PACELC

Extends CAP: if there is a Partition, choose Availability or Consistency; Else, in normal operation, choose Latency or Consistency. Waiting for more replicas to agree is always slower, even when nothing is broken.

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

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