Interview question MySQL transaction (TCL) isolation (four characteristics)

Technology provided by database （ Business ）（TCL classification ）

In actual business development , Some business operations need to access the database multiple times . One service needs to send multiple messages SQL Statement to the database . You need to
The operation of accessing the database is performed as a whole , Or all SQL All statements are executed successfully .
If one of them SQL Statement failure , Roll back the transaction , be-all SQL All statements failed to execute .

 Such as transfers :
	1   tom  1000	
	2   rose 1000	
 To complete the transfer business requires operations 2 Secondary database      
1. from tom Subtract 500
     update account set money = money - 500 where id = 1 ;
2. take 500 Add to rose Account number 
     update account set money = money + 500 where id = 2 ;

 How to control 2 strip sql Not all statements are successful , Or all of them are failures ？
 We have to use the transaction technology provided by the database to help us achieve our goal 

The role of Transaction Technology ：

You can control multiple steps of a complete thing

As long as one step goes wrong, the whole thing will fail , As long as all steps are successful, it is successful to control the current thing

demand ： Create account table , Complete the control of transfer through transaction technology

The transaction operations

mysql By default, each item sql Statements are submitted automatically
But we have to make every one sql Statements have to be submitted manually , So turn off automatic submission

Do transaction operations ：

start transaction; Start a transaction ( Turn off auto submit Manual submission is enabled )

update

update

commit; Commit the transaction and close the current manual commit

rollback; Rollback the transaction and close the current manual commit

Conclusion ： We can put a business that needs to operate multiple databases into one transaction ,

It is convenient for us to manage every impact of this business on database data

When a business is completed, it needs to operate multiple sql At the time of statement You have to use transactions …

Interview questions ： Four characteristics of transactions (ACID)

Atomicity （Atomicity）

​ A transaction is an indivisible unit , Either the operations in the transaction occur , Or none at all .

Uniformity （Consistency）

​ The integrity of the data before and after the transaction must be consistent

Isolation, （Isolation）

​ When multiple users access the database at the same time , The transaction of one user cannot be interfered by the transactions of other users , Data between multiple concurrent transactions should be isolated from each other , Don't influence each other .

persistence （Durability）

​ Once a transaction is committed , It changes the data in the database permanently

Isolation of transactions

problem : If the transaction does not have isolation , There will be reading problems when reading database data

1 Dirty reading : Read uncommitted data （ It must be avoided ）

​ One transaction reads uncommitted data from another transaction

2 It can't be read repeatedly ( in the light of update Modified )

​ The data contents read twice in a transaction are inconsistent

3 Virtual reading / Fantasy reading ( in the light of insert Inserted )

​ The amount of data read twice in a transaction is inconsistent

solve

Isolation level of database

read uncommitted			  Read uncommitted 	  The above three problems will happen 
read committed(oracle Default )	 Read submitted 	 Avoid dirty reading 
repeatable read(mysql Default )	 Repeated reading 	      Avoid dirty reading and non repeatable reading 
serializable			      Serialization        Avoid all problems 

Inquire about mysql Isolation level

show variables like "%isolation%";  

Set the isolation level of the transaction

set global transaction isolation level Level string ;

1. Read uncommitted ( The lowest level , None of the above can be guaranteed )

read uncommitted:

2. Read submitted ( It can avoid dirty reading .)

read committed

3. Repeatable ( Avoid dirty reading 、 Occurrence of non repeatable reading . mysql The default isolation level of )

repeatable read

4. Serialization ( Avoid dirty reading 、 It can't be read repeatedly 、 Fantasy reading ( Virtual reading ) What happened )

Use serializable Isolation level , A transaction is not completed , Other business SQL It can't be carried out

Security ：serializable>repeatable read>read committed>read uncommitted:

efficiency ：read uncommitted>read committed>repeatable read>serializable

In enterprise development , What must be avoided is dirty reading

mysql Data default level ：repeatable read

oracle Data default level ：read committed