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Quorum vs Consensus#

The Distinction#

Quorum is a number — the minimum nodes that must agree.

Consensus is a process — how nodes actually reach agreement.

They are related but different. Consensus algorithms use quorum internally.

Quorum#

A threshold. Nothing more.

3 node cluster → quorum = 2
"At least 2 nodes must confirm before this operation is valid"

Write request:
  Node A confirms ✓
  Node B confirms ✓  ← quorum reached → write successful
  Node C hasn't responded → doesn't matter

Read request (quorum read):
  Must read from at least 2 nodes
  Compare values → take the latest
  Guarantees you see the most recent write

R + W > N formula: 

N = total nodes
W = nodes that must confirm a write
R = nodes that must confirm a read

R + W > N → guaranteed to see latest write

Example: N=3, W=2, R=2
  2 + 2 > 3 ✓ → strong consistency guaranteed
  At least one node in the read set saw the write

Quorum is just the number — it doesn't say anything about how nodes communicate, how leaders are elected, or how conflicts are resolved.

Consensus#

The full protocol for nodes to agree on a value — including leader election, handling failures, and guaranteeing no split-brain.

Consensus answers:
  Who is the leader?
  How is a new leader elected when the current one fails?
  How do nodes agree on the order of operations?
  What happens when nodes disagree?
  How is split-brain prevented?

Famous consensus algorithms:

Algorithm Used In Key Idea
Raft etcd, CockroachDB, TiDB Leader-based, easier to understand
Paxos Google Chubby, Zookeeper (ZAB) Original, mathematically proven, complex

How They Relate#

Consensus algorithms use quorum internally:

Raft consensus:
  Leader election → candidate needs quorum of votes to become leader
  Write commit    → leader needs quorum of followers to confirm before committing
  Leader validity → leader steps down if it can't reach quorum

Quorum is a tool INSIDE consensus — not the same thing

Analogy: 

Quorum    → "we need 6 out of 10 board members to vote yes"
            just the threshold

Consensus → the entire board meeting:
            agenda, discussion, handling absent members,
            what if two people claim to be chairperson,
            how to record decisions, how to replay them

Consensus uses quorum as one of its rules

When to Mention Each in Interviews#

Mention QUORUM when talking about:
  → Read/write safety (R + W > N)
  → How many nodes must confirm a write
  → Why you need majority for split-brain prevention

Mention CONSENSUS when talking about:
  → Leader election
  → Distributed coordination
  → ZooKeeper, etcd, Raft
  → How distributed databases maintain consistency

Interview framing

"For write safety I'd use quorum — R + W > N ensures any read sees the latest write. For leader election and coordination I'd use a consensus-based system like etcd which implements Raft — nodes only become leader with quorum of votes, preventing split-brain automatically."