CAP vs. PACELC Theorem in Distributed Systems

When designing systems that span multiple servers or data centers, how do we ensure they remain reliable and performant?

This question is fundamental to System Design. Building these systems means constantly managing trade-offs. We need to manage communication across multiple nodes, handle inevitable network failures, and maintain data consistency, all while delivering a fast and seamless user experience.

Frameworks like the CAP and PACELC theorems provide a mental model for navigating these complexities.

They do not give us the answers, but they help us ask the right questions about the trade-offs we must make. We’ll discuss both of these frameworks in detail in this lesson. Let’s start by understanding the inherent trade-offs in distributed systems.

Balancing trade-offs in distributed systems

Building distributed systems involves orchestrating multiple computers to work together as a cohesive unit.

This approach is essential for achieving scalability and high availability, but it introduces significant challenges. Servers can crash, and network links between them can fail, creating communication breakdowns known as partitions.

In these moments of uncertainty, a system must make difficult choices to maintain its integrity.

Achieving a perfect balance between reliability, performance, and data correctness is impossible. Every architectural decision involves a trade-off. Do we prioritize making the system always available, even if it means some users might see slightly stale data?

Or do we enforce strict data consistency, even if it slows down the system or makes it temporarily unavailable during a network issue?

Theoretical frameworks are valuable for this reason. They provide a language and structure for discussing system behavior under different conditions. For anyone starting in System Design, understanding these concepts is a critical step toward making informed architectural decisions that align with business goals and user expectations.

Visualizing a typical distributed system helps to understand the inherent tensions:

This visualization sets the stage for our first framework, the CAP theorem, which formalizes the trade-offs a system must make when those network links break.

Introduction to CAP theorem

The CAP theorem, first proposed by computer scientist Eric Brewer, is a fundamental principle in distributed System Design. The CAP Theorem states that in the event of a network partition, a distributed system can provide either consistency or availability, but not both simultaneously:

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