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Intermediate Topic 22 of 51 Distributed Systems & Cloud

Paxos

Classic family of consensus protocols.

How it works

Paxos is a family of protocols, introduced by Leslie Lamport, that lets a set of nodes agree on a value despite crashes and message loss, as long as a majority remains. In the basic protocol, a proposer picks a ballot number and sends prepare messages; acceptors promise not to accept lower ballots and report any value already accepted; the proposer then proposes a value, which is chosen when a majority accept it. Multi-Paxos amortizes the prepare phase with a stable leader to agree on a sequence of values for replicated logs.

Where it is used

Google Chubby and Spanner, Apache Zookeeper's relatives and many distributed databases use Paxos variants to replicate state.

Common pitfall

It is famously hard to understand and implement; gaps between the paper and a production system include membership changes, log compaction and efficient leader handling. Dueling proposers can livelock without a leader. Safety holds even with failures, but liveness requires stability. Prefer Raft for new designs, or use well-tested libraries.

Where it fits in the course

Paxos is topic 22 of 51 in Distributed Systems & Cloud, rated Intermediate. It follows Consensus Problem. The next topic is Raft.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Distributed Systems & Cloud quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Paxos?
Classic family of consensus protocols. It belongs to Distributed Systems & Cloud and is an intermediate-level topic.
Where do people use Paxos?
Google Chubby and Spanner, Apache Zookeeper's relatives and many distributed databases use Paxos variants to replicate state.
What is the most common mistake with Paxos?
It is famously hard to understand and implement; gaps between the paper and a production system include membership changes, log compaction and efficient leader handling.
What should I learn before this topic?
Start with Consensus Problem, the topic just before it in Distributed Systems & Cloud.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Distributed Systems & Cloud quiz.
Is Paxos hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Paxos?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Paxos?
Consensus Problem, Raft, Logical Clocks are closely connected. The full list is in the related topics section of this page.
What should I study after Paxos?
Continue with Raft, the next topic in Distributed Systems & Cloud.

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