Clocks and Causality - Ordering Events in Distributed Systems

Logical clocks can produce timestamps that are reliably comparable, unlike conventional time-of-day timestamps, which can be undermined by clock drift and network delay assumptions. Lamport clocks use a single number and O(1) space, but they cannot deterministically order events with duplicate timestamps or tell whether two events are concurrent. Lamport origin clocks add the originating node id as a second sort key to resolve duplicate ordering, while vector clocks use an n-tuple with O(n) space so that concurrency can be detected by comparing entries. None of these clocks can order concurrent events in real time, so ordering happens after the fact.