allowEIO3 - Socket.io v4 argument

[mmilosevic]

I have a question regarding the allowEIO3 Socket.io v4 configuration option.

We have a client (an iOS app) that is using Swift client library v15 and the server with the following setup

• flask-socketio==4.3.2
• python-engineio==3.13.2
• python-socketio==4.6.1

With this configuration on the client and backend, everything works fine, but :).

We tried to upgrade flask-socketio to version 5.x sometime in January and we couldn't. python-socketio version used by flask-socketio 5.x and Swift client v15 could not communicate (EIO param on a handshake was omitted issue).
Obviously, we weren't the only one (The Socket.IO v4 server will now infer EIO=3 if the EIO query param is not included).

I am trying to do the upgrade process now and I still cannot make it work. My setup is

• client - still Swift client library v15
• backend - flask-socketio==5.1.1, python-engineio==4.2.1, python-socketio==5.4.1

Can someone tell me if the Swift client library (v15) and the latest flask-socketio version can work together if I set allowEIO3 to True? And if so, to provide me an example of how to initialize the Flask-SocketIO server properly? Thanks!

[miguelgrinberg]

The python-socketio package does not have dual EIO 3 and 4 support, you have to pick the one that you need. The 5.x supports EIO=4 only. The 4.x supports EIO=3. This is in the version compatibility table in the documentation.

[mmilosevic]

Thanks. Is there a chance that we will get a dual support at some point?

[miguelgrinberg]

I don't really have it as a priority, so it is not in my task list for the near future. I think it would make more sense to ask the developer of the client to upgrade to EIO=4.

[mmilosevic]

We can update the client, that is not a problem, problem are all older mobile applications after we upgrade the backend.