How cap theorem works

Disconnect two live replicas and observe which requests succeed, which are refused, and which return an outdated value.

When to use it

Decide how a replicated read/write service should behave when its replicas cannot communicate.

Explore the tradeoffs

CAP is not a menu where every healthy system must permanently discard one property. Coordination requires communication.

During a partition, this register cannot guarantee both linearizable responses and completion of every operation. Refusing work protects one history; local completion can expose different histories.

Later convergence cannot retroactively fix an earlier stale response. Keeping both versions requires application resolution; choosing one can overwrite acknowledged work.

Model boundaries

Two replicas and a deliberately strict protocol requiring both for coordinated operations. A real majority-based system can keep a majority partition available. CAP availability is a guarantee about operations, not an uptime percentage; an unavailable error does not satisfy it.