Initial proposal for websockets support.

pull/977/head
Richard van der Hoff 9 years ago
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WebSockets API
==============
Introduction
------------
This document is a proposal for a WebSockets-based client-server API. It is not
intended to replace the REST API, but rather to complement it and provide an
alternative interface for certain operations.
The primary goal is to offer a more efficient interface than the REST API: by
using a bidirectional protocol such as WebSockets we can avoid the overheads
involved in long-polling (SSL negotiation, HTTP headers, etc). In doing so we
will reduce the latency between server and client by allowing the server to
send events as soon as they arrive, rather than having to wait for a poll from
the client.
Handshake
---------
1. Instead of calling ``/sync``, the client makes a websocket request to
``/_matrix/client/rN/stream``, passing the query parameters ``access_token``
and ``since``, and optionally ``filter`` - all of which have the same
meaning as for ``/sync``.
* The client sets the ``Sec-WebSocket-Protocol`` to ``m.json``. (Servers may
offer alternative encodings; at present only the JSON encoding is
specified but in future we will specify alternative encodings.)
#. The server returns the websocket handshake; the socket is then connected.
If the server does not return a valid websocket handshake, this indicates that
the server or an intermediate proxy does not support WebSockets. In this case,
the client should fall back to polling the ``/sync`` REST endpoint.
Example
~~~~~~~
Client request:
.. code:: http
GET /_matrix/client/v2_alpha/stream?access_token=123456&since=s72594_4483_1934 HTTP/1.1
Host: matrix.org
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: m.json
Sec-WebSocket-Version: 13
Origin: https://matrix.org
Server response:
.. code:: http
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: m.json
Update Notifications
--------------------
Once the socket is connected, the server begins streaming updates over the
websocket. The server sends Update notifications about new messages or state
changes. To make it easy for clients to parse, Update notifications have the
same structure as the response to ``/sync``: an object with the following
members:
============= ========== ===================================================
Key Type Description
============= ========== ===================================================
next_batch string The batch token to supply in the ``since`` param of
the next /sync request. This is not required for
streaming of events over the WebSocket, but is
provided so that clients can reconnect if the
socket is disconnected.
presence Presence The updates to the presence status of other users.
rooms Rooms Updates to rooms.
============= ========== ===================================================
Example
~~~~~~~
Message from the server:
.. code:: json
{
"next_batch": "s72595_4483_1934",
"presence": {
"events": []
},
"rooms": {
"join": {},
"invite": {},
"leave": {}
}
}
Client-initiated operations
---------------------------
The client can perform certain operations by sending a websocket message to
the server. Such a "Request" message should be a JSON-encoded object with
the following members:
============= ========== ===================================================
Key Type Description
============= ========== ===================================================
id string A unique identifier for this request
method string Specifies the name of the operation to be
performed; see below for available operations
param object The parameters for the requested operation.
============= ========== ===================================================
The server responds to a client Request with a Response message. This is a
JSON-encoded object with the following members:
============= ========== ===================================================
Key Type Description
============= ========== ===================================================
id string The same as the value in the corresponding Request
object. The presence of the ``id`` field
distinguishes a Response message from an Update
notification.
result object On success, the results of the request.
error object On error, an object giving the resons for the
error. This has the same structure as the "standard
error response" for the Matrix API: an object with
the fields ``errcode`` and ``error``.
============= ========== ===================================================
Request methods
~~~~~~~~~~~~~~~
It is not intended that all operations which are available via the REST API
will be available via the WebSockets API, but a few simple, common operations
will be exposed. The initial operations will be as follows.
``ping``
^^^^^^^^
This is a no-op which clients may use to keep their connection alive.
The request ``params`` and the response ``result`` should be empty.
``send``
^^^^^^^^
Send a message event to a room. The parameters are as follows:
============= ========== ===================================================
Parameter Type Description
============= ========== ===================================================
room_id string **Required.** The room to send the event to
event_type string **Required.** The type of event to send.
content object **Required.** The content of the event.
============= ========== ===================================================
The result is as follows:
============= ========== ===================================================
Key Type Description
============= ========== ===================================================
event_id string A unique identifier for the event.
============= ========== ===================================================
The ``id`` from the Request message is used as the transaction ID by the
server.
``state``
^^^^^^^^^
Update the state on a room.
============= ========== ===================================================
Parameter Type Description
============= ========== ===================================================
room_id string **Required.** The room to set the state in
event_type string **Required.** The type of event to send.
state_key string **Required.** The state_key for the state to send.
content object **Required.** The content of the event.
============= ========== ===================================================
The result is as follows:
============= ========== ===================================================
Key Type Description
============= ========== ===================================================
event_id string A unique identifier for the event.
============= ========== ===================================================
Example
~~~~~~~
Client request:
.. code:: json
{
"id": "12345",
"method": "send",
"params": {
"room_id": "!d41d8cd:matrix.org",
"event_type": "m.room.message",
"content": {
"msgtype": "m.text",
"body": "hello"
}
}
}
Server response:
.. code:: json
{
"id": "12345",
"result": {
"event_id": "$66697273743031:matrix.org"
}
}
Alternative server response, in case of error:
.. code:: json
{
"id": "12345",
"error": {
"errcode": "M_MISSING_PARAM",
"error": "Missing parameter: event_type"
}
}
Rationale
---------
Alternatives to WebSockets include HTTP/2, CoAP, and simply rolling our own
protocol over raw TCP sockets. However, the need to implement browser-based
clients essentially reduces our choice to WebSockets. HTTP/2 streams will
probably provide an interesting alternative in the future, but current browsers
do not appear to give javascript applications low-level access to the protocol.
Concerning the continued use of the JSON encoding: we prefer to focus on the
transition to WebSockets initially. Replacing JSON with a compact
representation such as CBOR, MessagePack, or even just compressed JSON will be
a likely extension for the future. The support for negotiation of subprotocols
within WebSockets should make this a simple transition once time permits.
The number of methods available for client requests is deliberately limited, as
each method requires code to be written to map it onto the equivalent REST
implementation. Some REST methods - for instance, user registration and login -
would be pointless to expose via WebSockets. It is likely, however, that we
will increate the number of methods available via the WebSockets API as it
becomes clear which would be most useful.
Open questions
--------------
Throttling
~~~~~~~~~~
At least in v2 sync, clients are inherently self-throttling - if they do not
poll quickly enough, events will be dropped from the next result. This proposal
raises the possibility that events will be produced more quickly than they can
be sent to the client; backlogs will build up on the server and/or in the
intermediate network, which will not only lead to high latency on events being
delivered, but will lead to responses to client requests also being delayed.
We may need to implement some sort of throttling mechanism by which the server
can start to drop events. The difficulty is in knowing when to start dropping
events. A few ideas:
* Use websocket pings to measure the RTT; if it starts to increase, start
dropping events. But this requires knowledge of the base RTT, and a useful
model of what constitutes an excessive increase.
* Have the client acknowledge each batch of events, and use a window to ensure
the number of outstanding batches is limited. This is annoying as it requires
the client to have to acknowledge batches - and it's not clear what the right
window size is: we want a big window for long fat networks (think of mobile
clients), but a small one for one with lower latency.
* Start dropping events if the server's TCP buffer starts filling up. This has
the advantage of delegating the congestion-detection to TCP (which already
has a number of algorithms to deal with it, to greater or lesser
effectiveness), but relies on homeservers being hosted on OSes which use
sensible TCP congestion-avoidance algorithms, and more critically, an ability
to read the fill level of the TCP send buffer.
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