Random 64 bit ids are used in the process. So the probability of accidentally completing a tuple is very, very small (1 / 2^64 for every ack).

Counters don't work because of the asynchronous nature of Storm. For example, consider a topology that looks like this:

A -> B -> C

      \-> D

Let's say A emits 2 tuples (+2 differential), B processed those and emits 2 to C and 3 to D (+3 differential), C processes 2 tuples (-2 differential), and D processes 3 tuples (-3 differential).

Everything's asynchronous, so the acker could receive the acks in this order: A, C, B, D

The counter would then look like this: 2, 0, 3, 0

So it would think that the tuple was complete before it actually was, which means the counter algorithm doesn't work.