quorum
termcloudnetworking
The minimum number of members that must agree for a distributed system to act - normally a majority, so that two disjoint groups can never both decide.
The arithmetic is the whole safety argument. If a decision needs more than half the members, then any two decisions must share at least one member, and that member prevents them contradicting each other. It is also why cluster sizing is about odd numbers rather than round ones: five members tolerate two failures, six tolerate the same two and cost more. When a network splits, the side without quorum has to stop serving even though it is healthy, which feels wrong to operators the first time and is the only thing standing between them and split brain.
Also known as: majority