Explain the essence of the CAP theorem and its impact on choosing architecture for distributed systems.

The CAP theorem states that in a distributed system, it is impossible to simultaneously guarantee all three properties: Consistency, Availability, and Partition tolerance.

• 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 (when nodes cannot communicate).

In real-world conditions, network partitioning is inevitable, so a choice must be made between consistency and availability.

Impact on architecture:

• If strict consistency is important (e.g., banking transactions), CP systems (Consistency + Partition tolerance) are chosen, sacrificing availability.
• If high availability is crucial and the system can tolerate some delay in consistency (e.g., social networks), AP systems (Availability + Partition tolerance) are preferred.

This influences the choice of databases, replication protocols, and error handling strategies in distributed systems.