What is the CAP theorem and what is its essence?

The CAP theorem states that a distributed 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 long waiting.
• Partition tolerance: The system continues to operate despite loss or delay of communication between nodes.

The essence of the theorem is that during network partitions, developers must choose between consistency and availability.

For example, if the network is partitioned, one can:

• Sacrifice consistency to keep the system available (data may be temporarily unsynchronized).
• Sacrifice availability to guarantee consistency (some requests will wait or be rejected).

This is a fundamental limitation in designing distributed systems, affecting architectural decisions and trade-offs.