The standard mechanism for backend-aware service overrides is in and these services are already tagged. We have agreed on how to transfer data from one backend to another and my sandbox contains the first getTransferIterator implementation for config with more to follow. There's a small amendment in the works that makes the old backend available too.

More field renames are in the works. That's why I am not focusing on the MongoDB drivers just yet. Let's wait for beta. The drivers in a state where we can do some meaningful testing (which lead to making sure tests are modifiable before) and so we can make sure everything will work for us but writing the entity drivers themselves at this point is a waste of time -- let's wait for a (more) stable API. The transfer work started because MongoDB needed to solve taking over config storage and while we had it solved, there is really nothing MongoDB specific so I am pushing for core inclusion.

I didn't mention config devel in this series -- although I did previously on this blog -- I firmly believe this will be used widely and lead to a more joyous CMI experience. Reminder: everyone using CMI is what makes the Drupal on MongoDB only feasible in the first place.