Basic characteristics and isolation level of transactions

The nature of transactions

Transactions are strictly defined , It has to satisfy four characteristics at the same time , namely Atomicity 、 Uniformity 、 Isolation and persistence , It is also known as ACID characteristic , As follows .

1） Atomicity （Atomic）

Said is ⼀ All operations in a transaction are either successful , All or nothing .

2） Uniformity （Consistency）

It means that the database always starts from ⼀ individual ⼀ The state of causation changes to another ⼀ individual ⼀ The state of being sexual .

3） Isolation, （Isolation）

refer to ⼀ Modification of a transaction before final commit , For other matters is not ⻅ Of .

4） persistence （Durability）

refer to ⼀ Once the transaction is committed , The changes will be permanently saved to the database .

The isolation level of the transaction

Isolation level	Chinese name
read uncommint	Read uncommitted
read commit	Read submitted
repeatable read	Repeatable
serializable	Serialization

• read uncommit Read uncommitted , You may read uncommitted data from other transactions , It's also called Dirty reading .
  • ⽤ The user should have read id=1 Of ⽤ Household age Should be 10, As a result, transactions that have not been committed by other transactions are read , Results read results age=20, This is dirty reading .
• read commit Read submitted , The results of two readings are not ⼀ Cause , It's called non repeatable reading . Non repeatable reading solves the problem of dirty reading , He will only read transactions that have been committed .
  • ⽤ The user starts transaction reading id=1⽤ Household , Query to age=10,( Other transactions committed ) Read the result again =20, At the same time ⼀ One transaction ⾥ Same as ⼀ A query Reading different results is called non repeatable .
• repeatable read Repeatable , This is a mysql Of Default level , Is that every time you read the result ⼀ sample , But there are May produce unreal reading .
• serializable strand ⾏,⼀ Generally, it won't make ⽤ Of , He will give everyone ⼀⾏ Lock the read data , Meeting Lead to a lot of timeout and lock competition .