My take is and has been for a long time that the right solution is something like a key/value store on the backend + a SQLite4-style frontend, with a Lustre-style lock manage co-located with the k/v backend, and key/value pairs distributed by key ranges. This is... a lot like Spanner I think, though I don't know if Spanner uses a DLM like Lustre or if it does something else for synchronization.
This way you get the scalability benefits of a key/value store with all the benefits of a SQL, and you get always-consistent, and even ACID semantics (depending on how many failures there are at once and how many you design to).
This way you get the scalability benefits of a key/value store with all the benefits of a SQL, and you get always-consistent, and even ACID semantics (depending on how many failures there are at once and how many you design to).
EDIT: Oh, and check out https://github.com/rqlite/rqlite/ .