---
title: "Clocks, Ordering and Happens-Before"
description: "Separate machines have separate clocks that drift, so a timestamp cannot settle which of two events came first. What can be settled is causality: one event happened before another if a message carried"
canonical: https://lightmysky.com/learn/computing/clocks-ordering-and-happens-before-mt_ratia1eb_2
source: https://lightmysky.com/learn/computing/clocks-ordering-and-happens-before-mt_ratia1eb_2.md
retrieved: 2026-09-12
---

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# Clocks, Ordering and Happens-Before

Separate machines have separate clocks that drift, so a timestamp cannot settle which of two events came first. What can be settled is causality: one event happened before another if a message carried information from the first to the second.

Subject: Computing · Area: Networks & Security · Ages 20 to 21
Page: https://lightmysky.com/learn/computing/clocks-ordering-and-happens-before-mt_ratia1eb_2

## Ready when they can

- Show two events on different machines whose order no clock can decide
- Use the happens-before relation to order the events that can be ordered
- Explain what a logical clock counts and what it does not measure

## Lesson: What clocks cannot tell you

When chat messages from two friends arrive in a mixed order, you wonder which came first. You already know machines exchange messages over a network. Each machine has its own clock, and those clocks drift apart. So a timestamp cannot settle which of two events on different machines came first.

What can be settled is causality. One event happened before another if a message carried information from the first to the second, possibly through middlemen. Here A sends to B, then B sends to C, so each step caused the next.

**Example.** Take a different setup where A and C never exchange messages, directly or through B. Their events have no definite order: no clock and no message chain decides it. Events like these are called concurrent, meaning unordered, not simultaneous.

A logical clock counts causal steps, not seconds. It tells you which orders are possible, never what time anything happened. Use it to track cause, and never read real time off it. Passing one token round a ring links each handoff to the next, so rising counts mirror that order.

**Recap.** Drifting clocks cannot order distant events, but message chains define happened-before, and the rest is concurrent.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Race Conditions and Mutual Exclusion](https://lightmysky.com/learn/computing/race-conditions-and-mutual-exclusion-mt_hygT74jJDE)
- [The Web as a Protocol: Requests, State and Caching](https://lightmysky.com/learn/computing/the-web-as-a-protocol-requests-state-and-caching-mt_MDiW4Vt0bo)

## Opens up

- [Failure Models and Replication](https://lightmysky.com/learn/computing/failure-models-and-replication-mt_aYK210-NIS)
- [Stream Processing: Windows, Watermarks and Late Events](https://lightmysky.com/learn/computing/stream-processing-windows-watermarks-and-late-events-mt_CfzXL_cL0D)
- [Impossibility Under Asynchrony: What Consensus Cannot Promise](https://lightmysky.com/learn/computing/impossibility-under-asynchrony-what-consensus-cannot-promise-mt_le54TNOeiR)