Eventlet Fails Where Flask Development Server Works Fine

[Anti-Distinctlyminty]

I am experiencing an issue whereby when running in the Flask development server everything works perfectly fine, but if I use eventlet, messages are not received and the socket seems to time out, disconnect and reconnect.

Here's the gist of my server code (I've omitted the code invoking the socketio.emit)

# __init__.py

import simplejson

db = flask_sqlalchemy.SQLAlchemy()
api = flask_smorest.Api()
socketio = flask_socketio.SocketIO(json=simplejson, logger=True, engineio_logger=True)

# app.py

import flask_cors
import flask

from . import (api, db, socketio)

def get_app():

    app = flask.Flask(__name__)
    app.config.from_mapping(...)
    app.app_context().push()   

    flask_cors.CORS(app)

    # flask_sqlalchemy
    db.init_app(app)
    db.Model.metadata.reflect(bind=db.engine, extend_existing=True)

    # flask_smorest
    api.init_app(app)
    # omitted register blueprints

    #flask_socketio
    socketio.init_app(app)

    return app

def run(port):
    app = get_app()
    socketio.run(app, host="0.0.0.0", port=port)

if __name__ == "__main__":
    sys.exit(run())

And on the client side

class Listen():
    def __init__(self)
        self.sio = socketio.Client(logger=True)

    def run(self):
        @self.sio.event
        def connect():
            log.info("PostgresListen socket connection established")

        @self.sio.event
        def disconnect():
            log.info("PostgresListen socket disconnected")

        @self.sio.on("projects")
        def onProjectChanged(data):
            log.info("PosgresListen project changed")
            log.debug(data)
            self.postgresEvent.emit(data)

        self.sio.connect(nymus.socketAddress)

When running with eventlet installed, when the server broadcasts an event I get a pause, then PostgresListen socket disconnected followed by PostgresListen socket connection established

Is there something about this setup that wont work with eventlet?

[miguelgrinberg]

Are you familiar with using asynchronous frameworks and green threads? If not, I think you shouldn't use eventlet. There are many things that need to be done differently, and many libraries that are incompatible.

[Anti-Distinctlyminty]

Yes I'm trying to keep things cross platform where possible, so Windows is a consideration. One thing I noted is that I just tried replacing socketio.run(app, host="0.0.0.0", port=port) with waitress.serve(app, host="0.0.0.0", port=port) and it seems to work as with the Flask development server. If this is not an obviously wrong thing to do I could carry on. the only reason I didn't try this before was because waitress wasn't listed in the deployment section of the docs.

[miguelgrinberg]

I don't think waitress is compatible. It will work for long-polling requests, but I don't think it supports WebSocket connections, which is kind of the main point of using Socket.IO.

[Anti-Distinctlyminty]

My use case is that I'm only trying to broadcast events from the server and have clients receive the data that is pushed, not communicate anything back (I have a REST APi for that). Am I perhaps using the wrong tool for this job? Flask-SocketIO seemed to do this and be easy to implement.

[miguelgrinberg]

There are many ways to do what you want, Socket.IO is one of them, so it is right for the job. I wouldn't say that it is easy to implement. None of the options that allow the server to push data to clients are easy to implement.

[Anti-Distinctlyminty]

In which case I shall look into Gunicorn on WSL as you suggest. Thank you Miguel.