daphne/docs/asgi.rst

=======================================================
ASGI (Asynchronous Server Gateway Interface) Draft Spec
=======================================================

**NOTE: This is still in-progress, and may change substantially as development
progresses.**

Abstract
========

This document proposes a standard interface between network protocol
servers (particularly webservers) and Python applications, intended
to allow handling of multiple common protocol styles (including HTTP, HTTP2,
and WebSocket).

It is intended to replace and expand on WSGI, though the design
deliberately includes provisions to allow WSGI-to-ASGI and ASGI-to-WGSI
adapters to be easily written for the HTTP protocol.


Rationale
=========

The WSGI specification has worked well since it was introduced, and
allowed for great flexibility in Python framework and webserver choice.
However, its design is irrevocably tied to the HTTP-style
request/response cycle, and more and more protocols are becoming a
standard part of web programming that do not follow this pattern
(most notably, WebSocket).

ASGI attempts to preserve a simple application interface, but provide
an abstraction that allows for data to be sent and received at any time,
and from different application threads or processes.

It also lays out new, serialization-compatible formats for things like
HTTP requests and responses and WebSocket data frames, to allow these to
be transported over a network or local socket, and allow separation
of protocol handling and application logic into different processes.

Part of this design is ensuring there is an easy path to use both
existing WSGI servers and applications, as a large majority of Python
web usage relies on WSGI and providing an easy path forwards is critical
to adoption.


Overview
========

ASGI consists of three different components - *protocol servers*,
a *channel layer*, and *application code*. Channel layers are the core
part of the implementation, and provide an interface to both protocol
servers and applications.

A channel layer provides a protocol server or an application server
with a ``send`` callable, which takes a channel name and message
``dict``, and a ``receive_many`` callable, which takes a list of
channel names and returns the next message available on any named channel.

Thus, rather than under WSGI, where you point the protocol server to the
application, under ASGI you point both the protocol server and the application
to a channel layer instance. It is intended that applications and protocol
servers always run in separate processes or threads, and always communicate
via the channel layer.

Despite the name of the proposal, ASGI does not specify or design to any
specific in-process async solution, such as ``asyncio``, ``twisted``, or
``gevent``. Instead, the ``receive_many`` function can be switched between
nonblocking or synchronous. This approach allows applications to choose what's
best for their current runtime environment; further improvements may provide
extensions where cooperative versions of receive_many are provided.

The distinction between protocol servers and applications in this document
is mostly to distinguish their roles and to make illustrating concepts easier.
There is no code-level distinction between the two, and it's entirely possible
to have a process that does both, or middleware-like code that transforms
messages between two different channel layers or channel names. It is
expected, however, that most deployments will fall into this pattern.

There is even room for a WSGI-like application abstraction with a callable
which takes ``(channel, message, send_func)``, but this would be slightly
too restrictive for many use cases and does not cover how to specify
channel names to listen on; it is expected that frameworks will cover this
use case.


Channels and Messages
---------------------

All communication in an ASGI stack uses *messages* sent over *channels*.
All messages must be a ``dict`` at the top level of the object, and
contain only the following types to ensure serializability:

* Byte strings
* Unicode strings
* Integers (no longs)
* Lists (tuples should be treated as lists)
* Dicts (keys must be unicode strings)
* Booleans
* None

Channels are identified by a unicode string name consisting only of ASCII
letters, ASCII numerical digits, periods (``.``), dashes (``-``) and
underscores (``_``), plus an optional prefix character (see below).

Channels are a first-in, first out queue with at-most-once delivery
semantics. They can have multiple writers and multiple readers; only a single
reader should get each written message. Implementations must never deliver
a message more than once or to more than one reader, and must drop messages if
this is necessary to achieve this restriction.

In order to aid with scaling and network architecture, a distinction
is made between channels that have multiple readers (such as the
``http.request`` channel that web applications would listen on from every
application worker process) and *single-reader channels*
(such as a ``http.response!ABCDEF`` channel tied to a client socket).

