What is the CAP theorem?

The CAP theorem is a fundamental concept in distributed systems formulated by Eric Brewer. It states that in a distributed system, it is impossible to guarantee all three properties simultaneously:

• Consistency: All nodes see the same data at the same time.
• Availability: Every request receives a response (successful or with an error) without long waiting.
• Partition tolerance: The system continues to operate despite network partitions (when nodes cannot communicate with each other).

Under network partition conditions, the system must choose between consistency and availability:

• If consistency is chosen, some requests may be unavailable to prevent data inconsistency.
• If availability is chosen, data may be temporarily inconsistent.

Example: In NoSQL databases, availability and partition tolerance are often prioritized over strict consistency.

The CAP theorem helps in designing systems by understanding the trade-offs between these three properties.