Time, Clocks, and the Ordering of Events in a…

Lamport asks how separated computers can agree on a useful event order when no perfect global clock tells them what happened first. Read it to see how local event order and delivered messages define causality, how message-carried logical counters preserve that order, and why the resulting timestamps still cannot prove that two events influenced one another.

Reading focus: How same-process order, message send before receive, and transitivity define the happened-before partial order while unrelated events remain concurrent. Why each process increments a logical counter and advances it beyond a received timestamp before recording the receive event. How a fixed tie-breaker extends causality to a useful but arbitrary total order, and which delivery, participation, external-causality, and physical-time limits remain.

Communications of the ACM 1978. Leslie Lamport. 30 min read, easy difficulty.