MySQL - transaction attributes

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Atomicity （Actomicity）	A transaction is an atomic unit of operation , Its modification of data , Or do it all , Or none of them .
Uniformity （Consistent）	At the beginning and end of the transaction , Data must be consistent .
Isolation, （Isolation）	The database provides a certain isolation mechanism , Ensure that transactions are not affected by external concurrent operations “ Independent ” Environmental execution .
persistence （Durable）	After the transaction completes , Its modification of data is permanent , Even if there is a system failure, it can keep .

One 、 Atomicity （Atomicity）

A series of operations on the database , Either it's all success , Or it's all failure , There can be no partial success or partial failure .

Q： If the previous operation has been successful , Later operation failed , How to make it all fail ？
A： Roll back

Q： How to implement rollback ？
A： stay InnoDB It's through undo log To achieve , It records the values before data modification （ Logic log ）, Once something goes wrong , You can use it undo log To implement the rollback operation .

Two 、 Uniformity （consistent）

1、 The integrity constraints of the database itself

It means that the integrity constraints of the database are not broken , There are valid data states before and after the transaction execution .
For example, the primary key must be unique , The length of the field meets the requirements .

Example ： Transfer accounts

A Decrease in account balance 1000,B The account balance only increased 500, At this time, because of the success of both operations , According to our definition of atomicity , It's atomic , But it doesn't satisfy consistency , Because it leads to the imbalance of accounting subjects .

2、 User defined integrity

User defined integrity is usually controlled in the code .

Example ： Transfer accounts

A The account balance is 0, If the transfer is successful at this time ,A The balance of the account will become -1000, Although it satisfies the atomic , But we know that , The balance of debit card cannot be less than 0 Of , So it's also against consistency .

3、 ... and 、 Isolation, （isolation）

There are many transactions in the database , Operate the same table or row of data at the same time , There must be some concurrent or interfering operations , And these concurrent operations , It should be transparent , Not interfering with each other . In this way , We also guarantee the consistency of business data in the end .

Four 、 persistence （Durable）

Our arbitrary operation on the database , Additions and deletions , As long as the transaction is committed successfully , So the result is permanent , It is impossible because we have restarted the server of the database , It's back to its original state .

Q： How can persistence be achieved ？ Database crash recovery （crash-safe） By what means ？
A： Persistence is through redo log To achieve .
When we operate data , Will write to memory first buffer pool Inside , Simultaneous recording redo log, If something goes wrong before you brush the disc , It can be read after restart redo log The content of , Write to disk , Ensure data persistence .

summary

Atomicity , Isolation, , persistence , It's all about consistency