Transaction characteristics and isolation levels

Transaction features

Atomicity 、 Uniformity 、 persistence 、 Isolation,

• Atomicity （Atomicity） Atomicity means that a transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .
• Uniformity （Consistency） The integrity of the data before and after the transaction must be consistent .
• persistence （Durability） Persistence means that once a transaction is committed , It changes the data in the database permanently , Next, even if the database fails, it should not have any impact .
• Isolation, （Isolation） Transaction isolation refers to when multiple users operate the database concurrently , The transaction of one user cannot be interfered by the transactions of other users , Data between multiple concurrent transactions should be isolated from each other . Simply speaking : Transactions do not interfere with each other

Transaction concurrency

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 .

Isolation level

Level	name	Isolation level	Dirty reading	It can't be read repeatedly	Fantasy reading	Database default isolation level
1	Read uncommitted	read uncommitted	yes	yes	yes
2	Read submitted	read committed	no	yes	yes	Oracle
3	Repeatable	repeatable read	no	no	yes	MySQL
4	Serialization	serializable	no	no	no