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Intermediate Topic 21 of 51 Distributed Systems & Cloud

Consensus Problem

Getting nodes to agree on a single value.

How it works

The consensus problem asks a group of nodes, some of which may fail, to agree on a single value so that all correct nodes decide the same value, the value was proposed by some node and every correct node eventually decides. The FLP impossibility result shows that in an asynchronous system, no deterministic algorithm can guarantee consensus if even one node can crash, so practical protocols use timeouts, randomness or partial synchrony. Consensus underlies replicated state machines, leader election and atomic commit.

Where it is used

Coordination services such as ZooKeeper and etcd, distributed databases like Spanner and CockroachDB, and blockchains use consensus to keep replicas consistent.

Common pitfall

Getting consensus right is notoriously subtle; ad hoc protocols have subtle bugs. Majority quorums are needed, so a cluster of 2f plus 1 nodes tolerates f crashes, and losing a majority halts progress to preserve safety. Latency is at least a network round trip. Use proven implementations of Paxos or Raft, not custom ones.

Where it fits in the course

Consensus Problem is topic 21 of 51 in Distributed Systems & Cloud, rated Intermediate. It follows Logical Clocks. The next topic is Paxos.

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 Consensus Problem?
Getting nodes to agree on a single value. It belongs to Distributed Systems & Cloud and is an intermediate-level topic.
Where do people use Consensus Problem?
Coordination services such as ZooKeeper and etcd, distributed databases like Spanner and CockroachDB, and blockchains use consensus to keep replicas consistent.
What is the most common mistake with Consensus Problem?
Getting consensus right is notoriously subtle; ad hoc protocols have subtle bugs.
What should I learn before this topic?
Start with Logical Clocks, 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 Consensus Problem 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 Consensus Problem?
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 Consensus Problem?
Logical Clocks, Paxos, Clock Synchronization are closely connected. The full list is in the related topics section of this page.
What should I study after Consensus Problem?
Continue with Paxos, the next topic in Distributed Systems & Cloud.

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