Software Engineering Glossary

Quorum

Also known as: Majority Quorum Voting Quorum

Definition

A quorum is the smallest group of nodes that has to agree before a distributed operation counts as done. The most common rule is the majority quorum, N/2 + 1. It works because any two majorities share at least one node, and that shared node is what keeps the system consistent when others fail.

Key Takeaways

  • Majority quorum guarantees overlap. In a five node cluster, any group of three shares at least one node with any other group of three.
  • A quorum based system can lose up to N/2 minus 1 nodes at the same time and still stay safe.
  • Read and write quorums can be different sizes. As long as R plus W is greater than N, every read sees the latest committed write.
  • Quorum is the budget Paxos, Raft, Dynamo style replication, and ZooKeeper all spend on fault tolerance.

How It Works

  1. An operation is sent to all N replicas in a group.
  2. Each replica handles the operation and sends an acknowledgement back.
  3. Once the coordinator collects acknowledgements from a quorum, the operation is committed.
  4. Slow or failed replicas catch up later through repair, log replay, or hinted handoff.

Where It Is Used

  • Cassandra and DynamoDB let you pick the read and write quorum per query, like ONE, QUORUM, or ALL.
  • Raft and Paxos commit log entries only after a majority quorum has stored them.
  • Kafka’s in-sync replica set is a quorum that decides which producer writes are safe to commit.