ACID compliance is not ever a strict requirement, it's an abstraction guarantee. If you have a fully ACID compliant database with a sufficiently expressive quer

ACID compliance is not ever a strict requirement, it's an abstraction guarantee. If you have a fully ACID compliant database with a sufficiently expressive query interface, then the complexity of the datastore is a perfect abstraction. The ATM machines people use every day to exchange money are not fully ACID compliant, and they do not use "transactions". It's a calculated choice on the design of the system to account for the guarantees in different ways and balance discrepancies that occur after the fact. If you need full ACID compliance and need to operate at "Google scale", you reorganize the problem so that you don't need the compliance.