The way in which errors are propagated to the caller needs to be decided before anything is implemented. Exceptions are an option but they cannot be taken into use without first writing all modules to expect one to be throw in Endpoint::connect(). The refactoring in readwritesplit to use exceptions would make this easier but an overall plan on how to handle this needs to be chosen.

Currently the simplest solution seems to be to throw exceptions from newSession to handle failures. This seems to be about on par with a std::expected approach of returning multiple values except that it can be isolated mostly to the ServiceEndpoint class. A preliminary set of patches can be found here.

The notable changes to the APIs are:

• mxs::Target::connect() can now throw exceptions.
• The newSession, routeQuery, clientReply and handleError functions in both filters and routers are allowed to throw exceptions.

Not going to make it into the 24.02 release, still needs review of the exception approach. The commits will continue to be in the develop-markusjm-exceptions branch.