====================================
HTTP & WebSocket ASGI Message Format
====================================

**Version**: 2.1 (2019-03-20)

The HTTP+WebSocket ASGI sub-specification outlines how to transport HTTP/1.1,
HTTP/2 and WebSocket connections within ASGI.

It is deliberately intended and designed to be a superset of the WSGI format
and specifies how to translate between the two for the set of requests that
are able to be handled by WSGI.


Spec Versions
-------------

This spec has had two versions:

* ``2.0``: The first version of the spec, released with ASGI 2.0
* ``2.1``: Added the ``headers`` key to the WebSocket Accept response

Spec versions let you understand what the server you are using understands. If
a server tells you it only supports version ``2.0`` of this spec, then
sending ``headers`` with a WebSocket Accept message is an error, for example.

They are separate from the HTTP version or the ASGI version.


HTTP
----

The HTTP format covers HTTP/1.0, HTTP/1.1 and HTTP/2, as the changes in
HTTP/2 are largely on the transport level. A protocol server should give
different scopes to different requests on the same HTTP/2 connection, and
correctly multiplex the responses back to the same stream in which they came.
The HTTP version is available as a string in the scope.

Multiple header fields with the same name are complex in HTTP. RFC 7230
states that for any header field that can appear multiple times, it is exactly
equivalent to sending that header field only once with all the values joined by
commas.

However, RFC 7230 and RFC 6265 make it clear that this rule does not apply to
the various headers used by HTTP cookies (``Cookie`` and ``Set-Cookie``). The
``Cookie`` header must only be sent once by a user-agent, but the
``Set-Cookie`` header may appear repeatedly and cannot be joined by commas.
The ASGI design decision is to transport both request and response headers as
lists of 2-element ``[name, value]`` lists and preserve headers exactly as they
were provided.

The HTTP protocol should be signified to ASGI applications with a ``type``
value of ``http``.


Connection Scope
''''''''''''''''

HTTP connections have a single-request connection scope - that is, your
applications will be instantiated at the start of the request, and destroyed
at the end, even if the underlying socket is still open and serving multiple
requests.

If you hold a response open for long-polling or similar, the scope will
persist until the response closes from either the client or server side.

The connection scope contains:

* ``type`` (*Unicode string*) -- ``"http"``.

* ``asgi["version"]`` (*Unicode string*) -- Version of the ASGI spec.

* ``asgi["spec_version"]`` (*Unicode string*) -- Version of the ASGI HTTP spec
  this server understands; one of ``"2.0"`` or ``"2.1"``. Optional; if missing
  assume ``2.0``.

* ``http_version`` (*Unicode string*) -- One of ``"1.0"``, ``"1.1"`` or ``"2"``.

* ``method`` (*Unicode string*) -- The HTTP method name, uppercased.

* ``scheme`` (*Unicode string*) -- URL scheme portion (likely ``"http"`` or
  ``"https"``). Optional (but must not be empty); default is ``"http"``.

* ``path`` (*Unicode string*) -- HTTP request target excluding any query
  string, with percent-encoded sequences and UTF-8 byte sequences
  decoded into characters.

* ``raw_path`` (*byte string*) -- The original HTTP path component unmodified
  from the bytes that were received by the web server. Some web server
  implementations may be unable to provide this. Optional; defaults to ``None``.

* ``query_string`` (*byte string*) -- URL portion after the ``?``,
  percent-encoded.

* ``root_path`` (*Unicode string*) -- The root path this application
  is mounted at; same as ``SCRIPT_NAME`` in WSGI. Optional; defaults
  to ``""``.

* ``headers`` (*Iterable[[byte string, byte string]]*) -- An iterable of
  ``[name, value]`` two-item iterables, where ``name`` is the header name, and
  ``value`` is the header value. Order of header values must be preserved from
  the original HTTP request; order of header names is not important. Duplicates
  are possible and must be preserved in the message as received. Header names
  must be lowercased. Pseudo headers (present in HTTP/2 and HTTP/3) must be
  removed; if ``:authority`` is present its value must be added to the start of
  the iterable with ``host`` as the header name or replace any existing host
  header already present.

