PACELC Theorem

PACELC Theorem Definition

The PACELC theorem is an extension to the CAP theorem. Both theorems were developed to provide a framework for comparing distributed systems. Like the CAP theorem, the PACELC theorem states that in case of network partitioning (P) in a distributed computer system, one has to choose between availability (A) and consistency (C). PACELC extends the CAP theorem by introducing latency and consistency as additional attributes of distributed systems. The theorem states that, “else (E), even when the system is running normally in the absence of partitions, one has to choose between latency (L) and consistency (C).”

CAP theorem was originally formulated by a noted computer scientist, Eric Brewer. The theorem states that it is impossible for a distributed system to provide more than two of the following three guarantees:

• Consistency – the same response is given to all identical requests
• Availability – access continues even during a partial system failure
• Partition Tolerance – operations remain intact even when some nodes are unavailable.

PACELC was developed to address a key limitation of the CAP theorem: it makes no provision for performance or latency. For example, according to the CAP theorem, a database can be considered Available if a query returns a response after 30 days. Obviously, such latency would be unacceptable for any real-world application.

Martin Kleppmann has written a comprehensive critique of the CAP Theorem in which he states, “CAP is often interpreted as proof that eventually consistent databases have better availability properties than strongly consistent databases; although there is some truth in this, we show that more careful reasoning is required.” So, it is important to keep in mind that the CAP theorem is a simplified model for understanding a very complex topic. PACELC is one attempt to address this limitation of the CAP Theorem.

As a newer way to model databases, the PACELC theorem extends the CAP theorem model, showing that systems can either tend towards latency sensitivity or strong consistency. In this way, a NoSQL system like ScyllaDB will be defined as PA/EL(highly available and latency sensitive) while a NewSQL system such as Google Spanner is defined as PC/EC( focusing primarily on being strongly consistent).

[Learn about PACELC vs CAP theorem, straight from the inventor of PACELC, Dr. Daniel Abadi]