Rack Specification¶ ↑

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 test with Rack::Lint to catch possible violations of this specification.

The Application¶ ↑

A Rack application is a Ruby object that responds to call. It takes exactly one argument, the environment (representing an HTTP request) and returns a non-frozen Array of exactly three elements: the status, the headers, and the body (representing an HTTP response).

The Request Environment¶ ↑

Incoming HTTP requests are represented using an environment. The environment must be an unfrozen instance of Hash. The Rack application is free to modify the environment, but the modified environment should also comply with this specification. All environment keys must be strings.

CGI Variables¶ ↑

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

The CGI keys (named without a period) must have String values and are reserved for the Rack specification. If the values for CGI keys contain non-ASCII characters, they should use ASCII-8BIT encoding.

The server and 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 specification and the classes that ship with Rack.

REQUEST_METHOD¶ ↑

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

SCRIPT_NAME¶ ↑

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. If non-empty, the string must start with /, but should not end with /.

In addition, SCRIPT_NAME MUST not be /, but instead be empty, and one of SCRIPT_NAME or PATH_INFO must be set, e.g. PATH_INFO can be / if SCRIPT_NAME is empty.

PATH_INFO¶ ↑

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 PATH_INFO, if provided, must be a valid request target or an empty string, as defined by RFC9110.

• Only OPTIONS requests may have PATH_INFO set to * (asterisk-form).

• Only CONNECT requests may have PATH_INFO set to an authority (authority-form). Note that in HTTP/2+, the authority-form is not a valid request target.

• CONNECT and OPTIONS requests must not have PATH_INFO set to a URI (absolute-form).

• Otherwise, PATH_INFO must start with a / and must not include a fragment part starting with # (origin-form).

QUERY_STRING¶ ↑

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

SERVER_NAME¶ ↑

Must be a valid host, as defined by RFC3986.

When combined with SCRIPT_NAME, PATH_INFO, and QUERY_STRING, these variables can be used to reconstruct the original the request URL. Note, however, that HTTP_HOST, if present, should be used in preference to SERVER_NAME for reconstructing the request URL.

SERVER_PROTOCOL¶ ↑

The HTTP version used for the request. It must match the regular expression HTTP\/\d(\.\d)?.

SERVER_PORT¶ ↑

The port the server is running on, if the server is running on a non-standard port. It must consist of digits only.

The standard ports are:

• 80 for HTTP

• 443 for HTTPS

CONTENT_TYPE¶ ↑

The optional MIME type of the request body, if any.

CONTENT_LENGTH¶ ↑

The length of the request body, if any. It must consist of digits only.

HTTP_HOST¶ ↑

An optional HTTP authority, as defined by RFC9110.

HTTP_ Headers¶ ↑

Unless specified above, the environment can contain any number of additional headers, each starting with HTTP_. The presence or absence of these variables should correspond with the presence or absence of the appropriate HTTP header in the request, and those headers have no specific interpretation or validation by the Rack specification. However, there are many standard HTTP headers that have a specific meaning in the context of a request; see RFC3875 section 4.1.18 for more details.

For compatibility with the CGI specifiction, the environment must not contain the keys HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH. Instead, the keys CONTENT_TYPE and CONTENT_LENGTH must be used.

Rack-Specific Variables¶ ↑

In addition to CGI variables, the Rack environment includes Rack-specific variables. These variables are prefixed with rack. and are reserved for use by the Rack specification, or by the classes that ship with Rack.

rack.url_scheme¶ ↑

The URL scheme, which must be one of http, https, ws or wss. This can never be an empty string, and so is always required. The scheme should be set according to the last hop. For example, if a client makes a request to a reverse proxy over HTTPS, but the connection between the reverse proxy and the server is over plain HTTP, the reverse proxy should set rack.url_scheme to http.

rack.protocol¶ ↑

An optional Array of String values, containing the protocols advertised by the client in the upgrade header (HTTP/1) or the :protocol pseudo-header (HTTP/2+).

