Python SocketIO KeyError: 'Session is disconnected'

Question

On a small Flask webserver running on a RaspberryPi with about 10-20 clients, we periodically get this error:

Error on request:
Traceback (most recent call last):
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/werkzeug/serving.py", line 270, in run_wsgi
    execute(self.server.app)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/werkzeug/serving.py", line 258, in execute
    application_iter = app(environ, start_response)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/flask_socketio/__init__.py", line 43, in __call__
    start_response)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/engineio/middleware.py", line 47, in __call__
    return self.engineio_app.handle_request(environ, start_response)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/socketio/server.py", line 360, in handle_request
    return self.eio.handle_request(environ, start_response)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/engineio/server.py", line 291, in handle_request
    socket = self._get_socket(sid)
  File "/home/pi/3D_printer_control/env/lib/python3.7/site-packages/engineio/server.py", line 427, in _get_socket
    raise KeyError('Session is disconnected')
KeyError: 'Session is disconnected'

The error is generated automatically from inside python-socketio. What does this error really mean and how can I prevent or suppress it?

score 6 · Accepted Answer · answered Oct 27 '21 at 22:18

As far as I can tell, this usually means the server can't keep up with supplying data to all of the clients.

Some possible mitigation techniques include disconnecting inactive clients, reducing the amount of data sent where possible, sending live data in larger chunks, or upgrading the server. If you need a lot of data throughput, there may be also be a better option than socketIO.

I have been able to reproduce it by setting a really high ping rate and low timeout in the socketIO constructor:

from flask_socketio import SocketIO

socketio = SocketIO(engineio_logger=True, ping_timeout=5, ping_interval=5)

This means the server has to do a lot of messaging to all of the clients and they don't have long to respond. I then open around 10 clients and I start to see the KeyError.

Further debugging of our server found a process that was posting lots of live data which ran fine with only a few clients but starts to issue the occasional KeyError once I get up to about a dozen.

Python SocketIO KeyError: 'Session is disconnected'

1 Answers1