*Single-reader channel* names contain an exclamation mark
(``!``) character in order to indicate to the channel layer that it may
have to route the data for these channels differently to ensure it reaches the
single process that needs it; these channels are nearly always tied to
incoming connections from the outside world. The ``!`` is always preceded by
the main channel name (e.g. ``http.response``) and followed by the
per-client/random portion - channel layers can split on the ``!`` and use just
the right hand part to route if they desire, or can ignore it if they don't
need to use different routing rules.

Messages should expire after a set time sitting unread in a channel;
the recommendation is one minute, though the best value depends on the
channel layer and the way it is deployed.

The maximum message size is finite, though it varies based on the channel layer
and the encoding it's using. Channel layers may reject messages at ``send()``
time with a ``MessageTooLarge`` exception; the calling code should take
appropriate action (e.g. HTTP responses can be chunked, while HTTP
requests should be closed with a ``413 Request Entity Too Large`` response).
It is intended that some channel layers will only support messages of around a
megabyte, while others will be able to take a gigabyte or more, and that it
may be configurable.

Handling Protocols
------------------

ASGI messages represent two main things - internal application events
(for example, a channel might be used to queue thumbnails of previously
uploaded videos), and protocol events to/from connected clients.

As such, this specification outlines encodings to and from ASGI messages
for three common protocols (HTTP, WebSocket and raw UDP); this allows any ASGI
web server to talk to any ASGI web application, as well as servers and
applications for any other protocol with a common specification. It is
recommended that if other protocols become commonplace they should gain
standardized formats in a supplementary specification of their own.

The message formats are a key part of the specification; without them,
the protocol server and web application might be able to talk to each other,
but may not understand some of what the other is saying. It's equivalent to the
standard keys in the ``environ`` dict for WSGI.

The design pattern is that most protocols will share a few channels for
incoming data (for example, ``http.request``, ``websocket.connect`` and
``websocket.receive``), but will have individual channels for sending to
each client (such as ``http.response!kj2daj23``). This allows incoming
data to be dispatched into a cluster of application servers that can all
handle it, while responses are routed to the individual protocol server
that has the other end of the client's socket.

Some protocols, however, do not have the concept of a unique socket
connection; for example, an SMS gateway protocol server might just have
``sms.receive`` and ``sms.send``, and the protocol server cluster would
take messages from ``sms.send`` and route them into the normal phone
network based on attributes in the message (in this case, a telephone
number).


Extensions
----------

Extensions are functionality that is
not required for basic application code and nearly all protocol server
code, and so has been made optional in order to enable lightweight
channel layers for applications that don't need the full feature set defined
here.

There are three extensions defined here: the ``groups`` extension, which
is expanded on below, the ``flush`` extension, which allows easier testing
and development, and the ``statistics`` extension, which allows
channel layers to provide global and per-channel statistics.

There is potential to add further extensions; these may be defined by
a separate specification, or a new version of this specification.

If application code requires an extension, it should check for it as soon
as possible, and hard error if it is not provided. Frameworks should
encourage optional use of extensions, while attempting to move any
extension-not-found errors to process startup rather than message handling.


Groups
------

While the basic channel model is sufficient to handle basic application
needs, many more advanced uses of asynchronous messaging require
notifying many users at once when an event occurs - imagine a live blog,
for example, where every viewer should get a long poll response or
WebSocket packet when a new entry is posted.

This concept could be kept external to the ASGI spec, and would be, if it
were not for the significant performance gains a channel layer implementation
could make on the send-group operation by having it included - the
alternative being a ``send_many`` callable that might have to take
tens of thousands of destination channel names in a single call. However,
the group feature is still optional; its presence is indicated by the
``supports_groups`` attribute on the channel layer object.

Thus, there is a simple Group concept in ASGI, which acts as the
broadcast/multicast mechanism across channels. Channels are added to a group,
and then messages sent to that group are sent to all members of the group.
Channels can be removed from a group manually (e.g. based on a disconnect
event), and the channel layer will garbage collect "old" channels in groups
on a periodic basis.

How this garbage collection happens is not specified here, as it depends on
the internal implementation of the channel layer. The recommended approach,
however, is when a message on a single-listener channel expires, the channel
layer should remove that channel from all groups it's currently a member of;
this is deemed an acceptable indication that the channel's listener is gone.

