Implementing this in any meaningful manner quickly begins to look like every read becoming a globally synchronised write. Of course it doesn't have to be perfect, but even approximating perfection doesn't look much different. Also, can you imagine the kind of downtimes and complaints that would inevitably originate from a fully synchronous billing architecture?

> Of course it doesn't have to be perfect, but even approximating perfection doesn't look much different.

It's pretty easy to get right, if the provider allows you to go (slightly) negative before cutting you off.

> Also, can you imagine the kind of downtimes and complaints that would inevitably originate from a fully synchronous billing architecture?

Doesn't need to fully synchronous.