rack.session¶ ↑

An optional Hash-like interface for storing request session data. The store must implement:

• store(key, value) (aliased as []=) to set a value for a key,

• fetch(key, default = nil) (aliased as []) to retrieve a value for a key,

• delete(key) to delete a key,

• clear to clear the session,

• to_hash (optional) to retrieve the session as a Hash.

rack.logger¶ ↑

An optional Logger-like interface for logging messages. The logger must implement:

• info(message, &block),

• debug(message, &block),

• warn(message, &block),

• error(message, &block),

• fatal(message, &block).

rack.multipart.buffer_size¶ ↑

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

rack.multipart.tempfile_factory¶ ↑

An optional object for constructing temporary files for multipart form data. The factory must implement:

• call(filename, content_type) to create a temporary file for a multipart form field.

The factory must return an IO-like object that responds to << and optionally rewind.

rack.hijack?¶ ↑

If present and true, indicates that the server supports partial hijacking. See the section below on hijacking for more information.

rack.hijack¶ ↑

If present, an object responding to call that is used to perform a full hijack. See the section below on hijacking for more information.

rack.early_hints¶ ↑

If present, an object responding to call that is used to send early hints. See the section below on early hints for more information.

rack.input¶ ↑

If present, the input stream. See the section below on the input stream for more information.

rack.errors¶ ↑

The error stream. See the section below on the error stream for more information.

rack.response_finished¶ ↑

If present, an array of callables that will be run by the server after the response has been processed. The callables are called with environment, status, headers, error arguments and should not raise any exceptions. The callables would typically be called after sending the response to the client, but it could also be called if an error occurs while generating the response or sending the response (in that case, the error argument will be a kind of Exception). The callables will be invoked in reverse order.

The Input Stream¶ ↑

The input stream is an IO-like object which contains the raw HTTP request data. When applicable, its external encoding must be “ASCII-8BIT” and it must be opened in binary mode. 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 String values.

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

Hijacking¶ ↑

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 server to ignore any response generated by the application. It is intended to be used when applications need access to the 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?.

Early Hints¶ ↑

The application or any middleware may call the rack.early_hints with an object which would be valid as the headers of a Rack response.

If rack.early_hints is present, it must respond to call. If rack.early_hints is called, it must be called with valid Rack response headers.

The Response¶ ↑

Outgoing HTTP responses are generated from the response tuple generated by the application. The response tuple is an Array of three elements, which are: the HTTP status, the headers, and the response body. The Rack application is responsible for ensuring that the response tuple is well-formed and should follow the rules set out in this specification.

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 String objects. Special headers starting rack. are for communicating with the server, and must not be sent back to the client.

• The headers 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 value, or an Array of String values, such that each String value must not contain characters with an ASCII ordinal below 040 (32).

The content-type Header¶ ↑

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

The content-length Header¶ ↑

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

The rack.protocol Header¶ ↑

If the rack.protocol header is present, it must be a String, and must be one of the values from the rack.protocol array from the environment.

Setting this value informs the server that it should perform a connection upgrade. In HTTP/1, this is done using the upgrade header. In HTTP/2, this is done by accepting the request.

The Body¶ ↑

The Body is typically an Array of String values, an enumerable that yields String values, 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 either nil or a String. If a String is returned, it must be a 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.

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 respond to: read, write, <<, flush, close, close_read, close_write, and 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 may simply pass on real IO objects to the Streaming Body. In some cases (e.g. when using transfer-encoding or HTTP/2+), the server may need to provide a wrapper that implements the required methods, in order to provide the correct semantics.

Thanks¶ ↑

We’d like to thank everyone who has contributed to the Rack project over the years. Your work has made this specification possible. That includes everyone who has contributed code, documentation, bug reports, and feedback. We’d also like to thank the authors of the various web servers, frameworks, and libraries that have implemented the Rack specification. Your work has helped to make the web a better place.

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