Definition
In a network partition, a distributed store can keep either Consistency or Availability — not both.
How it works
When nodes can't talk (P, which you can't avoid), you choose: refuse writes to stay consistent (CP, e.g. etcd, Spanner) or keep serving and reconcile later (AP, e.g. Cassandra, Dynamo). When the network is healthy you get both — CAP only forces the choice during a partition. PACELC extends it to the no-partition case (latency vs consistency).
Common questions
What is CAP Theorem?
In a network partition, a distributed store can keep either Consistency or Availability — not both.
How does CAP Theorem work?
When nodes can't talk (P, which you can't avoid), you choose: refuse writes to stay consistent (CP, e.g. etcd, Spanner) or keep serving and reconcile later (AP, e.g. Cassandra, Dynamo). When the network is healthy you get both — CAP only forces the choice during a partition.…
What is CAP Theorem used for in system design?
When nodes can't talk (P, which you can't avoid), you choose: refuse writes to stay consistent (CP, e.g. etcd, Spanner) or keep serving and reconcile later (AP, e.g. Cassandra, Dynamo). When the network is healthy you get both — CAP only forces the choice during a partition.…