*Implementation of the group functionality is optional*. If it is not provided
and an application or protocol server requires it, they should hard error
and exit with an appropriate error message. It is expected that protocol
servers will not need to use groups.


Linearization
-------------

The design of ASGI is meant to enable a shared-nothing architecture,
where messages can be handled by any one of a set of threads, processes
or machines running application code.

This, of course, means that several different copies of the application
could be handling messages simultaneously, and those messages could even
be from the same client; in the worst case, two packets from a client
could even be processed out-of-order if one server is slower than another.

This is an existing issue with things like WSGI as well - a user could
open two different tabs to the same site at once and launch simultaneous
requests to different servers - but the nature of the new protocols
specified here mean that collisions are more likely to occur.

Solving this issue is left to frameworks and application code; there are
already solutions such as database transactions that help solve this,
and the vast majority of application code will not need to deal with this
problem. If ordering of incoming packets matters for a protocol, they should
be annotated with a packet number (as WebSocket is in this specification).

Single-reader channels, such as those used for response channels back to
clients, are not subject to this problem; a single reader should always
receive messages in channel order.


Specification Details
=====================

A *channel layer* must provide an object with these attributes
(all function arguments are positional):

* ``send(channel, message)``, a callable that takes two arguments: the
  channel to send on, as a unicode string, and the message
  to send, as a serializable ``dict``.

* ``receive_many(channels, block=False)``, a callable that takes a list of channel
  names as unicode strings, and returns with either ``(None, None)``
  or ``(channel, message)`` if a message is available. If ``block`` is True, then
  it will not return until after a built-in timeout or a message arrives; if
  ``block`` is false, it will always return immediately. It is perfectly
  valid to ignore ``block`` and always return immediately.

* ``new_channel(pattern)``, a callable that takes a unicode string pattern,
  and returns a new valid channel name that does not already exist, by
  adding a single random unicode string after the ``!`` character in ``pattern``,
  and checking for existence of that name in the channel layer. The ``pattern``
  MUST end with ``!`` or this function must error. This is NOT called prior to
  a message being sent on a channel, and should not be used for channel
  initialization, and is also not guaranteed to be called by the same channel
  client that then reads the messages, so you cannot put process identifiers in
  it for routing.

* ``MessageTooLarge``, the exception raised when a send operation fails
  because the encoded message is over the layer's size limit.

* ``extensions``, a list of unicode string names indicating which
  extensions this layer provides, or empty if it supports none.
  The names defined in this document are ``groups``, ``flush`` and
  ``statistics``.

A channel layer implementing the ``groups`` extension must also provide:

* ``group_add(group, channel)``, a callable that takes a ``channel`` and adds
  it to the group given by ``group``. Both are unicode strings. If the channel
  is already in the group, the function should return normally.

* ``group_discard(group, channel)``, a callable that removes the ``channel``
  from the ``group`` if it is in it, and does nothing otherwise.

* ``send_group(group, message)``, a callable that takes two positional
  arguments; the group to send to, as a unicode string, and the message
  to send, as a serializable ``dict``.

A channel layer implementing the ``statistics`` extension must also provide:

* ``global_statistics()``, a callable that returns a dict with zero
  or more of (unicode string keys):

  * ``count``, the current number of messages waiting in all channels

* ``channel_statistics(channel)``, a callable that returns a dict with zero
  or more of (unicode string keys):

  * ``length``, the current number of messages waiting on the channel
  * ``age``, how long the oldest message has been waiting, in seconds
  * ``per_second``, the number of messages processed in the last second

A channel layer implementing the ``flush`` extension must also provide:

* ``flush()``, a callable that resets the channel layer to a blank state,
  containing no messages and no groups (if the groups extension is
  implemented). This call must block until the system is cleared and will
  consistently look empty to any client, if the channel layer is distributed.



Channel Semantics
-----------------

Channels **must**:

* Preserve ordering of messages perfectly with only a single reader
  and writer, and preserve as much as possible in other cases.

* Never deliver a message more than once.

* Never block on message send.

