Yyds dry inventory MySQL learning - how transactions are isolated

Preface

mysql The transactions in are used to ensure that when we operate the database , The whole operation process is either all successful , All or nothing , There will be no first half success , The second half fails ;

For example, the common transfer operations in our daily life , If there is no transaction support , It may appear that all the money in your card has been deducted , But Xiao Wang didn't receive the information ;

The transaction function is implemented at the engine level of the database ,InnoDB Support transactions ,MyISAM I won't support it ;

Let's introduce the concepts related to transactions

Catalog

1. How to start a transaction
2. The isolation level of the transaction
3. Implementation of transaction isolation

Text

1. How to start a transaction

The startup mode is divided into explicit startup and implicit startup ;

• Explicit start

adopt begin perhaps start transaction Start transaction ;

adopt commit Commit transaction ;

adopt rollback Roll back the transaction ;

The benefits of explicit startup are startup and commit ( Roll back ) It always comes together , If you forget to submit, it will be easy to find out ;

This reduces the occurrence of long transactions ;

• Implicitly start

No need to pass begin perhaps start transaction Start transaction ;

When you execute a SQL When the sentence is （ such as select * from t）, A transaction is started ;

Now if you set autocommit = 0, Then the transaction will not be automatically committed , It needs to be manually commit Submit perhaps rollback Roll back ; This may lead to long transactions （ Because there is no matching Start transaction command , So it's easy to forget Submission of orders ）;

And if the set autocommit = 1, The transaction is automatically committed , There is no need to manually commit; Each statement will automatically start a transaction , The transaction will be automatically committed after the statement is successfully executed , The transaction will be rolled back automatically after the statement fails to execute ;

But set up autocommit=1 There is a problem ： It is the transaction that has lost its function , Data consistency cannot be guaranteed , For example, transfer operation ;

Recommended Practice ： It is recommended to use explicit startup to Operational transaction ; meanwhile set autocommit = 1, This can prevent long transaction problems caused by implicit startup ;

2. The isolation level of the transaction

Yes 4 Kind of ： Read uncommitted 、 Read the submission 、 Repeatable 、 Serialization ;

• Read uncommitted （READ-UNCOMMITED）

Definition ： During the current transaction , You can read the uncommitted data of other transactions

For example, I now start a transaction A, Read out id=1 This data , At first, I read the result as 1;

At this time, I will start another transaction B, To update id=1 This line of data ;

here , I'll go back to work A Read data from , Will read the latest data ;

Let's use an example to demonstrate ：

First, modify the isolation level to Read uncommitted ：

Log in and copy

mysql> set transaction_isolation='READ-UNCOMMITTED';
Query OK, 0 rows affected (0.00 sec)

     

      
1.
      
2.
     

View the current isolation level ： You can see that the setting is successful

Log in and copy

mysql> show variables like 'transaction_isolation';
+-----------------------+------------------+
| Variable_name         | Value            |
+-----------------------+------------------+
| transaction_isolation | READ-UNCOMMITTED |
+-----------------------+------------------+
1 row in set (0.05 sec)

     

      
1.
      
2.
      
3.
      
4.
      
5.
      
6.
      
7.
     

Start transactions explicitly A, Check at the same time id=1 The data of ：

Log in and copy

mysql> begin;
Query OK, 0 rows affected (0.00 sec)
 
mysql> select * from test where id = 1;
+----+------+
| id | name |
+----+------+
|  1 | 1    |
+----+------+
1 row in set (0.00 sec)

     

      
1.
      
2.
      
3.
      
4.
      
5.
      
6.
      
7.
      
8.
      
9.
      
10.
     

Start transactions explicitly B, Simultaneous updating id=1 The data of ：

Log in and copy

mysql> begin;
Query OK, 0 rows affected (0.00 sec)
 
mysql> update test set name = 2 where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

     

      
1.
      
2.
      
3.
      
4.
      
5.
      
6.
     

Back to business again A, Inquire about id=1 The data of ：

Log in and copy

mysql> select * from test where id = 1;
+----+------+
| id | name |
+----+------+
|  1 | 2    |
+----+------+
1 row in set (0.01 sec)

     

      
1.
      
2.
      
3.
      
4.
      
5.
      
6.
      
7.
     

You can see , Although we didn't commit the transaction B, But in business A You can still see the latest data in , This is the credit for reading uncommitted ;

The characteristic of this isolation level is that the latest data can always be read ; The disadvantages are obvious , In a transaction operation , Will read a different version of the data , This will give people an illusion of data disorder ;

The following examples of isolation levels are similar , The parameters of the isolation level are changed first , Then read and write in multiple transactions

• Read the submission

Definition ： During the current transaction , You can read data submitted by other transactions , You can't read without submitting

• Repeatable （ Default ）

Definition ： During the current transaction , The data read is always consistent , That is, what data is read when the transaction is started , This data is read throughout the transaction , It won't change again

• Serialization

Definition ： Understand literally , It is similar to single thread running , For example, there is a transaction writing a row of data , Then another transaction must wait for the previous transaction to commit before continuing ;

3. Implementation of transaction isolation

Transaction isolation is mainly realized through the concept of view ;

Suppose that the isolation level of the current transaction is repeatable ：

Log in and copy

mysql> show variables like 'transaction_isolation';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.05 sec)

     

      
1.
      
2.
      
3.
      
4.
      
5.
      
6.
      
7.
     

So when the transaction starts , Will create a view , Used to record current data ;

At this point, no matter how other transactions update the data , None of the previously initiated transactions will be affected ;

Then we can say Through different views , Realize transaction isolation ;

Above, we use the read uncommitted isolation level as an example , Because the read uncommitted isolation level is the lowest , So I can't see the effect of isolation ;

Those interested can use the default isolation level （ Repeatable ） Repeat the above example , You will see that no matter how it is changed , Any previously initiated transaction will not be affected by the new transaction ;

summary

The transaction can be started explicitly , Implicitly start ; recommend Explicit start , adopt begin start-up ,commit Submit ,rooback Roll back ; meanwhile set autocommit = 1, This can prevent long transaction problems caused by implicit startup ;

There are four levels of transaction isolation ： Read uncommitted 、 Read the submission 、 Repeatable 、 Serialization ; The isolation levels are incremented ;

Transaction isolation is achieved through Create different views To achieve ： Transactions started at different times , Will create different views ;