# CAP Theorem
Confidence: high
Last verified: 2026-05-22
Generation: human_only


## TL;DR

The CAP theorem (Eric Brewer, 2000; formally proven by Gilbert & Lynch, 2002) states: a distributed system can guarantee at most two of: Consistency (all nodes see same data), Availability (every request gets a response), Partition Tolerance (system works despite network partitions). Since partitions are inevitable, you must choose between C and A.

## Core Explanation

CP systems (MongoDB, HBase, ZooKeeper): sacrifice availability during partition — some requests may timeout. AP systems (Cassandra, DynamoDB, CouchDB): sacrifice consistency — may serve stale data, resolve conflicts later. 'CA' systems are impossible in distributed context (partition is inevitable). PACELC extends CAP: if Partition, choose A or C; Else, choose Latency or Consistency.

## Further Reading

- [Brewer's Conjecture and the Feasibility of Consistent, Available, Partition-Tolerant Web Services (Gilbert & Lynch, 2002)](undefined)