What is the CAP theorem?

Answer

The CAP theorem states that a distributed data store can provide at most two of three guarantees at the same time: Consistency (every read sees the latest write), Availability (every request gets a non-error response), and Partition tolerance (the system keeps working despite network failures between nodes). Because network partitions are unavoidable in real distributed systems, the practical choice is between consistency and availability during a partition. CP systems reject some requests to stay consistent, while AP systems stay available but may return stale data. In practice, partition tolerance is a given, so the trade-off is really CP versus AP.