---
slug: thinker-leslie-lamport
title: Leslie Lamport — Time, Clocks, and the Ordering of Events
canonical: https://miscsubjects.com/a/thinker-leslie-lamport
---

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# Leslie Lamport — Time, Clocks, and the Ordering of Events

## §SELF — thinker-leslie-lamport

**What this page is:** A summary of Leslie Lamport's three major contributions to distributed systems theory.
**What it explains:** Logical clocks, the Paxos consensus algorithm, and the TLA+ specification language.
**Why read it:** To understand how a distributed system establishes order without a shared clock, reaches agreement when parts fail, and verifies its own correctness.

### What Leslie Lamport Is

Leslie Lamport (born 1941) is a computer scientist at Microsoft Research. He received the Turing Award in 2013 for his work on distributed systems. A distributed system is a collection of computers that communicate by sending messages to each other, with no shared memory or global clock.

### Why It Matters

Before Lamport, distributed systems researchers treated physical time as the basis for reasoning about computation. Lamport showed that logical relationships between events (what happens before what) are sufficient. This shift made it possible to design systems that are correct regardless of clock synchronization, which is the foundation of modern distributed databases, blockchains, and consensus protocols.

### The Key Idea

The "happened-before" relation (written →) is the core concept. Event A happened-before event B if one of three conditions holds: (1) A and B occur on the same process and A comes before B; (2) A is the sending of a message and B is the receipt of that same message; (3) there exists some event C where A → C and C → B (transitivity). This relation is a partial order: not all pairs of events are comparable. If A did not happen-before B and B did not happen-before A, the events are concurrent.

Logical clocks assign each event a timestamp such that if A → B then timestamp(A) < timestamp(B). Each process maintains a counter. When a process executes an event, it increments its counter. When sending a message, it includes its current counter value. When receiving a message, the process sets its counter to the maximum of its own counter and the received counter, then increments. This guarantees the happened-before ordering is preserved without physical clocks.

### What They Got Right

**Logical clocks (1978).** The paper "Time, Clocks, and the Ordering of Events in a Distributed System" defined the happened-before relation and showed that logical timestamps capture everything a distributed system needs to know about time. Vector clocks (an extension by others) later captured concurrency explicitly.

**Paxos (1989, published 1998).** A protocol for a group of distributed nodes to agree on a single value even when some nodes fail or messages are lost. Paxos guarantees safety (no two nodes can agree on different values) and liveness (agreement is reached eventually, if enough nodes are operational). It is the basis for Raft, ZooKeeper, and every modern consensus system.

**TLA+ (1994+).** The Temporal Logic of Actions is a formal specification language for describing distributed systems as mathematical formulas. TLA+ includes a model checker (TLC) that exhaustively tests a specification against all possible execution paths. Amazon, Microsoft, and others use TLA+ to find bugs in production systems before deployment.

**Lamport timestamps in practice.** Version vectors in databases (Cassandra, Riak), causality tracking in message queues, and blockchain ordering all derive from logical clocks.

### What They Got Wrong or Left Unfinished

**Paxos is famously hard to understand.** Lamport presented it as a story about a parliament on the Greek island of Paxos. The story made the algorithm harder to follow, not easier. Diego Ongaro and John Ousterhout created Raft in 2014 specifically because Paxos was too difficult to implement correctly.

**TLA+ has a steep learning curve.** It requires writing in mathematical notation, which most software engineers do not use. Adoption remains limited to large organizations with dedicated formal methods teams.

**Logical clocks do not solve the total ordering problem.** They establish partial order (some events are concurrent). When a total order is required, additional mechanisms (like a centralized sequencer or vector clocks with conflict resolution) are needed.

### How It Connects to Other Ideas

**Consensus algorithms.** Paxos is the ancestor of Raft, PBFT (Practical Byzantine Fault Tolerance), and HotStuff (used in several blockchains). All share the principle: reach agreement through rounds of proposal and confirmation.

**Formal verification.** TLA+ is one of several formal methods (alongside Coq, Isabelle, and Alloy) for proving system correctness. It differs by focusing on temporal logic (reasoning about sequences of states over time).

**Blockchain.** Nakamoto consensus in Bitcoin replaces the synchronous assumptions of Paxos with proof-of-work and probabilistic finality. The underlying problem — agreeing on a sequence of events in an untrusted network — is the same.

### Sources

Lamport, L. (1978). "Time, Clocks, and the Ordering of Events in a Distributed System." *Communications of the ACM*, 21(7), 558-565.

Lamport, L. (1998). "The Part-Time Parliament." *ACM Transactions on Computer Systems*, 16(2), 133-169.

Lamport, L. (2002). *Specifying Systems: The TLA+ Language and Tools for Hardware and Software Engineers*. Addison-Wesley.

---

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## Related on this shelf

- [Alan Kay — The Big Idea Is Messaging](https://miscsubjects.com/a/thinker-alan-kay)
- [Alfred North Whitehead — Process and Reality](https://miscsubjects.com/a/thinker-alfred-north-whitehead)
- [J.L. Austin and John Searle — Speech Acts](https://miscsubjects.com/a/thinker-austin-searle)
- [Barbara Liskov — Abstract Data Types and Distributed Consensus](https://miscsubjects.com/a/thinker-barbara-liskov)
- [Bram Cohen — BitTorrent and Content-Addressed Protocol Design](https://miscsubjects.com/a/thinker-bram-cohen)
- [Butler Lampson — Protection and Access Control](https://miscsubjects.com/a/thinker-butler-lampson)
- [Carl Hewitt — The Actor Model](https://miscsubjects.com/a/thinker-carl-hewitt)
- [Charles Sanders Peirce — Signs, Abduction, and Pragmatism](https://miscsubjects.com/a/thinker-charles-peirce)

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