Brewer's keynote, the Gilbert paper, and many other treatments, places C, A and P on an equal footing as desirable properties of an implementation and effectively say 'choose two!'. However, this is often considered to be a misleading presentation, since you cannot build - or choose! - 'partition tolerance': your system either might experience partitions or it won't. CAP is better understood as describing the tradeoffs you have to make when you are building a system that may suffer partitions. In practice, this is every distributed system: there is no 100% reliable network. So (at least in the distributed context) there is no realistic CA system. You will potentially suffer partitions, therefore you must at some point compromise C or A.

https://github.com/henryr/cap-faq#10-why-do-some-people-get-annoyed-when-i-characterise-my-system-as-ca

简单的CAP定理指出，分布式系统不可能同时提供所有三个保证:

一致性

每个节点同时包含相同的数据

可用性

每次必须至少有一个节点可用以提供数据

分区容忍

系统的故障是非常罕见的

大多数情况下，每个系统只能保证至少两个特性:CA、AP或CP。

Brewer's keynote, the Gilbert paper, and many other treatments, places C, A and P on an equal footing as desirable properties of an implementation and effectively say 'choose two!'. However, this is often considered to be a misleading presentation, since you cannot build - or choose! - 'partition tolerance': your system either might experience partitions or it won't. CAP is better understood as describing the tradeoffs you have to make when you are building a system that may suffer partitions. In practice, this is every distributed system: there is no 100% reliable network. So (at least in the distributed context) there is no realistic CA system. You will potentially suffer partitions, therefore you must at some point compromise C or A.

https://github.com/henryr/cap-faq#10-why-do-some-people-get-annoyed-when-i-characterise-my-system-as-ca

根据CAP定理，分布式系统有三个特征:

一致性(C)表示所有系统组件具有相同的信息。

系统的可用性(A)意味着它不会因为另一个系统故障而停止工作。

分区容差(P)表示在任意网络包丢失的情况下，系统将继续工作。

根据CAP定理，一个系统最多只能有这三个特征中的两个。(ap, cp, ca)

根据上图C是断开的，但A,B, D可以继续工作。现在我们可以调用系统部分工作(分区容忍)。

假设一个特定的事务只需要a、B和d，系统可以执行它而不会产生任何不一致。

但是当C必须参与一个特定的事务时，系统可以以两种方式执行。

1.由于C不可用，A可以拒绝用户请求。

So the system has Partition-Tolerance and consistency (P,C).
But no availability, because of the rejection.

2.A可以将C接收到的消息保存在A的本地内存中，并在C连接回来时传输。

So the system has Partition-Tolerance and availability (P,A).
But no consistency.because C has not updated.

一致性:

对于给定的客户端，读操作保证返回最近的写操作(如ACID)。如果在此期间有任何请求，则必须等待节点之间/节点内的数据同步完成。

可用性:

每个节点(如果没有失败)总是执行查询，并且应该总是响应请求。它是否返回最新的副本并不重要。

Partition-tolerance:

当发生网络分区时，系统将继续工作。

关于AP，可用性(始终可访问)可以与(Cassendra)或 没有(RDBMS)分区容忍

图片来源