Characteristics of transactions -- atomicity (implementation principle)

The nature of transactions —— Atomicity （ Realization principle ）

Atomicity of a transaction means that all operations in a transaction are indivisible , Must be a logical unit , It can only be all successful or all failed .（ A typical example ： Transfer accounts ）. So how to guarantee the atomicity of transactions ？ Before that, we need to say MySQL Medium WAL Mechanism .

WAL Mechanism

WAL Its full name is Write-Ahead Logging, Pre write log system . It mainly refers to MySQL When performing a write operation, it is not immediately updated to the disk , But first record it in the log , Then update to disk at the right time . Logs are mainly divided into undo log、redo log、binlog.

How transaction atomicity is guaranteed ？

MySQL The atomicity of a transaction is through ***undo log*** To achieve . Disk data is stored randomly , This makes it necessary not only to record the stored data value when storing data , You also need to record the address where the data is stored , The storage speed is relatively slow . Log storage is continuous storage , Therefore, when saving data, you only need to record the first address , The remaining data record offset , Can further improve performance .
Every write transaction , Will be modified BufferPool, And the corresponding Redo/Undo journal , These log information will be recorded in the log file . stay MySQL in , whatever BufferPool Before the pages in are flushed to disk , Will be written to the log file first , If BufferPool Data submission in , At this point, the database is down , So after the database starts again , You can go through Redo Log to recover it , To ensure that the data will not be lost . If the data is not submitted , At this point, the database is down , It needs to pass Undo Log to achieve .
Every piece of data changes （insert/update/delete） The operation is accompanied by a undo log Generation , And the rollback log must be persisted to the disk before the data . The so-called rollback is to do the reverse operation according to the rollback log , such as delete The reverse operation of is insert,insert The reverse operation of is delete,update The reverse is update etc. .