Glossary · System coordination, integration and orchestration
Quorum
Also known as: Majority quorum
German: Quorum
In distributed systems, a quorum is the minimum number of nodes, usually a majority, that must agree or respond before an operation, such as a write or a leader election, is considered valid; it ensures that two conflicting decisions cannot both succeed.
- System integration
In one sentence
A quorum is the minimum number of nodes, usually a majority, that must agree before a distributed operation such as a write or election counts.
Example
A three-node redundant historian cluster accepts a write only when two nodes confirm it, so a single isolated node cannot accept conflicting data.
How it applies
- Engineering: Quorum-based systems need an odd number of voters or a tie-breaker (witness) to avoid ties. With three nodes, the system tolerates the failure of one; with five, the failure of two.
- Operation: Losing quorum typically makes the system read-only or unavailable by design, which is preferable to inconsistent data. Operators should understand this behavior before it happens.
- Safety context: Voting in safety systems, such as 2oo3 (two-out-of-three) sensor architectures, follows a similar idea but is a separate concept governed by functional safety standards.
- Documentation: Document the cluster size, the quorum rule, the witness location and the recovery procedure after a quorum loss in the system administration documentation.
Quorum vs. 2oo3 voting
A quorum decides whether a distributed IT system may commit an operation. 2oo3 voting in a safety instrumented system decides whether a safety function trips, based on independent sensor channels. Both use majorities, but their purposes, standards and evidence differ.