generated from master branch on 2012-04-14
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.
A Rack application is a 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 must be an instance of Hash 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.
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 I 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 and SERVER_PORT can never be empty strings, and so are always required.
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:
The Array [1,1], representing this version of Rack.
http or https, depending on the request URL.
See below, the input stream.
See below, the error stream.
true if the application object may be simultaneously invoked by another thread in the same process, false otherwise.
true if an equivalent application object may be simultaneously invoked by another process, false otherwise.
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).
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;
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)
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 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 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, read and rewind.
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.
rewind must be called without arguments. It rewinds the input stream back to the beginning. It must not raise Errno::ESPIPE: that is, it may not be a pipe or a socket. Therefore, handler developers must buffer the input data into some rewindable object if the underlying input stream is not rewindable.
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.
This is an HTTP status. When parsed as integer (to_i), it must be greater than or equal to 100.
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 be Strings, consisting of lines (for multiple header values, e.g. multiple Set-Cookie values) seperated by “n“. The lines must not contain characters below 037.
There must be a Content-Type, except when the Status is 1xx, 204, 205 or 304, in which case there must be none given.
There must not be a Content-Length header when the Status is 1xx, 204, 205 or 304.
The Body must respond to each and must only yield String values. The Body itself should not be an instance of String, as this will break in Ruby 1.9. If the Body responds to close, it will be called after iteration. If the Body responds to to_path, it must return a String identifying the location of a file 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 Body commonly is an Array of Strings, the application instance itself, or a File-like object.
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.