What is the transaction of MySQL? What is dirty reading and what is unreal reading? Not repeatable?

Tips ： Although the role of affairs in actual work is not great , But the written examination at school , The interview is the key examination topic of the database ！！！ Therefore, Baozi should keep up their spirit to read this article ！！！

Catalog

Preface ：

One 、 What is business

Two 、 The isolation level of the transaction

1、 Atomicity （Atomicity）

2、 Uniformity （Consistency）

3、 Isolation, （Isolation）

4、 persistence （Durability）

3、 ... and 、 Methods and steps of implementing transactions

Four 、 The isolation level of the transaction

Read Uncommited, Read uncommitted ：

Read Commited, Read operations ：

Repeatable Read, Repeated reading ：

Serializable, Serializable ：

---

Preface ：

Mysql Relational databases are transaction enabled by default , This article will use MySQL Database and Navicat Explain things for everyone . This article will introduce in detail what is a transaction , The four characteristics of affairs , The isolation level of the transaction , Let's start today's study ！

 

One 、 What is business

Let's first look at the official definition of affairs ：

Business , Is to put a group SQL Statements are executed in the same batch , If one SQL Statement error , Then all of the SQl Will be cancelled .

Be careful ：MySQL Only innoDB and BDB Database tables of type support transactions ！

 

If you don't understand the official concept , Don't panic ！ Let's take a look at the following example ：

Say 500 years ago m78 There is a man named Sophie in the nebula, although he is the captain of the universe guard , But he always gets beaten when he comes out , So I want to buy armor to make myself more resistant to beating , So ran went to the earth and found a Gamay armor ...

  Brother , Your bubble dragon and clothes look good , Give me ten stacks .

 

                                                         B: yes, you can ！ Brother has eyes , I'll pack it for you ！！！

  But Sophie is old and poor , I want to run with my armor w(ﾟДﾟ)w

Let's think about it , Is it OK for Sophie not to pay for her clothes ！ It's not going to work !

So Dega said :" You must pay and deliver with one hand , This is a complete deal !"

The embodiment of affairs ： Payment and delivery must be completed , Business is a success , Any activity fails , The whole transaction failed ！

 

  Let's take a look at two understandings of affairs ：

Understand one ：

Business can be seen as a big activity , It's made up of different little activities , These activities are either all successful , All or nothing .

Understanding two ：

Transaction can be regarded as a big operation , It consists of a series of operations , These operations are either all successful , All or nothing .
 

 

Two 、 The isolation level of the transaction

1、 Atomicity （Atomicity）

It means that all operations contained in the transaction are either successful , All or nothing , Therefore, if the transaction operation succeeds, it must be fully applied to the database , If the operation fails, there will be no impact on the database .

Let's think about it , Although zophe didn't have a lot of money , But he still has a little private money , So he said to Dega , Can you pay in installments , But Dega said , Big brother , Little brother, this is also a small business , Must be paid in a lump sum , No credit ！！！

The transaction reflects ： Atomicity is indivisibility , Money can't be paid in installments .

 

2、 Uniformity （Consistency）

It means that the transaction must change the database from one consistency state to another consistency state , That is to say, a transaction must be in a consistent state before and after execution .

Sophie really can't , You can only borrow money from his third brother Saiwen , Sophie said ：“ Look, big brother lent you 91 Give it back to you 1 Block row no , Give me a change ”

Saiwen was immediately surprised ：“ Although you are my big one , But you can't bully your little brother like this , Pay back as much as you have to borrow ！！！”
The transaction reflects ： In the process of borrowing money , How much you borrow must be consistent with how much you repay later .

 

3、 Isolation, （Isolation）

It refers to when multiple users access the database concurrently , For example, when operating the same table , A transaction opened by a database for each user , It cannot be interfered by the operation of other transactions , Multiple concurrent transactions should be isolated from each other .

Ask Saiwen to borrow , Sophie suddenly felt that she was suddenly rich , So I want to buy better armor , But the money is not enough , So he asked his second younger brother to borrow money ：“ Second brother ！ You see, brother is a little short of money recently , Why don't you borrow some, brother money, At that time, if you return the money to the third brother, you will pay back the money to the second brother ”

The first generation listen , I was at a loss ：“ Big brother ！ The third brother is the third brother , The money you borrowed from us is irrelevant ！ It's time to pay back ！！！”

The transaction reflects ： Does the money that Sophie borrowed from Saiwen and chudai have anything to do with , Obviously, it doesn't matter , And borrowing money from the early generation or repaying money does not affect borrowing money from Saiwen to repay money . This process is not affected by other parts .

4、 persistence （Durability）

Once a transaction is committed , So the change to the data in the database is permanent , Even in the case of database system failure, the transaction commit operation will not be lost .

Zuo Fei finally took the money and bought the armor he was thinking about , So I can't wait to find the little monster to fight alone , As a result, although he played 18 kinds of Martial Arts , But unexpectedly, the armor was smashed by a palm , At this time, even if Zuo Fei is sad , The armor has broken , Unable to recover .

The transaction reflects ： Once the transaction is committed , Changes to database data are permanent ！！！
 

 

3、 ... and 、 Methods and steps of implementing transactions

 

Four 、 The isolation level of the transaction

Read Uncommited, Read uncommitted ：

That is, one transaction can read the data of another uncommitted transaction ; Concurrent operations can lead to dirty reads

Read Commited, Read operations ：

That is, a transaction cannot read data until another transaction is committed ; Solve the dirty reading problem ; Concurrent operations can result in non repeatable reads

Repeatable Read, Repeated reading ：

Start reading data （ The transaction open ） when , Modification operations are no longer allowed ; Solve the problem of non repeatable reading ; Concurrent operations can cause unreal reads （ Corresponding insert operation ）

Serializable, Serializable ：

Highest transaction isolation level , At this level , Sequential execution of transactions ; Avoid dirty reading 、 No repetition or unreal reading ; But this level is inefficient , Compare consumption of database performance , Generally do not use .

Let's take a look at the concurrency problems that occur in several isolation situations ：

1. Dirty reading

One transaction reads another uncommitted data .

 

 2. It can't be read repeatedly

Within the scope of a transaction , Two identical queries return different data .

 3. Fantasy reading

Two identical queries within a transaction scope return different data , The corresponding is the insert operation .

  For the convenience of memory , I summarize the isolation level of transactions and concurrency problems in the following table ：

features || problem	Dirty reading	It can't be read repeatedly	Fantasy reading
Read uncommitted	√	√	√
Read the submission	√	√	×
Repeatable	√	×	×
Serializable	×	×	×

 

Finally, I wish you all treasure children will get something after reading ^_^！！！

also ！ also ！ Do you still believe in light ？？？