FastAPI Distributed Websocket

A library to implement websocket for distibuted systems based on FastAPI.

N.B.: This library is still at an early stage, use it in production at your own risk.

What it does

The main features of this libarary are:

• Easly implementing broadcasting, pub/sub, chat rooms, etc...
• Proxy websocket connections to other servers (e.g. from an api gateway)
• Authentication
• Clean exception handling
• An in memory broker for fast development

Problems of scaling websocket among multiple servers in production

Websocket is a relatively new protocol for real time communication over HTTP. It establishes a durable, stateful, full-duplex connection between clients and the server. It can be used to implement chats, real time notifications, broadcasting and pub/sub models.

Connections from clients

HTTP request/response mechanism fits very well to scale among multiple server instances in production. Any time a client makes a request, it can connect to any server instance and it's going to receive the same response. After the response has been returned to the client, it went disconnected and it can make another request without the need to hit the same instace as before. This thanks to the stateless nature of HTTP.

However, Websocket establishes a stateful connection between the client and the server and, if some error occurs and the connection went lost, we have to ensure that clients are going to hit the same server instance they were connected before, since that instance was managing the connection state.

Stateful means that there is a state that can be manipulated. In particular, a stateful connection is a connection that heavily relies on its state in order to work

Broadcasting and group messages

Another problem of scaling Websocket occurs when we need to send messages to multiple connected clients (i.e. broadcasting a message or sending a message to all clients subscribed to a specific topic).

Imagine that we have a chat server, and that when an user send a message in a specific chat room, we broadcast it to all users subscribed to that room. If we have a single server instance, all connection are managed by this instance so we can safely trust that the message will be delivered to all recipents. On the other hand, with multiple server instances, users subscribing to a chat room will probably connect to different instances. This way, if an user send a message to the chat room 'xyz' at the server A, users subscribed to the same chat room at the server B are not receiving it.

Documenting Websocket interfaces

Another common problem with Websocket, that's not even related to scaling, is about documentation. Due to the event driven nature of the Websocket protocol it does not fit well to be documented with openapi. However a new specification for asynchronous, event driven interfaces has been defined recently. The spec name is asyncapi and I'm currently studying it. I don't know if this has to be implemented here or it's better having a separate library for that, however this is surely something we have to look at.

Other problems

When I came first to think about this library, I started making a lot of research of common problems related to Websocket on stackoverflow, reddit, github issues and so on. I found some interesting resource that are however related to the implementation itself. I picked up best solutions and elaborated my owns converging all of that in this library.

Examples

Installation

$ pip install fastapi-distributed-websocket

Basic usage

This is a basic example using an in memory broker with a single server instance.

from fastapi import FastAPI, WebSocket, WebSocketDisconnect, status
from distributed_websocket import Connection, WebSocketManager

app = FastAPI()
manager = WebSocketManager('channel:1', broker_url='memory://')
...


app.on_event('startup')
async def startup() -> None:
    ...
    await manager.startup()


app.on_event('shutdown')
async def shutdown() -> None:
    ...
    await manager.shutdown()


@app.websocket('/ws/{conn_id}')
async def websocket_endpoint(
    ws: WebSocket,
    conn_id: str,
    *,
    topic: Optional[Any] = None,
) -> None:
    connection: Connection = await manager.new_connection(ws, conn_id)
    try:
        while True:
            msg = await connection.receive_json()
            await manager.broadcast(msg)
    except WebSocketDisconnect:
        await manager.remove_connection(connection)

The manger.new_connection method create a new Connection object and add it to the manager.active_connections list. Note that after a WebSocketDisconnect is raised, we call remove_connection: this method only remove the connection object from the manager.active_connections list, without calling connection.close, since the connection is already closed. If you need to close a connection at any other time, you can use manager.close_connection. If you use connection.iter_json, it already handles the WebSocketDisconnect exception, so you can simply call manager.remove_connection just after the loop (see next code block).

Note that here we are using manager.broadcast to send the message to all connections managed by the WebSocketManager instance. However, this method only work if we have a single server instance. If we have multiple server instances, we have to use manager.receive, to properly send the message to the broker.

