Tell about the CAP theorem.

The CAP theorem states that a distributed system cannot simultaneously guarantee three properties:

• Consistency — all nodes see the same data at the same time.
• Availability — every request receives a response (successful or with an error), without long delays.
• Partition tolerance — the system continues to operate despite network partitioning (loss of communication between nodes).

Under network partitions, the system can provide either consistency or availability, but not both at the same time. This forces designers to choose between CP (Consistency + Partition tolerance) and AP (Availability + Partition tolerance) systems.

Example:

• CP system: during a network partition, it blocks some nodes to maintain consistency but may become unavailable.
• AP system: continues to respond to requests, but data may be temporarily inconsistent.

The CAP theorem helps in understanding the trade-offs in designing distributed databases and services.