It’s not mentioned in the docs for Flask-SocketIO that Eventlet has an option max_size which by default limits the maximum number of client connections opened at any time to 1024. There is no way to pass it through flask run command, so the application should be run with socketio.run, for example:

...
if __name__ == '__main__':
    socketio.run(app, host='0.0.0.0', port='8080', max_size=int(os.environ.get('EVENTLET_MAX_SIZE', 1024)))

Socket.IO requires the sticky sessions, so when it’s runned in Docker Swarm environment, the Swarm load balancer can forward the request to any node where the service is launched and there will be the 400 errors. The solution is to limit the number of nodes for Socket.IO service to one or to use another load balancer like Traefik or HAProxy.

If you have some long background tasks, sometimes it’s useful to notify a user about the progress. In this article i put an example for flask, celery and socket.io. In short form it’s described in flask-socketio documentation.