CAP Theorem

In 2000, Eric Brewer published a paper stating that a Distributed system cannot simultaneously provide all three of the following properties:

• Consistency: A read sees all previously completed writes.
• Availability: Reads (Actual Data) and writes always succeed.
• Partition tolerance: Guaranteed properties are maintained even when network failures prevent some machines from communicating with others.
   

The CAP Theorem says that the Distributed system cannot have all 3 of the above, it can only have any two of the above properties (CA, AP, CP).