Distributed quorum NWR of the alchemy furnace of the Supreme Master

“

Too white Venus ： I heard that Lao Jun is practicing the magic pill recently , Can you tell me ？

Lord Lao Zi ： Lao Bai , I'm practicing six pills recently ： Two pieces Yannian Dan 、 Two pieces Jianbu Dan 、 Two pieces Recovery Dan .

Too white Venus ： These three Eight Trigram furnaces must be used to practice these three magic weapons ？

Lord Lao Zi ： It's just . And for the same pills , The efficacy and size must be exactly the same .

”

Three kinds of pills

One 、 How are the three alchemy furnaces distributed

“

Too white Venus ： Lao Jun , How do you distribute your Bagua stove ？

”

Let's uncover Lao Jun's Alchemy furnace , See how the six pills are distributed .

First of all, it's easy for us to guess how the alchemy furnace distributes alchemy ：

• Two pills in No.1 Dan furnace Yannian Dan .
• Two pills in No.2 Dan furnace Jianbu Dan .
• Two in No.3 Dan furnace Recovery Dan .

Taibai Jinxing's view of Danlu situation

So what's wrong with this distribution ？

Let's think about , If No.1 Dan furnace bursts because the fire is too high , So the two Yannian dans are bound to fail . It's the same thing as putting eggs in a basket . If the basket is upset , All the eggs fall out of it , It's all broken .

“

Lord Lao Zi ： Laobai , I'll show you the cover of the boiler .

”

• One pill in No.1 Dan furnace Yannian Dan And one Jianbu Dan .
• Make one in No.2 Dan furnace Yannian Dan And one Recovery Dan .
• Make one in No.3 Dan furnace Jianbu Dan And one Recovery Dan .

The actual distribution of Dan furnace

“

Too white Venus ： Lao Jun , Why distribute it like this , The temperature of each pill is not so easy to control ？

Lord Lao Zi ： Laobai , I'm a master of alchemy , The heat can't beat me .

Too white Venus ： I am worthy of being Lao Jun , In this way, even if there is a problem with a Dan furnace , At least one can be made into , Not both .

”

Mapping to our Internet system ： Dan furnace is similar to server node or database node , Through multiple nodes to backup data to each other to ensure the stability of the system High availability （High Availability）.

Two 、 How to ensure the same quality of pills

2.1 Uniformity

“

Too white Venus ： Lao Jun , You just mentioned , Two Yannian pills need to have the same effect , The same size ？

Lord Lao Zi ： Such is the case , The quality of pills must be consistent , What I make are all nine kinds of pills , The efficacy of the medicine is different .

”

Taishanglaojun said that the quality of keeping consistent, in the end what's going on ？

How to ensure the same quality of Yannian pill in No.1 Dan furnace and Yannian pill in No.2 Dan furnace ？

That's what we often say Distributed consistency Do you ？ Two pills are distributed in different furnaces , Need to be consistent in quality .

As shown in the figure below , These two Yannian pills are big and small , There are also different colors , That's why the quality is different .

The quality is different

And in architecture design , For example, request access to different databases , All the data are the same , This is consistency .

As shown in the figure below ： The browser accesses the database 1 And the database 2 Data in A, It turns out that all of them are A = 1.

Consistency in Distributed Systems

2.2 Final and strong consistency

Consistency in distribution is divided into Final consistency and Strong consistency .

So-called Strong consistency After the write operation is completed , Any subsequent access can read the updated value . This is it. CP Consistency and partition fault tolerance required by the system ..

How to understand it in alchemy ？

For example, Lao Jun added to Yannian Dan of No.1 Dan furnace Lotus This kind of Medicine , Yannian Dan, who gave it to No.2 Dan furnace, did the same thing , So the ingredients of the two Yannian pills that Lao Bai opened the stove cover were the same .

and Final consistency It doesn't guarantee that all subsequent visits can read the updated values , But after a while , Read it again , You get the same value . in other words , In the meantime , It's possible to read old data . This is it. AP Availability and partition fault tolerance required by the system .

How to understand it in alchemy ？

For example, Lao Jun added lotus to Yan Nian Dan of No.1 Dan stove , And after an hour , Just add snow lotus to No.2 Dan stove , So in this hour , The ingredients of the two Yannian pills are different . But after an hour , The final ingredients are the same .

3、 ... and 、 Controllable quality ：Quorum NWR agreement

“

If Yannian pill must ensure strong consistency of quality , Jianbu Dan only needs to ensure the final consistency of quality , How to control this ？

”

This is not difficult for Lao Jun , Because Lao Jun knows distributed protocol ：Quorum NWR.

Quorum The meaning of this word ：( Of the meeting ) A quorum . Mainly look at the last three capital letters ：N、W、R. from NWR To control consistency .

3.1 Parameters N

Let's take a look at the situation in Dan furnace , The two Yannian pills are backup to each other , It's equivalent to having two copies .

N It's called the number of copies , It's also called replication factor （Replication Factor）. How many copies of the same data are there , therefore ： Yannian Dan's N = 2. By analogy ： Jianbu Dan's N = 2, Restore Dan's N = 2. As shown in the figure below ：

The copies of pills are the same

that N Can it change ？

As shown in the figure below ： For example, I want to practice 3 Yan Nian Dan , That is to say, every Dan stove has a long life Dan , Then put N Change to 3 That's all right. . And Jianbu pill only needs to refine one, which is enough to , No.1 Dan furnace is OK , therefore N = 1.

