Quorums

With N replicas, writing to W and reading from R replicas guarantees that a read sees the latest write whenever R + W > N. With N = 3, QUORUM reads and writes (2 + 2 > 3) are consistent; ONE and ONE are not.

Where it shows up in real systems

  • Wide-column store

    Every node accepts writes and each row is stored on RF replicas. The consistency level decides how many replicas must answer: ONE is fast and stays available when nodes fail but can return stale data; QUORUM on both reads and writes guarantees you read your latest write, at the cost of latency and availability. This is PACELC in practice.

    In production: Apache Cassandra, ScyllaDB, Amazon Keyspaces, Azure Managed Instance for Cassandra

  • Coordination service

    Runs Raft or ZAB on an odd number of nodes: three nodes survive one failure, five survive two. Use it for small, critical data, never as a general database.

    In production: etcd, Apache ZooKeeper, Consul KV

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