What is a quorum? From meetings to distributed systems

A quorum is the minimum set that must agree before a decision is valid. The idea is centuries old in meeting rooms, and it is also one of the foundations of modern computing. It is the idea our company is named after.

Where the word comes from

Quorum is Latin for “of whom”. It comes from the wording of the commissions once issued to English justices of the peace, which named certain justices, “of whom” at least one had to be present before the others could act. The word came to mean the number of members who must be present for a body to do business at all.

The quorum in a meeting

A board, a council or an association usually sets a quorum in its rules: often a majority of the members. Below that number, the people in the room can talk, but they cannot decide. The rule protects the body from decisions made by whoever happened to turn up, and it means a decision, once made, carries the weight of the whole.

The same problem, in a computer system

Important data is rarely kept on one machine. It is copied to several, called replicas, so that the system survives when one of them fails. But copies create a new problem. Machines fail, messages are delayed, and networks split. At any moment, two replicas may hold different versions of the truth. Which one should the system believe?

The answer is a quorum. A change counts as committed only when more than half of the replicas have accepted it. With N replicas, a majority is ⌊N/2⌋ + 1: two of three, three of five.

Why a majority works: overlap

The key property is simple. Any two majorities of the same group must share at least one member. In a group of five, any three and any other three overlap in at least one. So whenever the system forms a new majority, to read the data or to make the next change, at least one member of it was part of the majority that accepted the last change. The latest committed decision cannot be lost or contradicted without someone noticing.

That overlap is what lets a replicated system keep a single, consistent history even when some of its machines are down or unreachable.

How many failures can it survive?

A majority quorum of 2f + 1 machines keeps working with up to f of them failed: three machines tolerate one failure, five tolerate two. This is why such systems usually run an odd number of replicas. Four machines tolerate only one failure, the same as three, because a majority of four is three.

Read and write quorums

Some replicated stores separate the two operations. A write must reach W replicas and a read must ask R of them, out of N. If R + W > N, every read overlaps every completed write in at least one replica, so a read always meets the latest value. The numbers can be tuned: writing to all replicas and reading from one makes reads fast and writes slow, while majorities on both sides balance the two.

Consensus: agreeing on a sequence of decisions

Consensus protocols such as Paxos, described by Leslie Lamport, and Raft, by Diego Ongaro and John Ousterhout, use majority quorums to agree not just on one value but on an ordered log of changes. A leader proposes each entry, and it is committed once a majority has stored it. Much of the infrastructure behind modern databases and cloud services relies on this idea.

There is a stricter version for systems in which some machines might not just fail but actively lie. Tolerating f such machines takes at least 3f + 1 of them, with quorums of 2f + 1, so that any two quorums still overlap in at least one honest machine.

Why we named the company Quorum

In a meeting and in a distributed system, the quorum marks the same moment: the point at which a decision becomes valid and the system can be trusted to act on it. That is the standard we want our work held to. We build software for specialist work, where results have to be right, and engineering systems that have to behave the same way every time.

Our mark carries the idea. It is a Q built as a machined rotor, and its shaft does not stop at the ring; it crosses into the bowl. Stopped at the ring, it would be a magnifying glass, still searching. Carried through, it is committed.

About Quorum Systems