alt Hacker NewsThe way you'd usually handle that AFAIK is to have the service ask the billing system for a "reservation" in its native units, likely with an attached TTL. Then, the service would translate those units to U.S. Dollars (or possibly Indian Rupees), taking your plan, discounts, vouchers, contracts, grandfathered pricing and all that into account. It would then "lock" the calculated amount of money, denying the reservation if total_spent + total_locked > spending_limit. After finishing the operation, the service would ask for actual billing and free the unused units.
Yes, thats one way to implement it. It still moves that global billing system into a ring 0 importance with 99.9999% availability and performance requirement, where before it wasn’t even in the picture. Not impossible, just suddenly every single AWS (or GCP or Azure etc) has a global single point of failure that gates all their performance and runtime actions.
Not to mention having no way to network isolate such service as every single service in every single network boundary needs to be able to contact such service, and every service needs (more or less) same auth permissions on all user accounts it’s running.
Again, solvable problems with enough code, but not simple problems by any means if you operate at a large scale.