* ``client`` (*Iterable[Unicode string, int]*) -- A two-item iterable of
  ``[host, port]``, where ``host`` the remote host's IPv4 or IPv6 address, and
  ``port`` is the remote port as an integer. Optional; defaults to ``None``.

* ``server`` (*Iterable[Unicode string, int]*) -- A two-item iterable of
  ``[host, port]``, where ``host`` is the listening address for this server,
  and ``port`` is the integer listening port. Optional; defaults to ``None``.


Request
'''''''

Sent to indicate an incoming request. Most of the request information is in
the connection scope; the body message serves as a way to stream large incoming
HTTP bodies in chunks, and as a trigger to actually run request code (as you
should not trigger on a connection opening alone).

Keys:

* ``type`` (*Unicode string*) -- ``"http.request"``.

* ``body`` (*byte string*) -- Body of the request. Optional; defaults to
  ``b""``. If ``more_body`` is set, treat as start of body and concatenate
  on further chunks.

* ``more_body`` (*bool*) -- Signifies if there is additional content
  to come (as part of a Request message). If ``True``, the consuming
  application should wait until it gets a chunk with this set to ``False``. If
  ``False``, the request is complete and should be processed. Optional;
  defaults to ``False``.


Response Start
''''''''''''''

Starts sending a response to the client. Needs to be followed by at least
one response content message. The protocol server must not start sending the
response to the client until it has received at least one *Response Body*
event.

Keys:

* ``type`` (*Unicode string*) -- ``"http.response.start"``.

* ``status`` (*int*) -- HTTP status code.

* ``headers`` (*Iterable[[byte string, byte string]]*) -- A iterable of
  ``[name, value]`` two-item iterables, where ``name`` is the header name, and
  ``value`` is the header value. Order must be preserved in the HTTP response.
  Header names must be lowercased. Optional; defaults to an empty list. Pseudo
  headers (present in HTTP/2 and HTTP/3) must not be present.


Response Body
'''''''''''''

Continues sending a response to the client. Protocol servers must
flush any data passed to them into the send buffer before returning from a
send call. If ``more_body`` is set to ``False`` this will
close the connection.

Keys:

* ``type`` (*Unicode string*) -- ``"http.response.body"``.

* ``body`` (*byte string*) -- HTTP body content. Concatenated onto any previous
  ``body`` values sent in this connection scope. Optional; defaults to
  ``b""``.

* ``more_body`` (*bool*) -- Signifies if there is additional content
  to come (as part of a Response Body message). If ``False``, response will
  be taken as complete and closed, and any further messages on the channel
  will be ignored. Optional; defaults to ``False``.


Disconnect
''''''''''

Sent to the application when a HTTP connection is closed or if ``receive``
is called after a response has been sent. This is mainly useful for
long-polling, where you may want to trigger cleanup code if the
connection closes early.

Keys:

* ``type`` (*Unicode string*) -- ``"http.disconnect"``.


WebSocket
---------

WebSockets share some HTTP details - they have a path and headers - but also
have more state. Again, most of that state is in the scope, which will live
as long as the socket does.

WebSocket protocol servers should handle PING/PONG messages themselves, and
send PING messages as necessary to ensure the connection is alive.

WebSocket protocol servers should handle message fragmentation themselves,
and deliver complete messages to the application.

The WebSocket protocol should be signified to ASGI applications with
a ``type`` value of ``websocket``.


Connection Scope
''''''''''''''''

WebSocket connections' scope lives as long as the socket itself - if the
application dies the socket should be closed, and vice-versa. The scope
contains the initial connection metadata (mostly from the HTTP handshake):

* ``type`` (*Unicode string*) -- ``"websocket"``.

* ``asgi["version"]`` (*Unicode string*) -- The version of the ASGI spec.

* ``asgi["spec_version"]`` (*Unicode string*) -- Version of the ASGI HTTP spec
  this server understands; one of ``"2.0"`` or ``"2.1"``. Optional; if missing
  assume ``"2.0"``.

* ``http_version`` (*Unicode string*) -- One of ``"1.1"`` or ``"2"``. Optional;
  default is ``"1.1"``.

