Linearizability
also called Atomic Consistency, Strong Consistency
The strongest single-object guarantee — every operation appears to take effect instantaneously at some point between its call and its return.
The property that makes a distributed store behave like a single copy of the data on one machine. If a write completes at 10:00:00.000, every read starting after that instant returns the new value — regardless of which replica serves it.
Two things follow that people often miss. It is a recency guarantee, not just an ordering one: it forbids reading stale data, which is why it costs coordination on every operation. And it is per object: linearizable registers do not compose into a serializable transaction across several objects. That is a different, stronger property.
Where it is genuinely required: anything used to make a mutually-exclusive decision — a lock, a leader lease, a uniqueness constraint, a balance check that gates a withdrawal. Where it is routinely over-applied: reads that inform rather than gate. The cost is a round trip to a quorum on every operation, which is a latency floor you cannot optimise away.