I am sitting on an algorithm for hard billing caps right now that seems it may have some holes, but gets close based on several very tricky distributed systems

I am sitting on an algorithm for hard billing caps right now that seems it may have some holes, but gets close based on several very tricky distributed systems problems. Making a billing cap that doesn't amount to "just use one single gateway server" (serializing everything and introducing tons of latency) seems to be harder than building a database or a filesystem, and most programmers would never attempt even those.