Review the past and know the new MySQL isolation level

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To understand isolation levels , First of all, we need to understand the affairs , Isolation level only works under transaction operation .

Business

The business is made up of a group of SQL A logical processing unit made up of statements , These operations are either all successful , All or nothing .

Transaction features

• Atomicity (Atomicity)： A transaction is an atomic unit of operation , Its modification of data , Or do it all , Or none of them .

• Uniformity (Consistency)： At the beginning and end of the transaction , Data must be consistent . This means that all relevant data rules must be applied to the modification of the transaction , To maintain the integrity of the data .

• Isolation, (Isolation)： The database system provides a certain isolation mechanism , Ensure that transactions are not affected by external concurrent operations “ Independent ” Environmental execution . This means that the intermediate state in the transaction process is not visible to the outside , vice versa .

• persistence (Durability)： After the transaction completes , Its modification of data is permanent , Even if there is a system failure, it can keep .

Problems caused by concurrent transaction processing

• Update missing ( Dirty write )： When multiple transactions modify the same record , The last operation overwrites the previous transaction operation , As a result, the previous operation update was lost . This phenomenon is called 「 Update missing 」.

• Dirty reading ： A transaction is modifying a record , Not submitted , When not locked, another transaction reads the uncommitted record and operates on it , This leads to inconsistent data . This phenomenon is called 「 Dirty reading 」.
  A friend of yours is transferring 10000 yuan to you , The transfer transaction is not over yet , You go to check the account balance and find that there is an extra 10000 , At this time, reading that the account balance is more than 10000 is dirty reading . If you spend this money , And this transaction is rolled back , There will be data inconsistencies .

• Fantasy reading ： A transaction re reads the previously retrieved data according to the same query criteria , However, it is found that other transactions have inserted new data satisfying their query criteria , This phenomenon is called 「 Fantasy reading 」.
  Check the account balance , You went to check the bill again this month , At this time, you are in the transaction of checking the bill . When I checked it for the first time, I saw that there was only one income this month , Inexplicably, you checked this month's bill again and found an extra income of onemillion , The extra record of onemillion is unreal reading .

• Don't reread ： A time after a transaction reads some data , Read the previously read data again , It is found that the read data has changed or some records have been deleted . This phenomenon is called 「 It can't be read repeatedly 」.
  Check the details of this onemillion bill , At this time, you are in the transaction of checking the bill details , I don't know who transferred it to you . At this time, the old man found that the transfer was wrong , Just call the bank to cancel the transfer . At the same time, when you want to check more information about this bill , It turns out that you don't have this bill , The bank withdrew . When you read the same record in the transaction of checking the details of the bill and get different results, you cannot read it repeatedly .

The loss of updates is a writing problem , Dirty reading 、 Fantasy reading 、 Unrepeatable reading is the problem of reading .

Isolation level

The isolation level is when multiple transactions make changes and execute queries at the same time , Fine tuning performance and reliability of results 、 Setting of a balance between consistency and reproducibility .

The isolation level consistency and protection are from low to high ：

• Read uncommitted (Read Uncommitted)
• Read submitted (Read committed)
• Repeatable (Repeatable Read)
• Serialization (SERIALIZABLE)

Read uncommitted (Read Uncommitted)

Read uncommitted is a transaction that modifies the data , But did not submit , Other transactions can read the uncommitted data , This time there will be dirty reading . This level is rarely used .

Business B Read to transaction A Uncommitted data , This causes dirty reading .

Read submitted (Read committed)

A transaction reads data that has been modified and committed by other transactions , Whenever other transactions commit modified data , Can be read by the current transaction . Read has been submitted to solve the dirty read problem , Phantom reading can occur 、 It can't be read repeatedly .

Business A After submission , Business B To find out a Value

Repeatable (Repeatable Read)

MySQL InnoDB The default isolation level of the engine . Business T1 When a record is first read , Even if other transactions have modified the record , Business T1 What I read is still the record I read for the first time , Instead of reading different values every time , This is repeatable reading . Repeatable reading solves dirty reading 、 It can't be read repeatedly , There will still be unreal reading ,MySQL adopt MVCC( Multi version concurrency control ) To some extent, it solves the problem of unreal reading , of MVCC The next article will introduce .

Repeatable read will generate a snapshot when the same transaction is queried for the first time , When the same condition is queried again in this transaction , The data queried is actually the data in the snapshot , In order to achieve repeatable reading .

Business A And transaction B All available a=1, Business A take a Is changed to 2, Then commit the transaction , Although a The value of is already 2 了 , But the business B Obtained before submission a Still 1, This is repeatable reading .

Serialization (SERIALIZABLE)

Highest isolation level , It forces transactions to execute serially . It is rarely used in practice , Only when there is a strong need to ensure data consistency and it is acceptable that there is no concurrency , Only consider . When serializing transactions, other transactions are not allowed to execute concurrently . Read completely serially , No writing as long as there is reading , But at the same time , It eliminates unreal reading .

The higher the transaction isolation level , The integrity and consistency of data are also higher , But the less concurrency , The greater the impact on the access speed of the system . Different applications may have different requirements for the isolation level of transactions , If the application is not sensitive to unreal reading , Is to use the current MySQL The default isolation level is ok .

Check the isolation level

1、 Check the system isolation level

show variables like 'transaction_isolation';

perhaps

select @@global.tx_isolation;

2、 View session isolation level

select @@tx_isolation;

Set isolation level

1、 Global isolation level setting

set global transaction isolation level  Isolation level ;

--  Set the isolation level to repeatable read 
set global transaction isolation level REPEATABLE READ;

2、 Session isolation level setting

set session transaction isolation level  Isolation level ;

set session transaction isolation level REPEATABLE READ;

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