* Be able to handle messages of at least 1MB in size when encoded as
  JSON (the implementation may use better encoding or compression, as long
  as it meets the equivalent size)

* Have a maximum name length of at least 100 bytes.

They are not expected to deliver all messages, but a success rate of at least
99.99% is expected under normal circumstances. Implementations may want to
have a "resilience testing" mode where they deliberately drop more messages
than usual so developers can test their code's handling of these scenarios.


Persistence
-----------

Channel layers do not need to persist data long-term; group
memberships only need to live as long as a connection does, and messages
only as long as the message expiry time, which is usually a couple of minutes.

That said, if a channel server goes down momentarily and loses all data,
persistent socket connections will continue to transfer incoming data and
send out new generated data, but will have lost all of their group memberships
and in-flight messages.

In order to avoid a nasty set of bugs caused by these half-deleted sockets,
protocol servers should quit and hard restart if they detect that the channel
layer has gone down or lost data; shedding all existing connections and letting
clients reconnect will immediately resolve the problem.

If a channel layer implements the ``groups`` extension, it must persist group
membership until at least the time when the member channel has a message
expire due to non-consumption. It should drop membership after a while to
prevent collision of old messages with new clients with the same random ID.


Message Formats
---------------

These describe the standardized message formats for the protocols this
specification supports. All messages are ``dicts`` at the top level,
and all keys are required unless otherwise specified (with a default to
use if the key is missing). Keys are unicode strings.

The one common key across all protocols is ``reply_channel``, a way to indicate
the client-specific channel to send responses to. Protocols are generally
encouraged to have one message type and one reply channel type to ensure ordering.

A ``reply_channel`` should be unique per connection. If the protocol in question
can have any server service a response - e.g. a theoretical SMS protocol - it
should not have ``reply_channel`` attributes on messages, but instead a separate
top-level outgoing channel.

Messages are specified here along with the channel names they are expected
on; if a channel name can vary, such as with reply channels, the varying
portion will be represented by ``!``, such as ``http.response!``, which matches
the format the ``new_channel`` callable takes.

There is no label on message types to say what they are; their type is implicit
in the channel name they are received on. Two types that are sent on the same
channel, such as HTTP responses and response chunks, are distinguished apart
by their required fields.


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 requests on the same connection different reply channels, and
correctly multiplex the responses back into the same stream as they come in.
The HTTP version is available as a string in the request message.

HTTP/2 Server Push responses are included, but must be sent prior to the
main response, and applications must check for ``http_version = 2`` before
sending them; if a protocol server or connection incapable of Server Push
receives these, it must drop them.

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.
For this reason, we can safely make the request ``headers`` a ``dict``, but
the response ``headers`` must be sent as a list of tuples, which matches WSGI.

Request
'''''''

Sent once for each request that comes into the protocol server.

Channel: ``http.request``

Keys:

* ``reply_channel``: Channel name for responses and server pushes, starting with
  ``http.response!``

* ``http_version``: Unicode string, one of ``1.0``, ``1.1`` or ``2``.

* ``method``: Unicode string HTTP method name, uppercased.

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

* ``path``: Byte string HTTP path from URL.

* ``query_string``: Byte string URL portion after the ``?``. Optional, default
  is ``""``.

* ``root_path``: Byte string that indicates the root path this application
  is mounted at; same as ``SCRIPT_NAME`` in WSGI. Optional, defaults
  to ``""``.

* ``headers``: A list of ``[name, value]`` pairs, where ``name`` is the
  byte string header name, and ``value`` is the byte string
  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.

* ``body``: Body of the request, as a byte string. Optional, defaults to ``""``.
  If ``body_channel`` is set, treat as start of body and concatenate
  on further chunks.

* ``body_channel``: Single-reader channel name that contains
  Request Body Chunk messages representing a large request body.
  Optional, defaults to ``None``. Chunks append to ``body`` if set. Presence of
  a channel indicates at least one Request Body Chunk message needs to be read,
  and then further consumption keyed off of the ``more_content`` key in those
  messages.

* ``client``: List of ``[host, port]`` where ``host`` is a unicode string of the
  remote host's IPv4 or IPv6 address, and ``port`` is the remote port as an
  integer. Optional, defaults to ``None``.

