Acid transaction theory

What is? ACID

A transaction has four basic characteristics , That's what we often say （ACID）：

• Atomicity（ Atomicity ）
  ： A business is an indivisible whole , All operations within the transaction are either successful , Or it's a total failure .

• Consistency（ Uniformity ）
  ： Before and after implementation , Data from one state to another must be consistent （A towards B Transfer accounts , Cannot appear A Buckle the money ,B Has received ）.

• Isolation（ Isolation, ）
  ：  Multiple concurrent transactions are isolated from each other , Don't interfere with each other .

• Durability（ persistence ）
  ： After the transaction completes , Changes to the database are permanent , Cannot be rolled back .

ACID By what guarantee ？

With MySQL For example ：

A Atomicity

from undo log Log guarantees , It records the log information that needs to be rolled back , When the transaction is rolled back, undo the successfully executed sql

C Uniformity

It is generally guaranteed by the code level

I Isolation,

from MVCC To guarantee

D persistence

By memory +redo log To guarantee ,mysql Modify data both in memory and redo log Record this operation , The transaction is committed through redo log Brush set , When it goes down, you can start from redo log recovery

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