* ``scheme`` (*Unicode string*) -- URL scheme portion (likely ``"ws"`` or
  ``"wss"``). Optional (but must not be empty); default is ``"ws"``.

* ``path`` (*Unicode string*) -- HTTP request target excluding any query
  string, with percent-encoded sequences and UTF-8 byte sequences
  decoded into characters.

* ``raw_path`` (*byte string*) -- The original HTTP path component unmodified
  from the bytes that were received by the web server. Some web server
  implementations may be unable to provide this. Optional; defaults to ``None``.

* ``query_string`` (*byte string*) -- URL portion after the ``?``. Optional;
  default is empty string.

* ``root_path`` (*byte string*) -- The root path this application
  is mounted at; same as ``SCRIPT_NAME`` in WSGI. Optional; defaults
  to empty string.

* ``headers`` (*Iterable[[byte string, byte string]]*) -- An iterable of
  ``[name, value]`` two-item iterables, where ``name`` is the header name and
  ``value`` is the header value. Order should be preserved from the original
  HTTP request; duplicates are possible and must be preserved in the message
  as received. Header names must be lowercased. Pseudo headers (present in
  HTTP/2 and HTTP/3) must be removed; if ``:authority`` is present its value
  must be added to the start of the iterable with ``host`` as the header name
  or replace any existing host header already present.

* ``client`` (*Iterable[Unicode string, int]*) -- A two-item iterable of
  ``[host, port]``, where ``host`` is the remote host's IPv4 or IPv6 address,
  and ``port`` is the remote port. Optional; defaults to ``None``.

* ``server`` (*Iterable[Unicode string, int]*) -- A two-item iterable of
  ``[host, port]``, where ``host`` is the listening address for this server,
  and ``port`` is the integer listening port. Optional; defaults to ``None``.

* ``subprotocols`` (*Iterable[Unicode string]*) -- Subprotocols the client
  advertised. Optional; defaults to empty list.


Connection
''''''''''

Sent when the client initially opens a connection and is about to finish the
WebSocket handshake.

This message must be responded to with either an *Accept* message
or a *Close* message before the socket will pass ``websocket.receive``
messages. The protocol server must send this message
during the handshake phase of the WebSocket and not complete the handshake
until it gets a reply, returning HTTP status code ``403`` if the connection is
denied.

Keys:

* ``type`` (*Unicode string*) -- ``"websocket.connect"``.


Accept
''''''

Sent by the application when it wishes to accept an incoming connection.

* ``type`` (*Unicode string*) -- ``"websocket.accept"``.

* ``subprotocol`` (*Unicode string*) -- The subprotocol the server wishes to
  accept. Optional; defaults to ``None``.

* ``headers`` (*Iterable[[byte string, byte string]]*) -- An iterable of
  ``[name, value]`` two-item iterables, where ``name`` is the header name, and
  ``value`` is the header value. Order must be preserved in the HTTP response.
  Header names must be lowercased. Must not include a header named
  ``sec-websocket-protocol``; use the ``subprotocol`` key instead. Optional;
  defaults to an empty list. *Added in spec version 2.1*. Pseudo headers
  (present in HTTP/2 and HTTP/3) must not be present.


Receive
'''''''

Sent when a data message is received from the client.

Keys:

* ``type`` (*Unicode string*) -- ``"websocket.receive"``.

* ``bytes`` (*byte string*) -- The message content, if it was binary mode, or
  ``None``. Optional; if missing, it is equivalent to ``None``.

* ``text`` (*Unicode string*) -- The message content, if it was text mode, or
  ``None``. Optional; if missing, it is equivalent to ``None``.

Exactly one of ``bytes`` or ``text`` must be non-``None``. One or both
keys may be present, however.


Send
''''

Sends a data message to the client.

Keys:

* ``type`` (*Unicode string*) -- ``"websocket.send"``.

* ``bytes`` (*byte string*) -- Binary message content, or ``None``.
   Optional; if missing, it is equivalent to ``None``.

