I have a question very similar to this one: Send data every 10 seconds via socket.io

But: My server is written in Python, the client is in JavaScript

The goal:

• Clients connect to server via socket.io
• Clients receive push messages ping from server every n seconds
• When a client sends a ping message, the server broadcasts a pong message

What works:

• Socket.io connection works fine
• Client ping is received by server, answered with pong, which is again received by client
• Server executes ping_in_intervals every 5 seconds

What doesn't work:

• When server executes ping_in_intervals (which triggers sending a ping), that ping is not received by any client
• When ping_in_intervals loop is active, socket connections crash every minute or so. If the method is commented out, then socket connection stays stable.

Observations:

• The thread, that ping_in_intervals is running in doesn't seem to properly work together with the wsgi server thread.
• The ping_in_intervals thread destabilizes the server thred, causes it to loose connections (which are reestablished right away, but they do drop every minute or so)
• I think, that I'm doing something terribly wrong with threading. I have very little experience with threading in Python and don't know, where to look for the problem

Server:

import eventlet
import socketio
import threading

sio = socketio.Server(cors_allowed_origins="*", async_mode='eventlet')
app = socketio.WSGIApp(sio)


def ping_in_intervals():
    threading.Timer(5.0, ping_in_intervals).start()
    print("send ping")
    sio.emit('ping')


@sio.on('ping')
def ping(*args):
    print("received ping - send pong")
    sio.emit('pong')


ping_in_intervals()
eventlet.wsgi.server(eventlet.listen(('', 8080)), app)

Client:

const socket = io.connect('localhost:8080', {secure: true, transports: ['websocket']});

socket.on('pong', () => {
    console.log('received pong');
});

socket.on('ping', () => {
    console.log('received ping');
});

socket.on('connect', () => {
    socket.emit('ping')
});