CAP Theorem


In theoretical computer science, the CAP theorem (also known as Brewer's Theorem) is a fundamental principle regarding distributed systems. It states that it's impossible for a distributed data store to simultaneously provide more than two of the following three guarantees:

  • Consistency: Every read request receives either the latest, most recently written data or an error. This implies that all nodes in the system see the same data at the same time.
  • Availability: Every request receives a non-error response, regardless of the state of any individual node. This does not guarantee the response contains the most recent data.
  • Partition Tolerance: The system continues to operate with data consistency or availability despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

Understanding CAP Theorem

The CAP theorem essentially presents a trade-off that needs to be considered when designing a distributed system. Here's why:

  • Networks: Distributed systems depend on networks for nodes (computers) to communicate and share data. Networks can be unreliable, and partitions (failures in communication between parts of the system) can occur.
  • Prioritization: When there's a network partition, a system has to choose. It can either:
    • Guarantee consistency by not allowing writes to one section of the system while the partition exists, making that section effectively unavailable.
    • Guarantee availability by allowing reads and writes on all sections of the system. If this occurs, writes that happen on different sides of the partition could result in inconsistencies once the partition is resolved.

Beyond CAP: The PACELC Theorem

A more nuanced extension of CAP theory is the PACELC theorem. It proposes:

  • If there is a partition (P), we must choose between availability (A) and consistency (C).
  • Otherwise (E), the trade-off is between latency (L) and consistency (C).

Importance of CAP Theorem

The CAP theorem presents a fundamental constraint for distributed systems architects. Understanding these trade-offs is crucial in choosing a database or designing a distributed system that matches the application's specific needs in terms of reliability, availability, and data consistency.