Lamport Timestamps — Total Order That Respects Causality Diagram

The Lamport Clock Rules

Every node keeps a local counter that starts at 0, and its timestamp is the pair (counter, nodeID)
Sending a message or doing local work: counter += 1, then stamp the event with (counter, nodeID)
Receiving a message: counter = max(local counter, seen counter) — the counter never goes backwards

What Happens at B

A's message (1,A) arrives first: max(0, 1) = 1, so B's counter becomes 1
C's message (1,C) arrives next: max(1, 1) = 1, so B's counter stays 1
B's reply is a send event: 1 + 1 = 2, so the reply carries (2, B)

Total Order — and Its Blind Spot

Compare counters first, break ties by node ID: (1, A) < (1, C) < (2, B)
The resulting total order is consistent with causality: a cause always sorts before its effect
Limitation: (1, A) and (1, C) are concurrent, but their ordering is indistinguishable from a causal one — detecting concurrency needs vector clocks