[Turn] CAP theorem of meaning

Date: July 16, 2018

Distributed systems (distributed system) is becoming increasingly important, large sites are almost always distributed.

The most difficult of distributed systems is how to synchronize the status of each node. CAP theorem is a fundamental theorem in this regard, and it is starting point for understanding distributed systems.

This article describes the theorem. It is actually easy to understand, but is obvious. The following mainly with reference to the Michael Whittaker's article .

Three indicators of a distributed system

In 1998, a computer scientist at the University of California, Eric Brewer proposed a distributed system has three indicators.

Consistency

Availability

Partition tolerance

They are the first letter C, A, P.

Eric Brewer said that the three indicators impossible at the same time. This conclusion is called CAP theorem.

二、Partition tolerance

Look at Partition tolerance, Chinese is called "partition fault tolerance."

Most distributed systems are distributed across multiple sub-networks. Each subnetwork is called a zone (partition). Fault-tolerant partition mean, range communication may fail. For example, a server in China, another server in the United States, which is the two areas could not communicate between them.

The figure above, G1 and G2 are two cross-server. Send a message to G1 G2, G2 may not be received. System design, it must take into account this situation.

In general, fault-tolerant partition can not be avoided, it can be considered the P-CAP always true. CAP theorem tells us that the rest of the C and A can not be done at the same time.

三、Consistency

Consistency Chinese called "consistency." You mean, read after write operation, must return the value. For example, a record is v0, the user initiates a write operation to the G1, which is to v1.