Unidirectional websocket connections where only the server pushes data to the clients

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Message text was: {data}")

Description

Hello, Is there a way I could send data to clients over websocket without listening for when clients send data back. I’m trying to have a websocket endpoint where the server is pushing data to the client in a unidirectional way without the option for the client to send responses back. There doesn’t seem to be any code that I could find that supports this since all the documentation seems to require that the server is listening for a websocket.recieve_text(). Any help would be much appreciated, thanks.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.81.0

Python Version

3.8.13

Additional Context

No response

[JarroVGIT]

Yes, but is has nothing to do with websockets.

The issue is that the while loop is ran multiple times during a second. That’s why I sleep for 1 second, so I know the time is at least in the next second when the while loop is run again.

This is not perfect logic either, if you require data to be send exactly every second, then you would have to design for that. But that was not the issue in this topic, that is more of ansubject to be asked on SO.

[JarroVGIT]

Your logic is flawed I am afraid (the modulo operation). The following works fine (and runs as-is):

import asyncio
import datetime

from fastapi import FastAPI, WebSocket, WebSocketDisconnect
from starlette.websockets import WebSocketClose
from websockets.exceptions import ConnectionClosedError

app = FastAPI()


@app.get("/")
async def root():
    return {"hello": "world"}


@app.websocket("/wsx")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        t = datetime.datetime.utcnow().time()
        try:
            if t.second % 5 == 0:
                print("Sending the time!")
                await websocket.send_text(str(t))
                await asyncio.sleep(1)
        except ConnectionClosedError:
            print("Client disconnected.")
            break


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app, host="0.0.0.0", port=8000)

Left is my client connecting with the websocket endpoint. Right is the logs of the above snippet. I disconnected using ctrl-c (otherwise it would run forever).