[potatosareok]

I've always wondered, (but apparently not enough to figure it out by reading the Spanner whitepaper), but how do these systems typically handle it?

I guess if you were using an append only log that recorded the exact timestamp of the transcation, your datastore would eventually reconcile that for example promo code 1 was applied twice. But what do you do then? Rollback the 2nd application of promo code and deduct the credit from user account?

Where would the logic for that be programmed?