What is Quorum?

A quorum is the minimum number of servers that must successfully perform an operation before the system declares that operation successful. Once that minimum number of servers agrees, the system treats the result as final, even if the remaining servers have not caught up yet.

Think of a jury in a court trial. A jury cannot deliver a verdict unless a minimum number of jurors agree. That minimum number is the quorum.

Distributed systems work the same way. Before an operation (like a read or a write) is considered successful, a minimum number of servers must agree on it.