CAP stands for Consistency, Availability, and Partition tolerance. The theorem simply states that any shared-data system can only achieve two of these three.
Consistency (all nodes see the same data at the same time)
Availability (a guarantee that every request receives a response about whether it was successful or failed)
Partition tolerance (the system continues to operate despite arbitrary message loss)
CAP原则的核心是只能满足其中两个，无法全部满足，这一点已被证明。
在NoSQL中，通常有两个层次的一致性：
第一种是强一致性，既集群中的所有机器状态同步保持一致。
第二种是最终一致性，既可以允许短暂的数据不一致，但数据最终会保持一致。
在分布式集群中，最终一致性通常是更合理的选择。