Tell me about the CAP theorem.

The CAP theorem is a fundamental concept in distributed systems, formulated by Eric Brewer. It states that it is impossible to 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).
• Partition tolerance: The system continues to operate despite network splits or delays between nodes.

In real-world conditions, when a network partition occurs, the system must choose between consistency and availability. For example, a database can either deny a response to maintain consistency or return outdated data to preserve availability.

This constraint helps in designing distributed systems by understanding what compromises are acceptable in specific cases.