This is the Balance service. It tracks the balance for a customer in particular for the following scenarios
Incrementing the balance when the customer tops up their account
Decrementing the balance when the customer uses the API
This means that it is designed for potentially high reqs/sec per customer - e.g. you'd expect high traffic customers to cause high traffic to the balance service. With this in mind it trades off accuracy for speed - it is not a ledger of debits and credits. In the event of failure it could under count usage which would mean that some API usage is unaccounted for which is a failure in favour of the customer. The record of customer payments is stored in our payments provider and as such, the danger of missing a customer payment is reduced.