* ``server``: List of ``[host, port]`` where ``host`` is the listening address
  for this server as a unicode string, and ``port`` is the integer listening port.
  Optional, defaults to ``None``.


Request Body Chunk
''''''''''''''''''

Must be sent after an initial Response.

Channel: ``http.request.body!``

Keys:

* ``content``: Byte string of HTTP body content, will be concatenated onto
  previously received ``content`` values and ``body`` key in Request.
  Not required if ``closed`` is True, required otherwise.

* ``closed``: True if the client closed the connection prematurely and the
  rest of the body. If you receive this, abandon processing of the HTTP request.
  Optional, defaults to ``False``.

* ``more_content``: Boolean value signifying if there is additional content
  to come (as part of a Request Body Chunk message). If ``False``, request will
  be taken as complete, and any further messages on the channel
  will be ignored. Optional, defaults to ``False``.


Response
''''''''

Send after any server pushes, and before any response chunks.

Channel: ``http.response!``

Keys:

* ``status``: Integer HTTP status code.

* ``headers``: A list of ``[name, value]`` pairs, where ``name`` is the
  byte string header name, and ``value`` is the byte string
  header value. Order should be preserved in the HTTP response. Header names
  must be lowercased.

* ``content``: Byte string of HTTP body content.
  Optional, defaults to empty string.

* ``more_content``: Boolean value signifying if there is additional content
  to come (as part of a Response Chunk message). If ``False``, response will
  be taken as complete and closed off, and any further messages on the channel
  will be ignored. Optional, defaults to ``False``.


Response Chunk
''''''''''''''

Must be sent after an initial Response.

Channel: ``http.response!``

Keys:

* ``content``: Byte string of HTTP body content, will be concatenated onto
  previously received ``content`` values.

* ``more_content``: Boolean value signifying if there is additional content
  to come (as part of a Response Chunk message). If ``False``, response will
  be taken as complete and closed off, and any further messages on the channel
  will be ignored. Optional, defaults to ``False``.


Server Push
'''''''''''

Must be sent before any Response or Response Chunk messages.

When a server receives this message, it must treat the Request message in the
``request`` field of the Server Push as though it were a new HTTP request being
received from the network. A server may, if it chooses, apply all of its
internal logic to handling this request (e.g. the server may want to try to
satisfy the request from a cache). Regardless, if the server is unable to
satisfy the request itself it must create a new ``http.response!`` channel for
the application to send the Response message on, fill that channel in on the
``reply_channel`` field of the message, and then send the Request back to the
application on the ``http.request`` channel.

This approach limits the amount of knowledge the application has to have about
pushed responses: they essentially appear to the application like a normal HTTP
request, with the difference being that the application itself triggered the
request.

If the remote peer does not support server push, either because it's not a
HTTP/2 peer or because SETTINGS_ENABLE_PUSH is set to 0, the server must do
nothing in response to this message.

Channel: ``http.response!``

Keys:

* ``request``: A Request message. The ``body``, ``body_channel``, and
  ``reply_channel`` fields MUST be absent: bodies are not allowed on
  server-pushed requests, and applications should not create reply channels.


Disconnect
''''''''''

Sent when a HTTP connection is closed. This is mainly useful for long-polling,
where you may have added the response channel to a Group or other set of
channels you want to trigger a reply to when data arrives.

Channel: ``http.disconnect``

Keys:

* ``reply_channel``: Channel name responses would have been sent on. No longer
  valid after this message is sent; all messages to it will be dropped.


WebSocket
---------

WebSockets share some HTTP details - they have a path and headers - but also
have more state. Path and header details are only sent in the connection
message; applications that need to refer to these during later messages
should store them in a cache or database.

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


Connection
''''''''''

Sent when the client initially opens a connection and completes the
WebSocket handshake.

Channel: ``websocket.connect``

Keys:

* ``reply_channel``: Channel name for sending data, start with ``websocket.send!``

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

* ``path``: Byte string HTTP path from URL.

* ``query_string``: Byte string URL portion after the ``?``. Optional, default
  is empty string.

