accepted specifications and must not be used otherwise.

The SERVER_PORT must be an Integer if set. The SERVER_NAME must be a valid authority as defined by RFC7540. The HTTP_HOST must be a valid authority as defined by RFC7540. The SERVER_PROTOCOL must match the regexp HTTP/\d(\.\d)?. The environment must not contain the keys HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH (use the versions without HTTP_). The CGI keys (named without a period) must have String values. If the string values for CGI keys contain non-ASCII characters, they should use ASCII-8BIT encoding. There are the following restrictions:

  • rack.url_scheme must either be http or https.

  • There may be a valid input stream in rack.input.

  • There must be a valid error stream in rack.errors.

  • There may be a valid hijack callback in rack.hijack

  • The REQUEST_METHOD must be a valid token.

  • The SCRIPT_NAME, if non-empty, must start with /

  • The PATH_INFO, if non-empty (or the request is something other than OPTIONS *), must start with /

  • The CONTENT_LENGTH, if given, must consist of digits only.

  • One of SCRIPT_NAME or PATH_INFO must be set. PATH_INFO should be / if SCRIPT_NAME is empty. SCRIPT_NAME never should be /, but instead be empty.


An array of callables run by the server after the response has been

processed. This would typically be invoked after sending the response to the client, but it could also be invoked if an error occurs while generating the response or sending the response; in that case, the error argument will be a subclass of Exception. The callables are invoked with env, status, headers, error arguments and should not raise any exceptions. They should be invoked in reverse order of registration.

The Input Stream

The input stream is an IO-like object which contains the raw HTTP POST data. When applicable, its external encoding must be “ASCII-8BIT” and it must be opened in binary mode, for Ruby 1.9 compatibility. The input stream must respond to gets, each, and read.

  • gets must be called without arguments and return a string, or nil on EOF.

  • read behaves like IO#read. Its signature is read([length, [buffer]]).

    If given, length must be a non-negative Integer (>= 0) or nil, and buffer must be a String and may not be nil.

    If length is given and not nil, then this method reads at most length bytes from the input stream.

    If length is not given or nil, then this method reads all data until EOF.

    When EOF is reached, this method returns nil if length is given and not nil, or “” if length is not given or is nil.

    If buffer is given, then the read data will be placed into buffer instead of a newly created String object.

  • each must be called without arguments and only yield Strings.

  • close can be called on the input stream to indicate that the

any remaining input is not needed.

The Error Stream

The error stream must respond to puts, write and flush.

  • puts must be called with a single argument that responds to to_s.

  • write must be called with a single argument that is a String.

  • flush must be called without arguments and must be called in order to make the error appear for sure.

  • close must never be called on the error stream.


The hijacking interfaces provides a means for an application to take control of the HTTP connection. There are two distinct hijack interfaces: full hijacking where the application takes over the raw connection, and partial hijacking where the application takes over just the response body stream. In both cases, the application is responsible for closing the hijacked stream.

Full hijacking only works with HTTP/1. Partial hijacking is functionally equivalent to streaming bodies, and is still optionally supported for backwards compatibility with older Rack versions.

Full Hijack

Full hijack is used to completely take over an HTTP/1 connection. It occurs before any headers are written and causes the request to ignores any response generated by the application.

It is intended to be used when applications need access to raw HTTP/1 connection.

If rack.hijack is present in env, it must respond to call and return an IO instance which can be used to read and write to the underlying connection using HTTP/1 semantics and formatting.

Partial Hijack

Partial hijack is used for bi-directional streaming of the request and response body. It occurs after the status and headers are written by the server and causes the server to ignore the Body of the response.

It is intended to be used when applications need bi-directional streaming.

If rack.hijack? is present in env and truthy, an application may set the special response header rack.hijack to an object that responds to call, accepting a stream argument.

After the response status and headers have been sent, this hijack callback will be invoked with a stream argument which follows the same interface as outlined in “Streaming Body”. Servers must ignore the body part of the response tuple when the rack.hijack response header is present. Using an empty Array instance is recommended.

The special response header rack.hijack must only be set if the request env has a truthy rack.hijack?.

The Response

The Status

This is an HTTP status. It must be an Integer greater than or equal to 100.

The Headers

The headers must be a unfrozen Hash. The header keys must be Strings. Special headers starting “rack.” are for communicating with the server, and must not be sent back to the client. The header must not contain a Status key. Header keys must conform to RFC7230 token specification, i.e. cannot contain non-printable ASCII, DQUOTE or “(),/:;<=>?@[]{}”. Header keys must not contain uppercase ASCII characters (A-Z). Header values must be either a String instance, or an Array of String instances, such that each String instance must not contain characters below 037.

The content-type

There must not be a content-type header key when the Status is 1xx, 204, or 304.

The content-length

There must not be a content-length header key when the Status is 1xx, 204, or 304.

The Body

The Body is typically an Array of String instances, an enumerable that yields String instances, a Proc instance, or a File-like object.

The Body must respond to each or call. It may optionally respond to to_path or to_ary. A Body that responds to each is considered to be an Enumerable Body. A Body that responds to call is considered to be a Streaming Body.

A Body that responds to both each and call must be treated as an Enumerable Body, not a Streaming Body. If it responds to each, you must call each and not call. If the Body doesn’t respond to each, then you can assume it responds to call.

The Body must either be consumed or returned. The Body is consumed by optionally calling either each or call. Then, if the Body responds to close, it must be called to release any resources associated with the generation of the body. In other words, close must always be called at least once; typically after the web server has sent the response to the client, but also in cases where the Rack application makes internal/virtual requests and discards the response.

After calling close, the Body is considered closed and should not be consumed again. If the original Body is replaced by a new Body, the new Body must also consume the original Body by calling close if possible.

If the Body responds to to_path, it must return a String path for the local file system whose contents are identical to that produced by calling each; this may be used by the server as an alternative, possibly more efficient way to transport the response. The to_path method does not consume the body.

Enumerable Body

The Enumerable Body must respond to each. It must only be called once. It must not be called after being closed, and must only yield String values.

The Body itself should not be an instance of String, as this will break in Ruby 1.9.

Middleware must not call each directly on the Body. Instead, middleware can return a new Body that calls each on the original Body, yielding at least once per iteration.

If the Body responds to to_ary, it must return an Array whose contents are identical to that produced by calling each. Middleware may call to_ary directly on the Body and return a new Body in its place. In other words, middleware can only process the Body directly if it responds to to_ary. If the Body responds to both to_ary and close, its implementation of to_ary must call close.

Streaming Body

The Streaming Body must respond to call. It must only be called once. It must not be called after being closed. It takes a stream argument.

The stream argument must implement: read, write, <<, flush, close, close_read, close_write, closed?

The semantics of these IO methods must be a best effort match to those of a normal Ruby IO or Socket object, using standard arguments and raising standard exceptions. Servers are encouraged to simply pass on real IO objects, although it is recognized that this approach is not directly compatible with HTTP/2.


Some parts of this specification are adopted from PEP 333 – Python Web Server Gateway Interface v1.0 I’d like to thank everyone involved in that effort.