The CAP theorem

The CAP theorem states that it is impossible for a distributed computing system to simultaneously provide all three of the following guarantees:

• Consistent: All clients see (immediately) the latest data even in the case of updates.
• Available: All clients can find a replica of some data even in the case of a node failure. This means that even if some part of the system goes down, the clients can still access consistent and valid data.
• Partition tolerance: The system continues to work regardless of arbitrary message loss or failure of part of the system.

The choice of which features to discard determines the nature of the system. For example, one could sacrifice consistency to get a scalable, simple, and high-performance database management system. Often, the main difference between a relational database and a NoSQL database is consistency. A relational database enforces atomicity, consistency, isolation, and durability (ACID) properties. In contrast, many NoSQL databases adopt the basically available, soft-state, eventual consistency (BASE) model.