Vector Clock
also: logical clock · Lamport clock
Counters that capture causality — decide whether two events are ordered or truly concurrent.
Wall clocks lie across machines, so distributed systems track causality instead: each node keeps a counter per node, increments its own on events, and merges on communication. Comparing vectors tells you A happened-before B, or they are concurrent — genuine conflicts needing resolution. Dynamo-style stores used them to detect conflicting writes; simpler Lamport clocks give total order without concurrency detection.