CAP Theorem with example?

The CAP Theorem is a concept in distributed database system design that states it is impossible for a distributed system to simultaneously provide all three of the following guarantees:

1. Consistency: Every read receives the most recent write or an error.
2. Availability: Every request receives a response, without guarantee that it contains the most recent write.
3. Partition tolerance: The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

In other words, in the presence of a network partition, one has to choose between consistency and availability.

For example, consider a distributed database for a banking system. If the system is partitioned into two halves, the system can either:

• Stop all operations until the partition is resolved (Consistency and Partition Tolerance, but not Availability), or
• Continue to process transactions independently in each half (Availability and Partition Tolerance, but not Consistency).

If the system chooses to continue processing transactions, it could result in a state where a customer could withdraw the same amount of money from their account from two different ATMs that are communicating with different halves of the system, resulting in an inconsistent state when the partition is resolved.