Strong consistency models

Network partitions are going to happen. Switches, NICs, host hardware, operating systems, disks, virtualization layers, and language runtimes, not to mention program semantics themselves, all conspire to delay, drop, duplicate, or reorder our messages. In an uncertain world, we want our software to maintain some sense of intuitive correctness.

Well, obviously we want intuitive correctness. Do The Right Thing™! But what exactly is the right thing? How might we describe it? In this essay, we’ll take a tour of some “strong” consistency models, and see how they fit together.

Correctness

There are many ways to express an algorithm’s abstract behavior–but just for now, let’s say that a system is comprised of a state, and some operations that transform that state. As the system runs, it moves from state to state through some history of operations.