The number of copies of multiple pills is different

3.2 Parameters W

Specifies the number of copies N after , You can read and write the replica data .

• Read operations ： Check the situation of pills in the furnace .
• Write operations ： Add herbs to pills 、 Raise the temperature .

How can multiple pills read and write ？ For write operations , We have W Parameters , For read operations , We have R Parameters .

W It's called the write consistency level （Write Consistency Level）, It means successful completion W Copy updates , To complete the write operation .

For example, the setting of Yannian Dan W = 2, When writing to Yannian Dan , It's done 2 When a copy is updated , To complete the write operation .

As shown in the figure below ： Yannian Dan capital in No. 1 and No. 2 Dan furnaces Joined the Lotus , And the longevity pill in No. 3 furnace Not joined Lotus . That is, only two copies have been updated , accord with W = 2 This condition , The write and play operation is complete .

Two Yannian dans joined the lotus

But you found no problem , Yannian Dan of No.3 Dan furnace has not been added to lotus , Then how to ensure that when taishanglaojun looks at the pills , I know that I have joined lotus ？ That is, how to ensure the strong consistency of reading and writing , This is going to use the third parameter ：R.

3.3 Parameters R

R It's called the read consistency level （Read Consistency Level）, When reading a data object , Need to read R Copies , Then return R The latest copy of the data in the copies .

Back to the problem of alchemy , Set Yannian Dan's R = 2, That is, when we look at the situation of Yannian Dan , Just look at the situation of Yannian Dan in the two Dan furnaces , Then go back to the latest situation of Yannian Dan .

• Suppose you are looking at Yannian Dan in No. 1 and No. 2 Dan furnace , The return situation is ： Joined lotus . This scenario is consistent .

• Suppose you are looking at Yannian Dan in No. 1 and No. 3 Dan furnace , Yannian Dan of No.1 Dan furnace has joined lotus , No.3 Dan stove is without lotus , But the last operation time of Yannian Dan in No.3 Dan furnace was earlier than that of No.1 Dan furnace , So return to the situation of Yannian Dan in No.1 Dan furnace ： Joined lotus . This scenario is also consistent .

Through the above two scenes , We know , By setting R = 2, Even if you read the third data that has not been updated , It can also return the updated data , Achieve strong consistency .

3.4 Parameter combination

Parameters N、W、R Different combinations of will bring different consistency effects .

• Take the example above ,N = 3,W = 2,R = 2,W + R > N, For clients , The whole system can guarantee strong consistency , It must be able to return the updated data .

• When W + R <= N when , For clients , The whole system can only guarantee the final consistency , Old data may be returned during data access .

Different parameters , Different effects , Distributed systems need to be configured according to different scenarios .

Four 、 application

InfluxDB Enterprise Edition is a time series database , It has four levels of write consistency ：

• any：W + R < N,W = 1, After any node writes successfully , Or write Hinted-handoff cache （ Wait for the next retransmission ）, Return success to client .
• one：W + R < N,W = 1, After any node writes successfully , Return success to client immediately , Does not include writing Hinted-handoff cache
• quorum：W + R > N, After most nodes write successfully , Return the success to the client .（ requirement N Greater than 2）
• all：W = N, After all nodes are successfully written , Return to success .

In addition to Time series database InfluxDB Come on , Read operations need to read a lot of data , To ensure efficient reading , It doesn't support read consistency levels （R = N）, But you can set the write consistency level to all, To achieve strong consistency .

InfluxDb Realized Quorum NWR, When the online business needs to make some temporary consistency adjustments , Set different write consistency levels to achieve fast switching .

5、 ... and 、 summary

This article talks about alchemy between Taishang Laojun and Taibai Jinxing , The distributed protocol of custom consistency is extended ：Quorum NWR agreement .

• Dan furnace is a metaphor for node , Pills are compared to data , Multiple pills are called copies .
  

• N Represents the number of copies ,W Represents how many copies to write ,R Represents how many copies to read .

• When N Greater than the number of nodes , There will be multiple copies of a node , When this node fails , Multiple copies will be affected .

• W + R > N when , Stands for strong consistency .

• W = N when , Good reading performance .R = N, Good writing performance .

• W = R = (N+1)/2, Good fault tolerance , Be tolerant of A few nodes （ That is to say (N-1)/2） Node failure .

• How to set up N、W、R value , It depends on where our system should be optimized .

• Quorum NWR Distributed algorithms provide flexibility for businesses to choose consistency levels on demand , Make up for AP The drawback of the lack of strong consistency of the system .

“

Too white Venus ： I wish you success in alchemy ！ I want to go Share and Leaving a message. 了 ！

”

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Author's brief introduction ： The wu is empty ,8 Years of experience in Internet development and architecture , Use stories to explain 、 Architecture design .《JVM Performance optimization practice 》 Columnist , Open source 《Spring Cloud actual combat PassJava》 project , Independently developed a PMP Brush topic small program . Pay attention to free brush topic .

I'm Wukong , Try to be strong , Become a super Saiya ！

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