Transaction database and its four characteristics, principle, isolation level, dirty read, unreal read, non repeatable read?

1. What is a database transaction ？

A transaction is a set of atomic operations , These operations either all happen , Or it doesn't happen at all . Transactions transform a database from one consistency state to another .

If Xiaoming wants to transfer money to Xiaohong 1000 element , There are two key operations involved in this transfer ： Reduce Xiao Ming's balance 1000 element , Increase the balance of Xiaohong 1000 element . In case of a sudden error between these two operations, such as the collapse of the banking system , As a result, Xiaoming's balance decreased while Xiaohong's balance did not increase , That's not right . Transaction is to ensure the success of both critical operations , Or they all have to fail .

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2. How transactions are implemented

Transactions are based on redo log files (redo log) And rollback logs (undo log) Realized .

Every time a transaction is committed, all logs of the transaction must be written to the redo log file for persistence , The database can ensure the atomicity and persistence of transactions through redo logs .

Whenever there is a modification transaction , There will be undo log, If you need to roll back , According to undo log The reverse statement of , such as insert A record is delete A record .undo log It mainly realizes the consistency of the database .

Talk about redo log file (redo log) And rollback logs (undo log)

undo log Before the beginning of a transaction , Before operating on any data , First, back up the data to be operated to a place .redo log Any data operated in a transaction , Back up the latest data in one place .

The purpose of the transaction log ： Instance or media failed , Transaction log files come in handy .

redo log
redo log Not written with the commit of the transaction , It's during the execution of the transaction , And start writing redo in . Specific drop strategies can be configured . Prevent at the point of failure , There are dirty pages not written to disk , It's restarting MySQL In service , according to redo log To redo , So as to achieve the feature of persistence of transaction's non disk data .RedoLog It's a product of the persistence of transactions .

undo log
undo log Used to roll back row records to a version . Before the transaction is committed ,Undo Saved version data before submission ,Undo The data in can be used as a snapshot of the old version of data for other concurrent transactions to read . It's a product of the atomicity of transactions , stay MySQL innodb Concurrent storage is used to implement version control engine .

Bold style

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3. Transaction has four characteristics

1. Atomicity .

A transaction is the logical unit of work of a database , The operations contained in the transaction either do , Either not

2. Uniformity .

things The result of service execution must be to change the database from one consistency state to another . So when the database contains only the results of a successful transaction commit , Say the database is in a consistent state . If the database system Failure in operation , Some transactions are interrupted before they are completed , Some of the changes made to the database by these unfinished transactions have been written to the physical database , The database is in an incorrect state , Or rather, A state of inconsistency .

3. Isolation, .

The execution of a transaction cannot be interfered by other transactions . That is, within a transaction // The operation and data used are isolated from other concurrent transactions , Transactions that execute concurrently cannot interfere with each other .

4. Continuity .

Also called permanence , Once a transaction is committed , Its changes to the data in the database should be permanent . The next operation or failure should not have any effect on its execution result .

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4.MySQL The four isolation levels

1.Read Uncommitted（ Read uncommitted content ）
At this isolation level , All transactions can see the results of other uncommitted transactions . This isolation level is rarely used in practical applications , Because its performance is not much better than other levels . Read uncommitted data , It's also called dirty reading （Dirty Read）.

2.Read Committed（ Read submissions ）
This is the default isolation level for most database systems （ But it's not MySQL default ）. It satisfies the simple definition of isolation ： A transaction can only see changes that have been committed by the transaction . This level of isolation Also support the so-called Of It can't be read repeatedly （Nonrepeatable Read）, Because other instances of the same transaction may have new commit, So the same select May return different results .

3.Repeatable Read（ Can be reread ）
This is a MySQL Default transaction isolation level for , It ensures that when multiple instances of the same transaction simultaneously read data , You will see the same data lines . But in theory , This can lead to another tricky problem ： Fantasy reading （Phantom Read）.

4.Serializable（ Serializable ）
By forcing transaction ordering , Make it impossible to conflict with each other , So as to solve the problem of unreal reading . in short , It is to add a shared lock to each read data line . At this level , It may lead to a large number of timeout phenomena and lock competition .

5. What is dirty reading ？ Fantasy reading ？ It can't be read repeatedly ？

1、 Dirty reading

Business A Read transaction B Updated data , then B Rollback operation , that A The data read is dirty

2、 It can't be read repeatedly

Business A Read the same data multiple times , Business B In the transaction A During multiple reads , The data has been updated and submitted , Cause transaction A When reading the same data multiple times , result atypism .

3、 Fantasy reading

System administrator A Change the scores of all students in the database from specific scores to ABCDE Grade , But the system administrator B At this time, a specific score record was inserted , When the system administrator A After the change, I found that there is another record that hasn't been changed , It's like an illusion , This is called Unreal reading .

Fantasy reading , It's not that the result sets obtained by two reads are different , Unreal reading focuses on one aspect or another select The data state represented by the result of the operation cannot support the subsequent business operation . More specifically ：select Whether a record exists , non-existent , Ready to insert this record , But enforcement insert Found that this record already exists , Can't insert , At this time, the unreal reading happened .

Unrepeatable reading focuses on revision , Unreal reading focuses on adding or deleting （ More or less lines ）, A dirty read is a transaction rollback that affects another transaction .