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.
<tt>REQUEST_METHOD</tt>:: The HTTP request method, such as
                          "GET" or "POST". This cannot ever
                          be an empty string, and so is
                          always required.
<tt>SCRIPT_NAME</tt>:: 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.
<tt>PATH_INFO</tt>:: 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.
<tt>QUERY_STRING</tt>:: The portion of the request URL that
                        follows the <tt>?</tt>, if any. May be
                        empty, but is always required!
<tt>SERVER_NAME</tt>:: When combined with <tt>SCRIPT_NAME</tt> and
                       <tt>PATH_INFO</tt>, these variables can be
                       used to complete the URL. Note, however,
                       that <tt>HTTP_HOST</tt>, if present,
                       should be used in preference to
                       <tt>SERVER_NAME</tt> for reconstructing
                       the request URL.
                       <tt>SERVER_NAME</tt> can never be an empty
                       string, and so is always required.
<tt>SERVER_PORT</tt>:: 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.
<tt>SERVER_PROTOCOL</tt>:: A string representing the HTTP version used
                           for the request.
<tt>HTTP_</tt> Variables:: Variables corresponding to the
                           client-supplied HTTP request
                           headers (i.e., variables whose
                           names begin with <tt>HTTP_</tt>). 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:
<tt>rack.url_scheme</tt>:: +http+ or +https+, depending on the
                           request URL.
<tt>rack.input</tt>:: See below, the input stream.
<tt>rack.errors</tt>:: See below, the error stream.
<tt>rack.hijack?</tt>:: See below, if present and true, indicates
                        that the server supports partial hijacking.
<tt>rack.hijack</tt>:: See below, if present, an object responding
                       to +call+ that is used to perform a full
                       hijack.
<tt>rack.protocol</tt>:: An optional +Array+ of +String+, containing
                         the protocols advertised by the client in
                         the +upgrade+ header (HTTP/1) or the
                         +:protocol+ pseudo-header (HTTP/2).
Additional environment specifications have approved to
standardized middleware APIs. None of these are required to
be implemented by the server.
<tt>rack.session</tt>:: 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);
<tt>rack.logger</tt>:: 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)
<tt>rack.multipart.buffer_size</tt>:: An Integer hint to the multipart parser as to what chunk size to use for reads and writes.
<tt>rack.multipart.tempfile_factory</tt>:: 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 <tt>rack.</tt>
is reserved for use with the Rack core distribution and other
accepted specifications and must not be used otherwise.

The <tt>SERVER_PORT</tt> must be an Integer if set.
The <tt>SERVER_NAME</tt> must be a valid authority as defined by RFC7540.
The <tt>HTTP_HOST</tt> must be a valid authority as defined by RFC7540.
The <tt>SERVER_PROTOCOL</tt> must match the regexp <tt>HTTP/\d(\.\d)?</tt>.
The environment must not contain the keys
<tt>HTTP_CONTENT_TYPE</tt> or <tt>HTTP_CONTENT_LENGTH</tt>
(use the versions without <tt>HTTP_</tt>).
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:
* <tt>rack.url_scheme</tt> must either be +http+ or +https+.
* There may be a valid input stream in <tt>rack.input</tt>.
* There must be a valid error stream in <tt>rack.errors</tt>.
* There may be a valid hijack callback in <tt>rack.hijack</tt>
* There may be a valid early hints callback in <tt>rack.early_hints</tt>
* The <tt>REQUEST_METHOD</tt> must be a valid token.
* The <tt>SCRIPT_NAME</tt>, if non-empty, must start with <tt>/</tt>
* The <tt>PATH_INFO</tt>, if provided, must be a valid request target or an empty string.
  * Only <tt>OPTIONS</tt> requests may have <tt>PATH_INFO</tt> set to <tt>*</tt> (asterisk-form).
  * Only <tt>CONNECT</tt> requests may have <tt>PATH_INFO</tt> set to an authority (authority-form). Note that in HTTP/2+, the authority-form is not a valid request target.
  * <tt>CONNECT</tt> and <tt>OPTIONS</tt> requests must not have <tt>PATH_INFO</tt> set to a URI (absolute-form).
  * Otherwise, <tt>PATH_INFO</tt> must start with a <tt>/</tt> and must not include a fragment part starting with '#' (origin-form).
* The <tt>CONTENT_LENGTH</tt>, if given, must consist of digits only.
* One of <tt>SCRIPT_NAME</tt> or <tt>PATH_INFO</tt> must be
  set. <tt>PATH_INFO</tt> should be <tt>/</tt> if
  <tt>SCRIPT_NAME</tt> is empty.
  <tt>SCRIPT_NAME</tt> never should be <tt>/</tt>, but instead be empty.
<tt>rack.response_finished</tt>:: 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.
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 <tt>IO#read</tt>.
  Its signature is <tt>read([length, [buffer]])</tt>.

  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
  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?+.

=== Early Hints

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

If <tt>rack.early_hints</tt> is present, it must respond to #call.
If <tt>rack.early_hints</tt> is called, it must be called with
valid Rack response headers.

== 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+ Header

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

==== The +content-length+ Header

There must not be a <tt>content-length</tt> 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+ 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.

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:
<tt>read, write, <<, flush, close, close_read, close_write, closed?</tt>

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.