Tell me about the CAP theorem.

The CAP theorem describes the limitations of distributed systems and states that a system cannot simultaneously guarantee all three properties:

• Consistency: All nodes see the same data at the same time.
• Availability: Every request receives a response (successful or error) without guarantee of data freshness.
• Partition tolerance: The system continues to operate despite network partitions.

In the event of network partitions, the system must choose between consistency and availability. For example, it may serve stale data to remain available or block responses to maintain consistency.

This is a fundamental constraint in designing distributed databases and services.