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}[https://peps.python.org/pep-0333/]), 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. This value may be
percent-encoded when originating from
a URL.
QUERY_STRING:: The portion of the request URL that
follows the ?, if any. May be
empty, but is always required!
SERVER_NAME:: 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.
SERVER_PORT:: 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.
SERVER_PROTOCOL:: 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}[https://tools.ietf.org/html/rfc3875#section-4.1.18]
for specific behavior.
In addition to this, the Rack environment must include these
Rack-specific variables:
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.hijack?:: See below, if present and true, indicates
that the server supports partial hijacking.
rack.hijack:: 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.
rack.session:: 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);
rack.logger:: 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)
rack.multipart.buffer_size:: An Integer hint to the multipart parser as to what chunk size to use for reads and writes.
rack.multipart.tempfile_factory:: 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)?.
If the HTTP_VERSION is present, it must equal the SERVER_PROTOCOL.
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, 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.
rack.response_finished:: 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.
=== 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 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.
== Thanks
Some parts of this specification are adopted from {PEP 333 – Python Web Server Gateway Interface v1.0}[https://peps.python.org/pep-0333/]
I'd like to thank everyone involved in that effort.