* ``root_path``: Byte string that indicates the root path this application
  is mounted at; same as ``SCRIPT_NAME`` in WSGI. Optional, defaults
  to empty string.

* ``headers``: Dict of ``{name: value}``, where ``name`` is the lowercased
  HTTP header name as byte string and ``value`` is the header value as a byte
  string. If multiple headers with the same name are received, they should
  be concatenated into a single header as per .

* ``client``: List of ``[host, port]`` where ``host`` is a unicode string of the
  remote host's IPv4 or IPv6 address, and ``port`` is the remote port as an
  integer. Optional, defaults to ``None``.

* ``server``: List of ``[host, port]`` where ``host`` is the listening address
  for this server as a unicode string, and ``port`` is the integer listening port.
  Optional, defaults to ``None``.

* ``order``: The integer value ``0``.


Receive
'''''''

Sent when a data frame is received from the client.

Channel: ``websocket.receive``

Keys:

* ``reply_channel``: Channel name for sending data, starting with ``websocket.send!``

* ``path``: Path sent during ``connect``, sent to make routing easier for apps.

* ``bytes``: Byte string of frame content, if it was bytes mode, or ``None``.

* ``text``: Unicode string of frame content, if it was text mode, or ``None``.

* ``order``: Order of this frame in the WebSocket stream, starting
  at 1 (``connect`` is 0).

One of ``bytes`` or ``text`` must be non-``None``.


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.

Channel: ``websocket.disconnect``

Keys:

* ``reply_channel``: Channel name that was used for sending data, starting
  with ``websocket.send!``. Cannot be used to send at this point; provided
  as a way to identify the connection only.

* ``path``: Path sent during ``connect``, sent to make routing easier for apps.

* ``order``: Order of the disconnection relative to the incoming frames'
  ``order`` values in ``websocket.receive``.


Send/Close
''''''''''

Sends a data frame to the client and/or closes the connection from the
server end.

Channel: ``websocket.send!``

Keys:

* ``bytes``: Byte string of frame content, if in bytes mode, or ``None``.

* ``text``: Unicode string of frame content, if in text mode, or ``None``.

* ``close``: Boolean saying if the connection should be closed after data
  is sent, if any. Optional, default ``False``.

A maximum of one of ``bytes`` or ``text`` may be provided. If both are
provided, the protocol server should ignore the message entirely.


UDP
---

Raw UDP is included here as it is a datagram-based, unordered and unreliable
protocol, which neatly maps to the underlying message abstraction. It is not
expected that many applications would use the low-level protocol, but it may
be useful for some.

While it might seem odd to have reply channels for UDP as it is a stateless
protocol, replies need to come from the same server as the messages were
sent to, so the reply channel here ensures that reply packets from an ASGI
stack do not come from a different protocol server to the one you sent the
initial packet to.


Receive
'''''''

Sent when a UDP datagram is received.

Channel: ``udp.receive``

Keys:

* ``reply_channel``: Channel name for sending data, starts with ``udp.send!``

* ``data``: Byte string of UDP datagram payload.

* ``client``: List of ``[host, port]`` where ``host`` is a unicode string of the
  remote host's IPv4 or IPv6 address, and ``port`` is the remote port as an
  integer.

* ``server``: List of ``[host, port]`` where ``host`` is the listening address
  for this server as a unicode string, and ``port`` is the integer listening port.
  Optional, defaults to ``None``.


Send
''''

Sent to send out a UDP datagram to a client.

Channel: ``udp.send!``

Keys:

* ``data``: Byte string of UDP datagram payload.


Protocol Format Guidelines
--------------------------

Message formats for protocols should follow these rules, unless
a very good performance or implementation reason is present:

* ``reply_channel`` should be unique per logical connection, and not per
  logical client.

* If the protocol has server-side state, entirely encapsulate that state in
  the protocol server; do not require the message consumers to use an external
  state store.

* If the protocol has low-level negotiation, keepalive or other features,
  handle these within the protocol server and don't expose them in ASGI
  messages.

* If the protocol has guaranteed ordering and does not use a specific channel
  for a given connection (as HTTP does for body data), ASGI messages should
  include an ``order`` field (0-indexed) that preserves the ordering as
  received by the protocol server (or as sent by the client, if available).
  This ordering should span all message types emitted by the client - for
  example, a connect message might have order ``0``, and the first two frames
  order ``1`` and ``2``.

