socket.io.emit() not running inside redis function

Question

I'm building a Flask application where I use Redis to receive data from a server, and then send that data to the frontend using Socket.IO. However, the emit() function is not sending the data to the frontend as expected.

Here is my Redis subscriber function:

def redis_subscriber():
    pubsub = r.pubsub()
    pubsub.subscribe('feed_channel')
    print("Subscribed to feed_channel")
    for message in pubsub.listen():
        if message['type'] == 'message':
            try:
                data = message['data'].decode('utf-8')
                # print(f"Emitting Redis data: {data}")
                socketio.emit('new_data', json.dumps({"message": "Connected to Flask server"}))

            except Exception as e:
                print(f"Error processing message: {e}")

The Redis subscription is working fine, and I can confirm that messages are being received. However, the socketio.emit() call doesn't seem to send the data to the frontend.

Redis is properly set up and publishing messages to the feed_channel. The Socket.IO server is running and connected to the frontend.

What could be causing the emit() function to fail in sending the data via WebSocket?

See the "troubleshooting" section of the Flask-SocketIO documentation to learn how to generate logging that can help with debugging this issue. Then please add the logs to your question. Also indicate if you are using gevent or eventlet, and if yes, if you also have applied monkey-patching to the standard library. Finally, indicate which web server you are using for this application. — Miguel Grinberg
– Miguel Grinberg, Commented Jan 11 at 10:03

avifen · Accepted Answer · 2025-01-10 16:22:53Z

0

pubsub.listen() is blocking, your redis_subscriber function gets stuck inside the for message in pubsub.listen(): loop. It waits indefinitely for a new message from Redis/Valkey.

You take the whole main thread for the subscriber and the socketio engine don't get the chance to actually do something with the data because the loop keep going.

If you want to have an async API, move to valkey-glide which will also give you better pub sub fault tolerance.

If you would like to keep using sync API, use socketio.start_background_task() (more recommended in most cases) or threading.Thread.

answered Jan 10 at 16:22

avifen

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