He suggester that they had both. My question was what to do when they disagree, and he wasn't able to either conceive of the 'two sources of truth problem' or wasn't able to suggest a solution. He pushed back saying: if they disagree then you've got buggy code, and that your job is to not write buggy code. "That's why we test". "For race conditions?" etc. etc.

Who owns the database in your locked example?

Revolut would own the database in my example.

How would a ledger work without having some kind of lock, how would you prevent a negative balance?

When someone checks their balance, would you to retrieve every transaction from the ledger and calculate the current balance?