Atomic State

Start with the idea

Atomic operations are indivisible but still need an ordering that describes cross-thread visibility. Use the weakest ordering justified by a written synchronization argument, not by habit.

An atomic value supports operations that are indivisible with respect to competing threads. fetch_add on AtomicUsize, for example, prevents two workers from losing an increment by reading and writing the same stale count. This makes atomics a good fit for simple counters, flags, and some carefully designed state machines. It does not turn a sequence of several atomic operations into one transaction. If two fields must change together, a mutex may express the invariant more clearly.