This page explains the PACELC Theorem, a foundational concept in distributed system design. Proposed by Daniel Abadi as an extension of the CAP Theorem, it provides a more complete framework for understanding system behavior during both failures and normal operations. While the CAP Theorem addresses only the scenario of network partitions—requiring a choice between two of three properties (Consistency, Availability, Partition Tolerance)—the PACELC Theorem bridges a critical gap. It recognizes that distributed systems must navigate trade-offs between latency and consistency even without failures, since most operational time occurs in healthy states. The acronym breaks down into decision points: P (Partition), A (Availability), C (Consistency), E (Else), L (Latency), and another C (Consistency), comprehensively addressing continuous system design trade-offs.
