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 an Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns an Array of exactly three values: The status, the headers, and the body.

The Environment

The environment must be an true instance of Hash (no subclassing allowed) that includes CGI-like headers. The application is free to modify the environment. The environment is required to include these variables (adopted from PEP333), except when they‘d be empty, but see below.

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.
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.
QUERY_STRING:	The portion of the request URL that follows the ?, if any. May be empty, but is always required!
SERVER_NAME, SERVER_PORT:	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 and SERVER_PORT can never be empty strings, and so are always required.
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.

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

rack.version:	The Array [0,1], representing this version of Rack.
rack.url_scheme:	http or https, depending on the request URL.
rack.input:	See below, the input stream.
rack.errors:	See below, the error stream.
rack.multithread:	true if the application object may be simultaneously invoked by another thread in the same process, false otherwise.
rack.multiprocess:	true if an equivalent application object may be simultaneously invoked by another process, false otherwise.
rack.run_once:	true if the server expects (but does not guarantee!) that the application will only be invoked this one time during the life of its containing process. Normally, this will only be true for a server based on CGI (or something similar).

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 must not be used otherwise. 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. There are the following restrictions:

• rack.version must be an array of Integers.
• rack.url_scheme must either be http or https.
• There must be a valid input stream in rack.input.
• There must be a valid error stream in rack.errors.
• The REQUEST_METHOD must be a valid token.
• The SCRIPT_NAME, if non-empty, must start with /
• The PATH_INFO, if non-empty, 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.

The Input Stream

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 must be called without or with one integer argument and return a string, or nil on EOF.
• each must be called without arguments and only yield Strings.
• close must never be called on the input stream.

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 Response

The Status

The status, if parsed as integer (to_i), must be greater than or equal to 100.

The Headers

The header must respond to each, and yield values of key and value. The header keys must be Strings. The header must not contain a Status key, contain keys with : or newlines in their name, contain keys names that end in - or _, but only contain keys that consist of letters, digits, _ or - and start with a letter. The values of the header must respond to each. The values passed on each must be Strings and not contain characters below 037.

The Content-Type

There must be a Content-Type, except when the Status is 1xx, 204 or 304, in which case there must be none given.

The Content-Length

There must be a Content-Length, except when the Status is 1xx, 204 or 304, in which case there must be none given.

The Body

The Body must respond to each and must only yield String values. If the Body responds to close, it will be called after iteration. The Body commonly is an Array of Strings, the application instance itself, or a File-like object.

Thanks

Some parts of this specification are adopted from PEP333: Python Web Server Gateway Interface v1.0 (www.python.org/dev/peps/pep-0333/). I‘d like to thank everyone involved in that effort.