[hinkley]

When a single system is receiving messages, you pick an observed order of events that meets some definition of fairness, and you stick with it all the way through a transaction. By pretending A happens before B (even if you're not entirely sure) you can return a self-consistent result. And once you have that you can simplify a lot of engineering and make a lot of optimizations, so that the requests aren't just reliable but also timely.

You throw three more observers in and how do you make sure that all of them observe the requests arriving in the same order? Not even the hardware can guarantee that packets arrive at 4 places in the same order, even if the hardware is arranged in a symmetrical fashion (which takes half the fun out of a clustered solution).