Tell me about the CAP theorem. What is it, and why is it needed?

The CAP theorem describes the limitations of distributed systems and states that 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 error).
• Partition tolerance — the system continues to operate despite network splits between nodes.

In real distributed systems, during network partitions, a choice must be made between consistency and availability. For example, in a database, when the connection between nodes is lost, it can either refuse to respond (maintaining consistency) or return outdated data (maintaining availability).

The CAP theorem helps in designing systems by understanding these trade-offs and prioritizing based on application requirements.