Using gunicorn raises KeyError wsgi.websocket

nginx flask wsgi gevent gunicorn

I had this issue; nginx needs to be configured to proxy web sockets: http://nginx.org/en/docs/http/websocket.html.

nginx flask wsgi gevent gunicorn

The reason might be an issue when using multiple gunicorn workers, see: https://github.com/abourget/gevent-socketio/issues/132

Try setting the number of workers to one to see if this is indeed the case.