Python 在FastAPIStarlette中使用WebSockets时如何运行后台任务

编写的代码只在调用端点时触发其他服务器，并等待来自Redis中特定通道的数据进入.我不想知道外部服务器的业务逻辑是由于延迟而完成的.

下面的call_external_server后台任务函数将通知外部服务器向Redis(发布/订阅)发送数据.然而，它不会执行.

以下是我的代码:

async def call_external_server(channel, text):
    print("call_external_server start")
    async with aiohttp.ClientSession() as session:
        async with session.get(f"http://localhost:9000/pub?channel={channel}&text={text}") as resp:
            print(resp)
    print("call_external_server finished")
    return {"response": "external_server is done"}


@app.websocket("/ws")
async def websocket_endpoint(channel: str, websocket: WebSocket, background_task: BackgroundTasks):
    await websocket.accept()
    client_info = dict(websocket.headers)
    text = client_info.get("text")
    redis_reader: redis.client.PubSub = await get_redis_pubsub()
    await redis_reader.subscribe(channel)
    

    # Problem is Here 
    background_task.add_task(call_external_server, channel, text)
    # Background task Doesn't work properly 

    try:
        while True:
            message = await redis_reader.get_message(ignore_subscribe_messages=True)
            if message is not None:
                decoded_msg = message["data"].decode()
                if decoded_msg == STOPWORD:
                    print("(Reader) STOP")
                    break
                await websocket.send_text(decoded_msg)
    except Exception as e:
        print(e)
        await websocket.close()
        return
    await websocket.close()
    return