Consensus
- Node.js course
- Expert
- Lesson 32/47
- 5 min
Learn Raft and Paxos consensus algorithms.
What is consensus?
Consensus is the process by which distributed nodes agree on a single value. Raft and Paxos are two common algorithms. Raft prioritizes understandability; Paxos prioritizes mathematical conciseness.
A node in Raft takes one of three roles: follower (passive), candidate (contesting an election) and leader (running writes). A consensusRole(term) function models these: term === 0 returns 'follower', term === 1 returns 'candidate', others return 'leader'. This maps Raft's term logic with a simple lookup; real Raft runs elections by majority vote but the roles stay the same.
Term logic
In Raft a term is a logical clock. Each election starts a new term. A node's role depends on the term it sits in. This drives monotonic progress and prevents stale leaders from returning.
Example: consensusRole(0) returns 'follower'; a fresh node is passive. consensusRole(1) returns 'candidate'; in term 1 it contests an election. consensusRole(2) returns 'leader'; it won a majority vote and leads. This model puts Raft's term-role link into one function. Paxos has a similar mechanism (proposal number); both algorithms rest on majority and monotonic terms.
Practice Write the consensus role function
Write function consensusRole(term). Add if (term === 0) return 'follower';, if (term === 1) return 'candidate'; and return 'leader';. Print console.log(consensusRole(0)) and console.log(consensusRole(2)).
Starter code
// Write the consensusRole function
// term 0 gives 'follower', 1 gives 'candidate', others give 'leader'
// Print calls with 0 and 2
// write hereSample solution
function consensusRole(term) {
if (term === 0) return 'follower';
if (term === 1) return 'candidate';
return 'leader';
}
console.log(consensusRole(0));
console.log(consensusRole(2));Expected output
follower leader
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the leader branch
if (term === 0) return 'follower';
if (term === 1) return 'candidate';
return '____';if (term === 0) return 'follower';
if (term === 1) return 'candidate';
return 'leader';Explanation: Other terms return 'leader'.
2 Match each concept to its role
| Term | Meaning |
|---|---|
Raft |
Understandable algorithm |
Paxos |
Mathematical algorithm |
follower |
Passive node |
leader |
Drives writes |
3 What happens when a node wins an election in Raft?
consensusRole(2)- Follower olur
- Candidate olur
- Leader olur Correct answer
- Client olur
Explanation: The winner becomes the leader.