@app.websocket('/ws/{conn_id}')
async def websocket_endpoint(
    ws: WebSocket,
    conn_id: str,
    *,
    topic: Optional[Any] = None,
) -> None:
    connection: Connection = await manager.new_connection(ws, conn_id)
    # This is the preferred way of handling WebSocketDisconnect
    async for msg in connection.iter_json():
        await manager.receive(connection, msg)
    await manager.remove_connection(connection)

Proxy from an API gateway

Let's say we are developing a chat service and that all our services are behind an API gateway. If we want to keep our websocket service behind it too, then fastapi-distributed-websocket provides us with WebSocketProxy.

from distributed_websocket import WebSocketProxy
# skipped other imports for brevity

app = FastAPI()


WS_TARGET_ENDPOINT = 'ws://websocket_service:8000/wshandler'

@app.websocket('/ws')
async def websocket_proxy(websocket: WebSocket):
    await websocket.accept()
    ws_proxy = WebSocketProxy(websocket, WS_TARGET_ENDPOINT)
    await ws_proxy()

This will forward all messages from the client to the target endpoint and all messages from the target endpoint to the client.

Now let's assume that our websocket service code is the code of our previous example. Our API Gateway code will be:

from distributed_websocket import WebSocketProxy
# skipped other imports for brevity

app = FastAPI()


WS_TARGET_ENDPOINT = 'ws://websocket_service:8000/ws/{}'

@app.websocket('/ws/{conn_id}')
async def websocket_endpoint(
    ws: WebSocket,
    conn_id: str,
) -> None:
    await websocket.accept()
    ws_proxy = WebSocketProxy(websocket, WS_TARGET_ENDPOINT.format(conn_id))
    await ws_proxy()

API Reference

Connection

Connection objects wrap the websocket connection and provide a simple interface to send and receive messages. They have a topics attribute to store subscriptions patterns and implement pub/sub models.

• async accept(self) -> None
  Accept the connection.
• async close(self, code: int = 1000) -> None
  Close the connection with the specified status.
• async receive_json(self) -> Any
  Receive a JSON message.
• async send_json(self, data: Any) -> None
  Send a JSON message over the connection.
• async iter_json(self) -> AsyncIterator[Any]
  Iterate over the messages received over the connection.

Messages

Message objects store the message type, the topic and the data. They provides an easy serialization/deserialization mechanism. Remeber that messages returned by connection.iter_json are already deserialized into dict objects, so here we call deserialization the process of converting a dict object into a Message object.

• type: str
  The message type.

• topic: str
  The message topic.

• conn_id: str | list[str]
  The connection id or list of connection ids that the message should be sent to.

• data: Any
  The message data.

• classmethodfrom_client_message(cls, *, data: Any) -> Message
  Create a message from a client message.

• __serialize__(self) -> dict
  Serialize the message into a dict object.

Subscriptions

You can bind topics to connection objects to implement pub/sub models, notification and so on. The topics attribute is a set of strings that follows the pattern matching syntax of MQTT. This library share connection objects state between server instances, so you may find references to terms like channel, publish, subscribe and unsubscribe referring to the same concepts but applied to the underlying server/broker communication.
This may be confusing, but remember to keep separated the communication between the server and the clients, that you are developing and the communication between the server and the broker, that you usually don't deal with.

• subscribe(connection: Connection, message: Message) -> None
  Subscribe a connection to message.topic.
• unsubscribe(connection: Connection, message: Message) -> None
  Unsubscribe a connection from message.topic.
• hanlde_subscription_message(connection: Connection, message: Message) -> None
  Calls subscribe or unsubscribe depending on the message type.
• matches(topic: str, patterns: set[str]) -> bool
  Check if topic matches any of the patterns in patterns.

Authentication

Authentication is provided with the WebSocketOAuth2PasswordBearer class. It inherits from FastAPI OAuth2PasswordBearer and overrides __call__ method to accept a WebSocket object.

• async __call__(self, websocket: WebSocket) -> str | None
  Authenticate the websocket connection and return the Authorization header value.
  If the authentication fails, return None if the objects has been initialized with auto_error=False
  or close the connection with the WS_1008_POLICY_VIOLATION code.

Exceptions and Exception Handling

fastapi-distributed-websocket provides exception handling via decorators. You can use the apposite decorators passing an exception class and a handler callable. Exception handlers should accept only the exception object as argument.
Why this is useful?
Because sometimes the same type of exception can be raised by different parts of the application, this way you can decorate the higer level function in the call stack to handle the exception at any level.
A base WebSocketException class is provided to bind connection objects to the exception, so your handler function can easily access it. If you need to access connection objects from the exception handler, your custom exceptions should inherit from WebSocketException, no matter if they are really network related or not.

• WebSocketException(self, message: str, *, connection: Connection) -> None

• InvalidSubscription(self, message: str, *, connection: Connection) -> None
  Raised when a subscription pattern use an invalid syntax. Inherits from WebSocketException.

