Vector clock.

also: vector clocks · version vector · version vectors

A map of per-node counters attached to data that shows whether one version happened before another or both are concurrent.

the two-minute refresher

Each node increments its own counter on a write and sends the whole vector along.
V1 < V2 if every entry ≤ and at least one <: V1 happened before.
{A:1, B:2} vs {A:2, B:1}: neither dominates, so they are concurrent, a real conflict.
Detects conflicts; the application (or a CRDT) still decides how to merge.

also: vector clocks · version vector · version vectors

A map of per-node counters attached to data that shows whether one version happened before another or both are concurrent.

Each node increments its own counter on a write and sends the whole vector along.
V1 < V2 if every entry ≤ and at least one <: V1 happened before.
{A:1, B:2} vs {A:2, B:1}: neither dominates, so they are concurrent, a real conflict.
Detects conflicts; the application (or a CRDT) still decides how to merge.