You replicate your data five ways for safety. A write lands on the leader — so which replicas must confirm it before you call it durable? Answer “one” and a crash loses data. Answer “all” and a single slow node freezes the system. The right answer is a majority, and the reason is a small, beautiful piece of arithmetic.

The Problem: Two Truths

The danger in a distributed system is split-brain — two halves of a cluster that lose contact and each decide they’re in charge. Both accept writes, and now you have two divergent versions of reality with no way to merge them.

To prevent this, every decision needs the agreement of a quorum: a subset of nodes large enough that two quorums can never both form at the same time.

The Majority Trick

Set the quorum to a strict majority — more than half. In a 5-node cluster, that’s 3. The key property:

text
   any two majorities of 5 must share at least one node

   {A B C}   and   {C D E}   overlap at C
   {A B C}   and   {A B D}   overlap at A, B

Because any two majorities overlap in at least one node, two conflicting decisions can’t both gather a quorum — the shared node would have to vote for both, and it won’t. Split-brain becomes mathematically impossible.

Read and Write Quorums

The same overlap logic lets you tune reads vs writes. With N replicas, a write quorum W and read quorum R, you get strong consistency when:

text
   W + R > N      →  every read quorum overlaps every write quorum
   W > N / 2      →  writes can't conflict with each other

For N = 5: W = 3, R = 3 is the balanced choice. Want faster writes? W = 2 breaks the guarantee. Want fast reads at the cost of slow writes? W = 5, R = 1 — but now any down node blocks all writes.

text
   N=5, W=3, R=3
   write → {A B C}
   read  → {C D E}     reads see C, which has the latest write ✓

Why Clusters Are Odd-Sized

A majority of 4 is 3 — the same as a majority of 5. So a 4-node cluster tolerates one failure, exactly like 3 nodes, while costing more and being likelier to have a failure. Odd sizes give you the best failure tolerance per node:

text
   nodes   majority   failures tolerated
     3        2              1
     4        3              1     ← extra node buys nothing
     5        3              2

This is why Raft and Paxos clusters are almost always 3 or 5.

Takeaways

  • A quorum is a subset big enough that any two quorums overlap — which is what makes conflicting decisions impossible.
  • A strict majority is the smallest such quorum; it prevents split-brain because two majorities always share a node.
  • W + R > N and W > N/2 give strong consistency; tune W/R to trade read vs write speed.
  • Use odd cluster sizes — an even size costs more without tolerating more failures.