CAP Theorem
You can only guarantee 2 of the 3 following items:
- Consistency - every read receives the latest write or produces an error
- Availability - every request receives a response; with no guarantee of freshness of response
- Partition tolerance - system continues to operate despite hardware, network failures (arbitrary partitions)
Network and hardware failures will occur and are not within our control. For this reason, systems will generally always support partition tolerance; leaving us with a choice of one of the other two.
CP
This is good for systems that require or heavily rely on atomic reads and writes.
AP
Writes will take longer to propagate through the system, but every request will yield a response; though the response may or may not be the latest value. This is a good choice if a system can support eventual consistency of its data and needs to function despite external errors.