Flask, FlaskSocketIO - RuntimeError: Cannot obtain socket from WSGI environment

Question

When I try to use the functionality that uses websockets in my application, I get this error in the console:

File "/Users/user/venv/lib/python3.7/site-packages/simple_websocket/ws.py", line 138, in __init__
    raise RuntimeError('Cannot obtain socket from WSGI environment.')
RuntimeError: Cannot obtain socket from WSGI environment.

I also get this error in the browser console: WebSocket connection to 'ws://localhost:5000/socket.io/?EIO=4&transport=websocket&sid=40NYzDgGYStMR0CEAAAJ' failed:

I tried using gevent, gevent-websocket, and eventlet, but this created other issues, and I'm not sure that I need to use gevent or eventlet for this use case.

Here's the rest of the relevant code:

__ init __.py

from flask_socketio import SocketIO
...

socketio = SocketIO()

def create_app(config_class=Config):
    app = Flask(__name__)
    app.config.from_object(Config)

    socketio.init_app(app, cors_allowed_origins='*')

    ...

    return app

app.py

from app import create_app, socketio

app = create_app()

if __name__ == '__main__':
    socketio.run(app)

routes.py This only accepts POST requests because I send data to this route from a Celery task

from app import socketio

...

@main_bp.route('/send_message', methods=['POST'])
def send_message():
    
    ...

    socketio.emit('test_message', {'msg':'Test'}, namespace='/test')

    return 'Results Sent'

index.html

        var socket = io.connect('http://localhost:5000/test');
        socket.on('connect', function(){
            console.log('Connected to socket')

        });

        socket.on('test_message', function(message){
            console.log('Received test message');
            }
        )

Note that in the browser console I'll see "Connected to socket", but not "Received test message"

Answer 1

You are using the simple-websocket package. This package has a list of supported web servers. The error indicates that you are using a web server that is not in the supported list.

Supported web servers are:

• The Flask dev server (for development purposes only, of course)
• Gunicorn
• Eventlet
• Gevent

From this list, it seems your only choice for a production web server is Gunicorn, since you say that eventlet/gevent won't work for your needs.

The use of Gunicorn is covered in the documentation. In that section, simple-websocket is the third option mentioned. Here is the example start up command:

gunicorn -w 1 --threads 100 module:app