Quorum

A quorum is the smallest group of cluster members whose agreement is required to elect a leader, commit a write or change membership. In most systems it is a strict majority: more than half of the voting nodes.

Why it matters

Requiring a majority guarantees that two disjoint groups can never both act, because they cannot both hold more than half the votes. That is how clusters survive a network partition without split brain. It also defines your fault tolerance: the cluster keeps working only while a quorum is reachable.

Example

Majority is floor(n / 2) + 1.

nodes   quorum   failures tolerated
3       2        1
5       3        2
4       3        1

Notice that 4 nodes tolerate no more failures than 3, which is why odd cluster sizes are preferred. If a five-node cluster loses three nodes, the remaining two cannot form a quorum and the cluster stops accepting writes rather than risk divergence.

Common mistakes

• Choosing an even node count and paying for hardware that adds no fault tolerance.
• Placing all voters in one zone, so one zone failure destroys quorum.
• Forcing a minority to continue during an incident without understanding the data-loss risk.

Related

• Split brain
• Failover
• CAP theorem
• Availability

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