* If the protocol is datagram-based, one datagram should equal one ASGI message
  (unless size is an issue)


Approximate Global Ordering
---------------------------

While maintaining true global (across-channels) ordering of messages is
entirely unreasonable to expect of many implementations, they should strive
to prevent busy channels from overpowering quiet channels.

For example, imagine two channels, ``busy``, which spikes to 1000 messages a
second, and ``quiet``, which gets one message a second. There's a single
consumer running ``receive_many(['busy', 'quiet'])`` which can handle
around 200 messages a second.

In a simplistic for-loop implementation, the channel layer might always check
``busy`` first; it always has messages available, and so the consumer never
even gets to see a message from ``quiet``, even if it was sent with the
first batch of ``busy`` messages.

A simple way to solve this is to randomize the order of the channel list when
looking for messages inside the channel layer; other, better methods are also
available, but whatever is chosen, it should try to avoid a scenario where
a message doesn't get received purely because another channel is busy.


Strings and Unicode
-------------------

In this document, *byte string* refers to ``str`` on Python 2 and ``bytes``
on Python 3. If this type still supports Unicode codepoints due to the
underlying implementation, then any values should be kept within the lower
8-byte range.

*Unicode string* refers to ``unicode`` on Python 2 and ``str`` on Python 3.
This document will never specify just *string* - all strings are one of the
two types.

Some serializers, such as ``json``, cannot differentiate between byte
strings and unicode strings; these should include logic to box one type as
the other (for example, encoding byte strings as base64 unicode strings with
a preceding special character, e.g. U+FFFF).

Channel and group names are always unicode strings, with the additional
limitation that they only use the following characters:

* ASCII letters
* The digits ``0`` through ``9``
* Hyphen ``-``
* Underscore ``_``
* Period ``.``
* Exclamation mark ``!`` (only to deliniate single-reader channel names,
  and only one per name)


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 servers or
applications with ASGI.

The adaptability works in two ways:

* WSGI Server to ASGI: A WSGI application can be written that transforms
  ``environ`` into a Request message, sends it off on the ``http.request``
  channel, and then waits on a generated response channel for a Response
  message. This has the disadvantage of tying up an entire WSGI thread
  to poll one channel, but should not be a massive performance drop if
  there is no backlog on the request channel, and would work fine for an
  in-process adapter to run a pure-ASGI web application.

* ASGI to WSGI application: A small wrapper process is needed that listens
  on the ``http.request`` channel, and decodes incoming Request messages
  into an ``environ`` dict that matches the WSGI specs, while passing in
  a ``start_response`` that stores the values for sending with the first
  content chunk. Then, the application iterates over the WSGI app,
  packaging each returned content chunk into a Response or Response Chunk
  message (if more than one is yielded).

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

* ``REQUEST_METHOD`` is the ``method`` key
* ``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 around ``body``
* ``wsgi.errors`` is directed by the wrapper as needed

The ``start_response`` callable maps similarly to Response:

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

It may even be possible to map Request Body Chunks in a way that allows
streaming of body data, though it would likely be easier and sufficient for
many applications to simply buffer the whole body into memory before calling
the WSGI application.


Common Questions
================

1. Why are messages ``dicts``, rather than a more advanced type?

   We want messages to be very portable, especially across process and
   machine boundaries, and so a simple encodable type seemed the best way.
   We expect frameworks to wrap each protocol-specific set of messages in
   custom classes (e.g. ``http.request`` messages become ``Request`` objects)


TODOs
=====

* Maybe remove ``http_version`` and replace with ``supports_server_push``?

* ``receive_many`` can't easily be implemented with async/cooperative code
  behind it as it's nonblocking - possible alternative call type?
  Asyncio extension that provides ``receive_many_yield``?

* Possible extension to allow detection of channel layer flush/restart and
  prompt protocol servers to restart?

* Maybe WSGI-app like spec for simple "applications" that allows standardized
  application-running servers?


Copyright
=========

This document has been placed in the public domain.