Flask-SocketIO - WebSocket is closed before the connection is established. [Heroku]

[KarteekAti]

While running Flask on my local server everything was fine, but after moving to Heroku I started getting "WebSocket is closed before the connection is established."

Heroku Logs

2021-10-26T08:23:02.637338+00:00 app[web.1]: [2021-10-26 08:23:02 +0000] [4] [CRITICAL] WORKER TIMEOUT (pid:43)
2021-10-26T08:23:02.637839+00:00 app[web.1]: [2021-10-26 08:23:02 +0000] [43] [INFO] Worker exiting (pid: 43)
2021-10-26T08:23:02.729197+00:00 app[web.1]: [2021-10-26 08:23:02 +0000] [45] [INFO] Booting worker with pid: 45
2021-10-26T08:23:03.105363+00:00 heroku[router]: at=info method=GET path="/socket.io/?EIO=4&transport=websocket" host=chatroom-private.herokuapp.com request_id=c6a836a6-5fbd-4e6b-8d21-bd97d72513e4 fwd="103.86.182.226" dyno=web.1 connect=0ms service=15569ms status=101 bytes=129 protocol=https
2021-10-26T08:23:03.105683+00:00 app[web.1]: Traceback (most recent call last):
2021-10-26T08:23:03.105716+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/gevent/pywsgi.py", line 999, in handle_one_response
2021-10-26T08:23:03.105717+00:00 app[web.1]:     self.run_application()
2021-10-26T08:23:03.105719+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/geventwebsocket/handler.py", line 75, in run_application
2021-10-26T08:23:03.105719+00:00 app[web.1]:     self.run_websocket()
2021-10-26T08:23:03.105728+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/geventwebsocket/handler.py", line 52, in run_websocket
2021-10-26T08:23:03.105729+00:00 app[web.1]:     list(self.application(self.environ, lambda s, h, e=None: []))
2021-10-26T08:23:03.105731+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/flask/app.py", line 2091, in __call__
2021-10-26T08:23:03.105731+00:00 app[web.1]:     return self.wsgi_app(environ, start_response)
2021-10-26T08:23:03.105742+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/flask_socketio/__init__.py", line 43, in __call__
2021-10-26T08:23:03.105742+00:00 app[web.1]:     return super(_SocketIOMiddleware, self).__call__(environ,
2021-10-26T08:23:03.105749+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/middleware.py", line 63, in __call__
2021-10-26T08:23:03.105750+00:00 app[web.1]:     return self.engineio_app.handle_request(environ, start_response)
2021-10-26T08:23:03.105757+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/socketio/server.py", line 589, in handle_request
2021-10-26T08:23:03.105758+00:00 app[web.1]:     return self.eio.handle_request(environ, start_response)
2021-10-26T08:23:03.105760+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/server.py", line 379, in handle_request
2021-10-26T08:23:03.105760+00:00 app[web.1]:     r = self._handle_connect(environ, start_response,
2021-10-26T08:23:03.105769+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/server.py", line 554, in _handle_connect
2021-10-26T08:23:03.105769+00:00 app[web.1]:     ret = s.handle_get_request(environ, start_response)
2021-10-26T08:23:03.105776+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/socket.py", line 103, in handle_get_request
2021-10-26T08:23:03.105776+00:00 app[web.1]:     return getattr(self, '_upgrade_' + transport)(environ,
2021-10-26T08:23:03.105783+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/socket.py", line 158, in _upgrade_websocket
2021-10-26T08:23:03.105783+00:00 app[web.1]:     return ws(environ, start_response)
2021-10-26T08:23:03.105790+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/async_drivers/eventlet.py", line 16, in __call__
2021-10-26T08:23:03.105791+00:00 app[web.1]:     raise RuntimeError('You need to use the eventlet server. '
2021-10-26T08:23:03.105807+00:00 app[web.1]: RuntimeError: You need to use the eventlet server. See the Deployment section of the documentation for more information.
2021-10-26T08:23:03.105880+00:00 app[web.1]: 2021-10-26T08:23:03Z {'REMOTE_ADDR': '10.1.95.245', 'REMOTE_PORT': '29183', 'HTTP_HOST': 'chatroom-private.herokuapp.com', (hidden keys: 39)} failed with RuntimeError
2021-10-26T08:23:03.105881+00:00 app[web.1]: 
2021-10-26T08:23:03.515546+00:00 heroku[router]: at=info method=GET path="/socket.io/?EIO=4&transport=websocket" host=chatroom-private.herokuapp.com request_id=27a8989f-fda3-46c1-9e5f-c9e3a5354c49 fwd="103.86.182.226" dyno=web.1 connect=0ms service=1ms status=101 bytes=129 protocol=https
2021-10-26T08:23:03.512553+00:00 app[web.1]: Traceback (most recent call last):
2021-10-26T08:23:03.512578+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/gevent/pywsgi.py", line 999, in handle_one_response
2021-10-26T08:23:03.512579+00:00 app[web.1]:     self.run_application()
2021-10-26T08:23:03.512581+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/geventwebsocket/handler.py", line 75, in run_application
2021-10-26T08:23:03.512581+00:00 app[web.1]:     self.run_websocket()
2021-10-26T08:23:03.512590+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/geventwebsocket/handler.py", line 52, in run_websocket
2021-10-26T08:23:03.512590+00:00 app[web.1]:     list(self.application(self.environ, lambda s, h, e=None: []))
2021-10-26T08:23:03.512592+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/flask/app.py", line 2091, in __call__
2021-10-26T08:23:03.512593+00:00 app[web.1]:     return self.wsgi_app(environ, start_response)
2021-10-26T08:23:03.512613+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/flask_socketio/__init__.py", line 43, in __call__
2021-10-26T08:23:03.512613+00:00 app[web.1]:     return super(_SocketIOMiddleware, self).__call__(environ,
2021-10-26T08:23:03.512620+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/middleware.py", line 63, in __call__
2021-10-26T08:23:03.512620+00:00 app[web.1]:     return self.engineio_app.handle_request(environ, start_response)
2021-10-26T08:23:03.512628+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/socketio/server.py", line 589, in handle_request
2021-10-26T08:23:03.512628+00:00 app[web.1]:     return self.eio.handle_request(environ, start_response)
2021-10-26T08:23:03.512630+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/server.py", line 379, in handle_request
2021-10-26T08:23:03.512630+00:00 app[web.1]:     r = self._handle_connect(environ, start_response,
2021-10-26T08:23:03.512639+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/server.py", line 554, in _handle_connect
2021-10-26T08:23:03.512639+00:00 app[web.1]:     ret = s.handle_get_request(environ, start_response)
2021-10-26T08:23:03.512646+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/socket.py", line 103, in handle_get_request
2021-10-26T08:23:03.512646+00:00 app[web.1]:     return getattr(self, '_upgrade_' + transport)(environ,
2021-10-26T08:23:03.512653+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/socket.py", line 158, in _upgrade_websocket
2021-10-26T08:23:03.512653+00:00 app[web.1]:     return ws(environ, start_response)
2021-10-26T08:23:03.512660+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/engineio/async_drivers/eventlet.py", line 16, in __call__
2021-10-26T08:23:03.512661+00:00 app[web.1]:     raise RuntimeError('You need to use the eventlet server. '
2021-10-26T08:23:03.512675+00:00 app[web.1]: RuntimeError: You need to use the eventlet server. See the Deployment section of the documentation for more information.
2021-10-26T08:23:03.512745+00:00 app[web.1]: 2021-10-26T08:23:03Z {'REMOTE_ADDR': '10.1.23.225', 'REMOTE_PORT': '13370', 'HTTP_HOST': 'chatroom-private.herokuapp.com', (hidden keys: 39)} failed with RuntimeError
2021-10-26T08:23:03.512746+00:00 app[web.1]: 
2021-10-26T08:23:03.513320+00:00 app[web.1]: Traceback (most recent call last):
2021-10-26T08:23:03.513345+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/eventlet/hubs/hub.py", line 476, in fire_timers
2021-10-26T08:23:03.513345+00:00 app[web.1]:     timer()
2021-10-26T08:23:03.513352+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/eventlet/hubs/timer.py", line 59, in __call__
2021-10-26T08:23:03.513353+00:00 app[web.1]:     cb(*args, **kw)
2021-10-26T08:23:03.513361+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.9/site-packages/eventlet/semaphore.py", line 152, in _do_acquire
2021-10-26T08:23:03.513362+00:00 app[web.1]:     waiter.switch()
2021-10-26T08:23:03.513363+00:00 app[web.1]:   File "src/gevent/greenlet.py", line 910, in gevent._gevent_cgreenlet.Greenlet.run
2021-10-26T08:23:03.513373+00:00 app[web.1]:   File "src/gevent/greenlet.py", line 875, in gevent._gevent_cgreenlet.Greenlet._Greenlet__report_result
2021-10-26T08:23:03.513381+00:00 app[web.1]:   File "src/gevent/_gevent_cgreenlet.pxd", line 45, in gevent._gevent_cgreenlet.get_my_hub
2021-10-26T08:23:03.513393+00:00 app[web.1]: TypeError: Cannot convert greenlet.greenlet to gevent._gevent_c_greenlet_primitives.SwitchOutGreenletWithLoop

