Websocket Client not receiving any messages

I have Python client which opens a websocket connection to a server and subscribes to particular topic using STOMP protocol, subscription goes just fine as i see on the server all is fine. However, When the server publishes a few messages the client does not receive any. Here are the codes used:

Client

# coding: utf-8
import websocket
import stomp
import stomper
token = "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJhZG1pbiIsInByaW5jaXBhbF9uYW1lIjoiYWRtaW4iLCJpc3MiOiJBdGhlbmEiLCJ1c2VydHlwZSI6IkxPQ0FMIiwiYW9zX3ZlcnNpb24iOiJldXBocmF0ZXMtNS4xMS1zdGFibGUiLCJyZWdpb24iOiJlbi1VUyIsImV4cCI6MTczNDI4MDI3NywidXVpZCI6ImI4MzhjOGRkLWI4NmQtNGNkZS05ZTE4LTUxM2E1OTk4ODhhYyIsImlhdCI6MTU3NjYwMDI3NywiYXV0aG9yaXRpZXMiOiJST0xFX0NMVVNURVJfQURNSU4sUk9MRV9NVUxUSUNMVVNURVJfQURNSU4sUk9MRV9VU0VSX0FETUlOLFJPTEVfQ0xVU1RFUl9WSUVXRVIiLCJqdGkiOiI1NTU1ZjEwZC04NGQ5LTRkZGYtOThhNC1mZmI1OTM1ZTQwZWEifQ.LOMX6ppkcSBBS_UwW9Qo2ieWZAGrKqADQL6ZQuTi2oieYa_LzykNiGMWMYXY-uw40bixDcE-aVWyrIEZQbVsvA"
headers = {"Authorization": "Bearer " + token}
uri = "ws://127.0.0.1:8765/notifications/websocket"
def on_msg(ws, msg):
    print(msg)

def on_error(ws, err):
    print(err)

def on_closed(ws):
    print("#Closed#")

def on_open(ws):
    sub = stomper.subscribe("/user/queue/alert", "MyuniqueId", ack="auto")
    ws.send(sub)

headers = {"Authorization": "Bearer " + token}



websocket.enableTrace(True)
ws = websocket.WebSocketApp(uri, header=headers, on_message=on_msg, on_error=on_error, on_close=on_closed)
ws.on_open = on_open
ws.run_forever()

Code server uses to publish the message:

    for (WatchesSubscription s : subscriptions) {
            template.convertAndSendToUser(s.getSession().getUser(), destination, dto);
        }

When i checked out the value of the above variables i saw that destination was as expected queue/alerts. I have java client to test out as well and it works just fine. I have even tried this by subscribing to /topic/alerts and sending to it via template.convertAndSend(/topic/alerts), here too i received nothing. I am a drawing a complete blank on this and would appreciate any sort of help!

Answer

After many days of hair pulling I finally figured out the reason and the fix!

1. The java client I used was WebSocketStompClient stompClient = new WebSocketStompClient(transport);.The stompClient.connect(URL, webSocketHttpHeaders, sessionHandler); method implicitly sends a stomp CONNECTnnx00n
2. The Springboot server which has been configured for STOMP understands this as a connection request and responds with a CONNECT_ACK.
3. When this ACK is sent it also updates it’s local UserRegistry with the new user. So the internal message broker knows that there is a user who has subscribed to so-and-so topic.
4. In my Python code, i had merely opened a Websocket connection and after that directly sent a SUBSCRIBE message. So the broker never got a CONNECT so the user was never stored! This resulted in the messages later on being published to be merely discarded by the broker.
5. The fix was to send a CONNECTnnx00n after opening up the connection and before the subscription. Here is the code:

def on_open(ws):
    #The magic happens here!
    ws.send("CONNECTnaccept-version:1.0,1.1,2.0nnx00n")
    sub = stomper.subscribe("/user/queue/alert", "MyuniqueId", ack="auto")
    ws.send(sub)