• InvalidSubscriptionMessage(self, message: str, *, connection: Connection) -> None
  Like InvalidSubscription it could be raised for bad syntax, but it could also be raised
  when the message type is not subscribe or unsubscribe. Inherits from WebSocketException.

• handle(exc: BaseException, handler: Callable[..., Any]) -> Callable[..., Any]
  Decorator to handle exceptions. If you decorate a function with this decorator, at any time
  an exception of type exc is raised or propagated to the function, it will be handled by handler.
  Use this decorator only if both your handler and the function are not async.

• async ahandle( exc: BaseException, handler: Callable[..., Coroutine[Any, Any, Any]] ) -> Callable[..., Any]
  Decorator to handle exceptions, same ad handle, but the handler is a coroutine function.
  Use this if your handler is a coroutine function, while the decorated function could be
  either a sync or an async function.

Broker Interfaces

Connections' state is shared between server instances using a pub/sub broker. By default, the broker is a reids.asyncio.Redis instance (ex aioredis.Redis), but you can use any other implementation. fastapi-distributed-websocket provides an InMemoryBroker class for development purposes. You can inherit from BrokerInterface and override the methods to implement your own broker.

• async connect(self) -> Coroutine[Any, Any, None]
  Connect to the broker.
• async disconnect(self) -> Coroutine[Any, Any, None]
  Disconnect from the broker.
• async subscribe(self, channel: str) -> Coroutine[Any, Any, None]
  Subscribe to a channel.
• async unsubscribe(self, channel: str) -> Coroutine[Any, Any, None]
  Unsubscribe from a channel.
• async publish(self, channel: str, message: Any) -> Coroutine[Any, Any, None]
  Publish a message to a channel.
• async get_message(self, **kwargs) -> Coroutine[Any, Any, Message | None]
  Get a message from the broker.

WebSocketManager

The WebSocketManager class is where the main logic of the library is implemented.
It keeps track of the connection objects and starts the broker connection. It spawn a main task, a listener that wait (non-blocking) for messages from the broker, and send them to the connection objects (broadcasting or checking for subscriptions) spawning a new task for each send.
The broker initialisation is done in the constructor while calls to broker.connect and broker.disconnect are handled in the startup and shutdown methods.

• async new_connection( self, websocket: WebSocket, conn_id: str, topic: str | None = None ) -> Coroutine[Any, Any, Connection]
  Create a new connection object, add it to self.active_connections and return it.
• async close_connection( self, connection: Connection, code: int = status.WS_1000_NORMAL_CLOSURE ) -> Coroutine[Any, Any, None]
  Close a connection object and remove it from self.active_connections.
• remove_connection(self, connection: Connection) -> None
  Remove a connection object from self.active_connections.
• set_conn_id(self, connection: Connection, conn_id: str) -> None
  Set the connection id and notify the client.
• send(self, message: Message) -> None
  Send a message to all the connection objects subscribed to message.topic.
  It spawns a new task wrapping the coroutine resulting from self._send.
• broadcast(self, message: Message) -> None
  Send a message to all the connection objects.
  It spawns a new task wrapping the coroutine resulting from self._broadcast.
• send_by_conn_id(self, message: Message) -> None
  Send a message to all the connection objects with id equals to message.conn_id.
  It spawns a new task wrapping the coroutine resulting from self._send_by_conn_id
  if conn_id is a string or from _send_multi_by_conn_id if it is a list.
• send_msg(self, message: Message) -> None
  Based on the message type, it calls send, send_by_conn_id or broadcast.
• async receive( self, connection: Connection, message: Any ) -> Coroutine[Any, Any, None]
  Receive a message from a connection object. It passes the message down to
  a private method that handle eventual subscriptions and then publish the message
  to the broker.
• async startup(self) -> Coroutine[Any, Any, None]
  Start the broker connection and the listener task.
• async shutdown(self) -> Coroutine[Any, Any, None]
  Close the broker connection and the listener task.
  It also takes care to cancel all the tasks spawned by send_msg and
  close all the connection objects before.

WebSocketProxy

The WebSocketProxy class initialise callable objects that can be used to start proxyng websocket messages from client to a server and viceversa. It's initialised with a two parameters:

• client: a WebSocket object
• server_endpoint: a str containing the endpoint of the server

Notice that the target server could be a remote server or the same server that starts the proxy.

• async __call__(self) -> Coroutine[Any, Any, None]
  Start a websocket connection to server_endpoint and spawn two tasks:
  one that forwards the messages from the client to the target and the other that
  forwards the messages from the target to the client.