Right -- that's why I said most RDBMS. To get ACID and CAP-consistent (which, as an aside, are not the same thing) cross-shard transactions requires some form of migrating your production system onto such a distributed-capable RDBMS that you describe (of which, Spanner arguably fits the description of).
The thing I'm confused about is why migrating your production system onto a custom in-house data store (that you've written and you maintain) is easier / less work than migrating onto something you can use off the shelf which solves the problem you have.
I hear the argument that migrating from (say), postgres -> postgres + books is easier than moving everything into cockroachdb. But why is postgres + books easier than postgres + cockroach? The latter doesn't require you to write your own database from scratch. (And writing your own database is fun, so if you're doing it for that reason I understand, but thats not what the post says.)
Edit: Ah, I see - my confusion was because I missed the part which mentioned that this is just built on top of spanner. That makes sense!
For a fun fact, CockroachDB was actually started by ex-Squares, so we're definitely very familiar with this exact same argument internally, back in the day when neither CockroachDB nor Spanner existed as a viable option :)
