CAP Theorem Explained: Why Distributed Systems Must Choose Between Consistency and Availability

CAP theorem, introduced by computer scientist Eric Brewer in 2000, describes a fundamental trade-off in distributed systems involving three properties: Consistency, Availability, and Partition Tolerance. Consistency requires all nodes to reflect the same data at the same time, while Availability ensures the system keeps responding even if some parts fail. Partition Tolerance means the system continues operating even when network communication between nodes breaks down. Because network partitions are unavoidable in real-world distributed systems, engineers must ultimately choose between prioritizing Consistency or Availability during such failures, leading to either CP or AP system designs. The theorem is less about picking two out of three properties and more about deciding how a system should behave when its components can no longer communicate with each other.