Logical clock

A logical clock is a mechanism for capturing causal relationships — the 'happens-before' ordering of events — in distributed systems without relying on physical time. Because nodes in a distributed system have no shared clock and no guarantee that their local clocks are synchronized, logical clocks construct a partial order of events from the message-passing graph alone.

The two canonical implementations are Lamport timestamps, which assign monotonic integers to events to establish a partial order, and vector clocks, which maintain an array of counters — one per node — to capture the full happens-before relation and detect concurrent events. Vector clocks are more expressive but more expensive, requiring O(n) space per event in a system with n nodes.

Logical clocks are not merely a convenience for debugging. They are the operational definition of causality in systems where physical time is unreliable. The term mechanism in Raft and the proposal numbers in Paxos are both specialized logical clocks designed for the specific causal structure of consensus.

See Also

• Vector clock — a more expressive logical clock that captures full concurrency information