[lolinder]

For the one DB file per user question, I'll take a stab at a solution:

* Global store of user identifiers mapped to user IDs. Identifiers can change but the underlying user ID is immutable.

* One DB per user ID.

The per-user DB doesn't really need to know the user identifier, and at auth time you don't need to access the business logic, so cross-DB updates should not be a problem.

In the general case, similar logic applies: you can't partition just anywhere, but where you can partition is pretty well defined.