python server

app = Flask(__name__,instance_relative_config=False)
CORS(app)
app.config.from_object('config.Config')
app.config['CORS_HEADERS'] = 'Content-Type'
socketio = SocketIO(app,cors_allowed_origins='*')

@app.route("/",methods=['GET', 'POST'])
def index():
  return render_template('index.html',)

@socketio.on('message')
def handleMessage(msg):
	print('Message: ' + msg)
	send(msg, broadcast=True)

if __name__ == '__main__':
	port = int(os.environ.get('PORT', 5000))
	socketio.run(app,port=port)  

client

<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.3.2/socket.io.js" crossorigin="anonymous">
var connectionOptions =  {
            "force new connection" : true,
            "reconnectionAttempts": "Infinity", //avoid having user reconnect manually in order to prevent dead clients after a server restart
            "timeout" : 10000, //before connect_error and connect_timeout are emitted.
            "transports" : ["websocket"]
        };
$(document).ready(function() {
	const socket = io(connectionOptions);

Procfile

web: gunicorn -k geventwebsocket.gunicorn.workers.GeventWebSocketWorker -w 1 app:app

[miguelgrinberg]

Do you see the discrepancy? You are running Gunicorn with the gevent-websocket worker, but Flask-SocketIO thinks it is configured to use eventlet:

RuntimeError: You need to use the eventlet server. See the Deployment section of the documentation for more information.

Which one do you want to use? If you want gevent, then add async_mode='gevent' to your SocketIO() object, to override the default. Or as an alternative, make sure eventlet is not installed in your virtualenv, and then Flask-SocketIO is going to go with the next option, which is gevent.

[KarteekAti]

Thanks, after adding async_mode='gevent' at SocketIO() it works absolutely fine!