* ``text`` (*Unicode string*) -- Text message content, or ``None``.
   Optional; if missing, it is equivalent to ``None``.

Exactly one of ``bytes`` or ``text`` must be non-``None``. One or both
keys may be present, however.


Disconnection
'''''''''''''

Sent when either connection to the client is lost, either from the client
closing the connection, the server closing the connection, or loss of the
socket.

Keys:

* ``type`` (*Unicode string*) -- ``"websocket.disconnect"``

* ``code`` (*int*) -- The WebSocket close code, as per the WebSocket spec.


Close
'''''

Tells the server to close the connection.

If this is sent before the socket is accepted, the server
must close the connection with a HTTP 403 error code
(Forbidden), and not complete the WebSocket handshake; this may present on some
browsers as a different WebSocket error code (such as 1006, Abnormal Closure).

If this is sent after the socket is accepted, the server must close the socket
with the close code passed in the message (or 1000 if none is specified).

* ``type`` (*Unicode string*) -- ``"websocket.close"``.

* ``code`` (*int*) -- The WebSocket close code, as per the WebSocket spec.
  Optional; defaults to ``1000``.


WSGI Compatibility
------------------

Part of the design of the HTTP portion of this spec is to make sure it
aligns well with the WSGI specification, to ensure easy adaptability
between both specifications and the ability to keep using WSGI
applications with ASGI servers.

WSGI applications, being synchronous, must be run in a threadpool in order
to be served, but otherwise their runtime maps onto the HTTP connection scope's
lifetime.

There is an almost direct mapping for the various special keys in
WSGI's ``environ`` variable to the ``http`` scope:

* ``REQUEST_METHOD`` is the ``method``
* ``SCRIPT_NAME`` is ``root_path``
* ``PATH_INFO`` can be derived from ``path`` and ``root_path``
* ``QUERY_STRING`` is ``query_string``
* ``CONTENT_TYPE`` can be extracted from ``headers``
* ``CONTENT_LENGTH`` can be extracted from ``headers``
* ``SERVER_NAME`` and ``SERVER_PORT`` are in ``server``
* ``REMOTE_HOST``/``REMOTE_ADDR`` and ``REMOTE_PORT`` are in ``client``
* ``SERVER_PROTOCOL`` is encoded in ``http_version``
* ``wsgi.url_scheme`` is ``scheme``
* ``wsgi.input`` is a ``StringIO`` based around the ``http.request`` messages
* ``wsgi.errors`` is directed by the wrapper as needed

The ``start_response`` callable maps similarly to ``http.response.start``:

* The ``status`` argument becomes ``status``, with the reason phrase dropped.
* ``response_headers`` maps to ``headers``

Yielding content from the WSGI application maps to sending
``http.response.body`` messages.


WSGI encoding differences
-------------------------

The WSGI specification (as defined in PEP 3333) specifies that all strings
sent to or from the server must be of the ``str`` type but only contain
codepoints in the ISO-8859-1 ("latin-1") range. This was due to it originally
being designed for Python 2 and its different set of string types.

The ASGI HTTP and WebSocket specifications instead specify each entry of the
``scope`` dict as either a byte string or a Unicode string. HTTP, being an
older protocol, is sometimes imperfect at specifying encoding, so some
decisions of what is Unicode versus bytes may not be obvious.

* ``path``: URLs can have both percent-encoded and UTF-8 encoded sections.
  Because decoding these is often done by the underlying server (or sometimes
  even proxies in the path), this is a Unicode string, fully decoded from both
  UTF-8 encoding and percent encodings.

* ``headers``: These are byte strings of the exact byte sequences sent by the
  client/to be sent by the server. While modern HTTP standards say that headers
  should be ASCII, older ones did not and allowed a wider range of characters.
  Frameworks/applications should decode headers as they deem appropriate.

* ``query_string``: Unlike the ``path``, this is not as subject to server
  interference and so is presented as its raw byte string version,
  percent-encoded.

* ``root_path``: Unicode string to match ``path``.


Version History
===============

* 2.0 (2017-11-28): Initial non-channel-layer based ASGI spec


Copyright
=========

This document has been placed in the public domain.
