Upgrading to Flask-SocketIO 5.x from the 4.x releases

The Socket.IO protocol recently introduced a series of backwards incompatible changes. The 5.x releases of Flask-SocketIO adopted these changes, and for that reason it can only be used with clients that have also been updated to the current version of the protocol. In particular, this means that the JavaScript client must be upgraded to a 3.x release, and if your client hasn't been upgraded to the latest version of the Socket.IO protocol, then you must use a Flask-SocketIO 4.x release.

The following protocol changes are of importance, as they may affect existing applications:

The default namespace '/' is not automatically connected anymore, and is now treated in the same way as other namespaces.
Each namespace connection has its own sid value, different from the others and different from the Engine.IO sid.
Flask-SocketIO now uses the same ping interval and timeout values as the JavaScript reference implementation, which are 25 and 5 seconds respectively.
The ping/pong mechanism has been reversed. In the current version of the protocol, the server issues a ping and the client responds with a pong.
The default allowed payload size for long--polling packets has been lowered from 100MB to 1MB.
The io cookie is not sent to the client anymore by default.