Simple definition
Quorum determines whether a cluster has enough votes to continue making decisions without allowing multiple partitions to become active at the same time.
Technical definition
Cluster nodes and, in many designs, a witness have votes. The quorum mechanism defines the majority required to retain authority. Models vary by platform and can involve node votes, disk witnesses, file-share witnesses, cloud witnesses, or dynamic voting rules.
How it works / role
When a network partition or failure separates cluster members, each side evaluates the votes it still owns. Only the partition that retains quorum is allowed to keep cluster resources online; the other side must stop or isolate its roles to prevent split-brain.
What is it used for?
Maintain consistency for highly available services and prevent two groups of nodes from modifying the same clustered resources simultaneously.
Practical example
In a two-node cluster with a witness, the surviving node can keep the service online if the node plus witness still represent the required majority.
Common issues
- The witness is unavailable at the same time as a node
- The witness is placed in the same failure domain as the nodes
- A network partition is mistaken for a node failure
- Votes are changed manually without understanding the quorum model
Key takeaway: Quorum is not simply ‘half the nodes’; it depends on the configured voting and witness model.