Flask SocketIO Server Emit works intermittently #1917

csharmatech · 2022-11-27T17:45:16Z

csharmatech
Nov 27, 2022

Hi Miguel,
I am using Flask SocketIO with Nginx, gunicorn for catching the client events(triggered by button click) on the server side which does some processing and then emits the response back to client asynchronously. For sending asynchronous updates from server, I am using threading such that long running task (which is run in a thread) is executed and once completes sends it to client. The problem I am facing is that server emit works intermittently. Not sure where the problem lies? In the logs, I do see the server emit data.

Client side Javascript

    <script>
        var socket = io();
        $(document).ready(function() {
        var socket = io.connect('http://flask:5000');
        });

function myupdate() {
        socket.emit("my_event", function(msg) {
       });
}
socket.on("final", function(msg) {
document.getElementById("two").innerHTML=msg.data1;
document.getElementById("four").innerHTML=msg.data2;
});
</script>
<body>
<button onclick="myupdate();">Websocket Asynchronous update</button>
<pre id="place1"></pre>
<pre id="place2"></pre>
</body>

Server Side Python Code

def longdata():
    listing1 = subprocess.run(['ping','-c3', 'google.com'],stdout=subprocess.PIPE,universal_newlines=True)
    longrunning = listing1.stdout
    return longrunning

@app.route('/')
def index():
    return render_template('base.html')

@socketio.on('my_event')
def handle_my_custom_event():
        mythread = threading.Thread(target=longdata) 
        mythread.start()
        while mythread.is_alive():
            emit('final', {'data1': "Getting ping results....", 'data2': "Hold on!!\n"})
            if not mythread.is_alive():
                emit('final', {'data1': longdata(), 'data2': "Here are the results"})
                break;

csharmatech · 2022-11-27T18:14:47Z

csharmatech
Nov 27, 2022
Author

I am using gevent as below:

gunicorn -k geventwebsocket.gunicorn.workers.GeventWebSocketWorker -w 1 myapp:app --bind 0.0.0.0:5000

3 replies

miguelgrinberg Nov 27, 2022
Maintainer

It would be useful to see logs from the client and server sides for a session, so that we can see where events are getting lost. One possible source of problems is that you are mixing gevent with subprocess. Not sure how reliable that is. I assume you are aware that when using gevent all the blocking functions in the Python standard library are replaced with gevent's own implementations. This is the so called monkey patching, that I believe gunicorn should be doing for you.

csharmatech Nov 28, 2022
Author

Thanks Miguel for your feedback.

I tried to start the gunicorn using threaded worker as below (instead of using gevent) and it seems to work consistently now emitting the results from server.I tested using 15 attempts and have 100% success rate. When using gevent with same number of attempts, the success rate is 66%.

gunicorn -w 1 --threads 100 myapp:app --bind 0.0.0.0:5000

I will try to use monkey patched gevent to see how it performs.

miguelgrinberg Nov 28, 2022
Maintainer

Oh interesting, so then it seems I was right with my guess. Note that using gevent without monkey patching cannot work, you cannot mix regular threads with gevent greenlets, its either one or the other. The monkey patching will convert your long running thread to a greenlet. As far as I know, gunicorn automatically does this monkey patching.

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Flask SocketIO Server Emit works intermittently #1917

csharmatech Nov 27, 2022

Replies: 1 comment · 3 replies

csharmatech Nov 27, 2022 Author

miguelgrinberg Nov 27, 2022 Maintainer

csharmatech Nov 28, 2022 Author

miguelgrinberg Nov 28, 2022 Maintainer

csharmatech
Nov 27, 2022

Replies: 1 comment 3 replies

csharmatech
Nov 27, 2022
Author

miguelgrinberg Nov 27, 2022
Maintainer

csharmatech Nov 28, 2022
Author

miguelgrinberg Nov 28, 2022
Maintainer