quorum
also: consensus requirement, quorum-based consensus
A minimum number of nodes or processes required to be active and in agreement for a distributed system to operate and make valid decisions. Without quorum, the system halts to prevent split-brain scenarios.
Quorum is a consensus mechanism used in distributed systems, databases, and clustering software to ensure data consistency and prevent conflicting decisions. When a quorum requirement exists, a majority (typically >50%) of nodes must agree and be reachable before the system can proceed with operations.
For example, in a 5-node cluster, quorum is 3 nodes. If the network partitions and only 2 nodes can communicate, they cannot form quorum and will not accept writes—preventing conflicting updates across the partition. The 3-node partition continues operating normally.
Common in etcd, Kubernetes control planes, Pacemaker high-availability clusters, and distributed databases like Consul. This prevents a dangerous split-brain condition where isolated partitions make conflicting decisions.