IMHO, the plan to support Custom Data Stores is a welcome enhancement that should enable a diverse ecosystem of stores (SQL, NOSQL, etc.). That said, a brief search of the internet finds a surprising lack of implementations. Fundamentally, it appears this capability is NOT ready for general adoption. Documentation is extremely sparse (or missing) and it appears important key properties are not accessible (e.g. in DefaultSnapshot and BackingData). As a next step, I'd like to propose the Apple team implement an actual custom DataStore using an actual SQL DB, enhance the documentation and code based on the experience, and publish so the broader community can help cary this forward.