CAP Theorem
In theoretical computer science, the CAP theorem, also named Brewer's theorem after computer scientist Eric Brewer, states that it is impossible for a distributed computer system to simultaneously provide all three of the following guarantees:
- Consistency: all nodes see the same data at the same time
- Availability: a guarantee that every request receives a response about whether it succeeded or failed
- Partition tolerance: the system continues to operate despite arbitrary partitioning due to network failures
因為在分散式系統中，網路是不穩定的，P是一定要選擇的，要選擇C或A就要看需求．
- 選擇CP：response有可能會timeout．
- 選擇AP：放棄C，退而要求Eventually Consistency．選擇A，response會在一定時間內回應，不管成功或失敗．
Cassandra可以動態切換成CP/AP．