Software Architect's Handbook

CAP theorem

The Consistency, Availability, and Partition tolerance (CAP) theorem, also known as Brewer's Theorem after Eric Brewer who published it, states that a distributed system can only achieve two of the following three guarantees, but not all three:

• Consistency: Every read either returns the latest data or an error. Every transaction either completes successfully and is committed or is rolled back due to a failure.
• Availability: A system always provides a response to every request.
• Partition tolerance: In a distributed system (data is partitioned to different servers), if one of the nodes fails, the system should still be able to function.

Databases will stress some of these guarantees over others. A traditional relational database ...