Comment by gregw2

There is a very simple but often missed pair of solutions to this REST+sync-state problem.

1) When fetching data from the source (to appear on your web form that someone will edit), ensure that you always also fetch a "version number" (or version number dictionary for multi-tables) attached with the source data (a la MVCC). (This does imply you control the backend database and can have a version# field in each table or your CRDT event table.)

Only apply updates/deletes on your backend when the version number still matches, otherwise return an error saying the state has changed, refresh the UI and have the user retry their data entry.

OR

2) When sending information back to the server in your REST PUT, include both old+new values. On the backend, only perform the UPDATE or DELETE operations if/WHERE the old values are still present (preferrably wrapped within a transaction). Otherwise return an error saying the underlying data has changed and refresh the UI and have the user retry their data entry.