majorityquorum

Majority quorum is a voting threshold used in distributed systems to determine when a decision can be considered valid. In a group of N members, the majority quorum size is floor(N/2) + 1. For example, a 3-node cluster requires 2 votes, a 5-node cluster requires 3, and a 6-node cluster requires 4. This threshold is designed so that any two majorities have at least one member in common, which helps preserve safety properties in consensus protocols.

Rationale and use: Majority quorums underpin consensus mechanisms such as Paxos and Raft. Decisions made with a majority quorum are intended to be compatible with decisions made by any other majority, ensuring progress and preventing conflicting choices when multiple proposals occur. By requiring a majority, the system can tolerate a certain number of faulty or unavailable nodes while still moving forward.

Fault tolerance and partitions: In a system of N nodes, up to floor((N−1)/2) nodes can fail or be unreachable while the majority quorum remains obtainable. In the event of a network partition, only the side containing a majority can make progress, while the minority side is halted to preserve consistency.

Dynamic membership: Many implementations support changing the set of participating nodes. Reconfiguration must be handled carefully to maintain a valid majority and avoid split-brain scenarios, typically by coordinating between old and new configurations.

Relation to other thresholds: Majority quorum differs from supermajority thresholds (such as 2/3) that require a larger, non-minimal portion of nodes. While majority quorums enable fault tolerance with minimal requirements, some systems may adopt higher thresholds for stronger safety or different operational guarantees.