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2024

Managing Exceptions in WebSockets with FastAPI

In this post, we delve into the management of exceptions in WebSockets, focusing on a potent but often overlooked feature: the WebSocketException offered by Starlette.

Understanding WebSocketException

Conceptually, WebSocketException enables you to close a WebSocket connection with a specific code and reason by raising this exception in your WebSocket route. Here's an illustrative example:

from fastapi import FastAPI, WebSocket
from fastapi.exceptions import WebSocketException

app = FastAPI()

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    raise WebSocketException(code=1008, reason="Closing the connection...")

In this instance, a WebSocketException is raised bearing the code 1008 and the explicit reason for closure: "Closing the connection...".

To run this application, first install FastAPI, Uvicorn, and WebSockets:

pip install fastapi uvicorn websockets

Run the application using Uvicorn:

uvicorn main:app --reload

On opening a WebSocket connection to ws://localhost:8000/ws, you will find the connection being closed with focal code 1008 and the attributed reason.

I use wscat for testing WebSocket connections; you can install it with the following command:

npm install -g wscat

A connection is opened with:

wscat -c ws://127.0.0.1:8000/ws

Handle custom exceptions

The app.exception_handler can be used to handle custom exceptions. Consider the following sample:

from fastapi import FastAPI, WebSocket
from fastapi.exceptions import WebSocketException

app = FastAPI()

class CustomException(Exception): ...

@app.exception_handler(CustomException)
async def custom_exception_handler(websocket: WebSocket, exc: Exception):
    await websocket.close(code=1008, reason="Custom exception")

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    raise CustomException()

In this example, the client receives a WebSocket closure with code 1008 and the reason "Custom exception" when the CustomException is raised.

Feel free to connect with me on LinkedIn for any questions or discussions on this topic.

Happy coding! 🚀