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This article mainly introduces the important theories of NoSQL, which can be used for reference. I hope you can learn a lot after reading this article. Let's take a look at it.

1.CAP theory

There is no doubt that CAP theory is the most important reason for the transformation of technology trend from relational database system to NoSQL system.

CAP (Consistency,Availability,Patition tolerance) theory states that in any distributed system, it is only possible to satisfy both of consistency, availability and partition tolerance, but not all of them. So you don't have to spend time and energy on how to satisfy all three.

The proof of the principle is simply that in the case of ensuring partition tolerance, consistency and availability can not be achieved at the same time, high consistency has to sacrifice availability, high availability has to sacrifice consistency. (why should partition tolerance be guaranteed? Because today, with the increasing application of the network, data partitioning is a basic requirement.

Proof process: Brewer's CAP Theorem

two。 Consistent hash

Needless to say, anyone who has used MC should know that it is directly above:

What are the important theories of NoSQL?

3.MapReduce

The idea of MapReduce is divided into two parts: Map and Reduce. To put it simply, Map is to slice a large amount of computation in order to calculate in parallel, and Reduce is to combine the results of parallel computing in order to get a final output.

For a more detailed description, see wikipedia:MapReduce

Google documentation on MapReduce PDF version: MapReduce: Simplified Data Processing on Large Clusters

4.Gossip

Gossip is a theory applied to P2P (not the current popular Gossip Girl [Gossip Girl]). Its main process is to communicate with all other NMY nodes through each node in an N-node cluster to achieve data synchronization. Gossip is based on the fact that it does not require the existence of a Master in the cluster and can communicate the changes of one node to all other nodes in the way of virus transmission. However, the cost of increasing or decreasing a node in the system is almost zero.

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