What is the CAP theorem and can you give examples of CP and AP systems?

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

• 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.

In the event of network partitions, the system must choose between consistency and availability.

Examples:

• CP systems (Consistency + Partition tolerance): for example, databases with strict consistency, such as HBase or MongoDB in write concern mode, where the system may become unavailable during a network split to maintain consistency.

• AP systems (Availability + Partition tolerance): for example, Cassandra or DynamoDB, which provide availability and partition tolerance but may allow temporary data inconsistency (eventual consistency).

The choice depends on the application's requirements: whether instant consistency is critical or continuous availability is more important.