Glossary

CAP Theorem

The CAP theorem (Brewer's theorem) states that a distributed system cannot simultaneously guarantee all three properties — Consistency, Availability, and Partition Tolerance. When a network partition (P) occurs, a choice must be made between C and A.

Three properties

Consistency — every request receives the most recent result or an error
Availability — every request receives a response (not necessarily the most recent data)
Partition Tolerance — the system continues operating despite network failures between nodes

In practice

Partition Tolerance is mandatory for any real distributed system — the network will always fail at some point. So the choice is: CP (MySQL, HBase, ZooKeeper) or AP (Cassandra, CouchDB, DynamoDB). MongoDB and Redis can be configured for either model.