This kind of thing only works if your whole universe belongs to your database. Transactions and conditional-updates work smoothly if it's your customer browsin

This kind of thing only works if your whole universe belongs to your database. Transactions and conditional-updates work smoothly if it's your customer browsing your shop in your database - up to a point. But I usually end up with partner integrations where those techniques don't work. For instance, partners will just tell you true facts about what happened - a customer quit, or a product was removed from the catalogue. Your system can't reject these just because your Db can't or won't accept that state change.