Websocket cannot respond immediately in v0.111.0, but it can in v0.110.2. #11588

nowfox · 2024-05-16T07:25:07Z

nowfox
May 16, 2024

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Example Code

from fastapi import FastAPI, WebSocket, WebSocketDisconnect
from fastapi.responses import HTMLResponse
import time

app = FastAPI()

html = """
<!DOCTYPE html>
<html>
    <head>
        <title>Chat</title>
    </head>
    <body>
        <h1>WebSocket Chat</h1>
        <h2>Your ID: <span id="ws-id"></span></h2>
        <form action="" onsubmit="sendMessage(event)">
            <input type="text" id="messageText" autocomplete="off"/>
            <button>Send</button>
        </form>
        <ul id='messages'>
        </ul>
        <script>
            var client_id = Date.now()
            document.querySelector("#ws-id").textContent = client_id;
            var ws = new WebSocket(`ws://localhost:${window.location.port}/ws/${client_id}`);
            ws.onmessage = function(event) {
                var messages = document.getElementById('messages')
                var message = document.createElement('li')
                var content = document.createTextNode(event.data)
                message.appendChild(content)
                messages.appendChild(message)
            };
            function sendMessage(event) {
                var input = document.getElementById("messageText")
                ws.send(input.value)
                input.value = ''
                event.preventDefault()
            }
        </script>
    </body>
</html>
"""


class ConnectionManager:
    def __init__(self):
        self.active_connections: list[WebSocket] = []

    async def connect(self, websocket: WebSocket):
        await websocket.accept()
        self.active_connections.append(websocket)

    def disconnect(self, websocket: WebSocket):
        self.active_connections.remove(websocket)

    async def send_personal_message(self, message: str, websocket: WebSocket):
        await websocket.send_text(message)

    async def broadcast(self, message: str):
        for connection in self.active_connections:
            await connection.send_text(message)
            time.sleep(5) # new code


manager = ConnectionManager()


@app.get("/")
async def get():
    return HTMLResponse(html)


@app.websocket("/ws/{client_id}")
async def websocket_endpoint(websocket: WebSocket, client_id: int):
    await manager.connect(websocket)
    try:
        while True:
            data = await websocket.receive_text()
            await manager.send_personal_message(f"You wrote: {data}", websocket)
            await manager.broadcast(f"Client #{client_id} says: {data}")
    except WebSocketDisconnect:
        manager.disconnect(websocket)
        await manager.broadcast(f"Client #{client_id} left the chat")

Description

FastAPI 0.111.0

Open the browser and call the endpoint '/'.Enter any content in the input box, then click the send button.I will receive a response in 5 seconds.

FastAPI 0.110.2

When I click the send button, I will immediately receive a response.

I expect to receive an immediate response from websocket in the latest version.
After I execute pip uninstall uvloop, it works.
uvloop is installed during the installation of FastAPI.uvloop is a fast.But it seems to conflict with WebSocket.

Operating System

Linux, macOS

Operating System Details

This issue exists on both Linux and macOD.

FastAPI Version

0.111.0

Pydantic Version

2.7.1

Python Version

3.9.19

Additional Context

No response

YuriiMotov · 2024-05-16T07:28:59Z

YuriiMotov
May 16, 2024

Use await asyncio.sleep(5) instead of blocking sleep(5) inside async functions.

    async def broadcast(self, message: str):
        for connection in self.active_connections:
            await connection.send_text(message)
            await asyncio.sleep(5) # NOT time.sleep(5)

Using time.sleep() inside async function blocks event loop.

2 replies

nowfox May 16, 2024
Author

time.sleep(5) is just a simulation. In my project, it is a time-consuming operation.
When I need to send messages to multiple websockets in an async method,websocket cannot respond immediately in v0.111.0, but it can in v0.110.2.

YuriiMotov May 16, 2024

await asyncio.sleep(5) also simulates time-consuming operation, but in a right way for async functions. It releases event loop (doesn't block it).

If your operation is blocking (as time.sleep() is) then you need either to insert some await asyncio.sleep(0) in this code or run this blocking code in the thread executor in order not to block event loop.

The fact that this code works on previous version and doesn't work on current version doesn't mean that current version is worse. Something was changed in the implementation and now your blocking code inside async function has more impact on the app perfomance.
But initial problem is that you use blocking code inside async function.

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Websocket cannot respond immediately in v0.111.0, but it can in v0.110.2. #11588

nowfox May 16, 2024

First Check

Commit to Help

Example Code

Description

FastAPI 0.111.0

FastAPI 0.110.2

Operating System

Operating System Details

FastAPI Version

Pydantic Version

Python Version

Additional Context

Replies: 1 comment · 2 replies

YuriiMotov May 16, 2024

nowfox May 16, 2024 Author

YuriiMotov May 16, 2024

nowfox
May 16, 2024

Replies: 1 comment 2 replies

YuriiMotov
May 16, 2024

nowfox May 16, 2024
Author