CAP theorem

When a network partition separates nodes, a distributed system must choose between consistency (refuse requests it cannot serve correctly) and availability (answer, possibly with stale data). Partitions are not optional, so the real choice is C or A during one.

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

See it break, then fix it

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