How to realize the four isolation levels of MySQL (brief)

The four properties of a transaction

Namely Atomicity 、 Uniformity 、 Isolation, 、 persistence .

• Atomicity ：undolog Log to ensure , It records the log information that needs to be rolled back , When the transaction is rolled back, undo the executed sql.

• Uniformity ： It is guaranteed by three other features

• Isolation, ：MVCC To guarantee , Multi version concurrency control

• persistence ： from redolog To guarantee ,mysql When modifying data, it will be in redolog Record a log data in , Even if the data is not saved successfully , As long as the log is saved successfully , Data is still not lost .MySQL Collapse recovery hero —Redo Log

Concurrency of transactions

Dirty reading ： Business A Transaction read B Data not submitted in , When a transaction B After rollback , Business A The data read is dirty .

It can't be read repeatedly ： Business A Read the same data multiple times , But the value read each time is different . It's because of the business B Update and submit the data many times .

Fantasy reading ： Business A During operation , Table data has been added or deleted many times , And the newly added or deleted data affects the transaction A The result of the operation .

Non repeatable reading focuses on data modification , Unreal reading focuses on adding or deleting data . Solve the problem of non repeatable reading , Only lock it , And solve unreal reading , You need to lock the whole table .

mysql Isolation level

The isolation level for database transactions is 4 individual , From low to high

Read uncommitted( Read uncommitted )： Dirty reading 、 It can't be read repeatedly 、 Fantasy reading All exist .

Read committed（ Read submitted ）： Solved the problem of dirty reading , There is Non repeatability 、 The problem of unreal reading .

Repeatable read（ Repeatable ）： Solved dirty reading 、 Unrepeatable read problem , There is Fantasy reading .

Serializable（ Serialization ）： Resolve all , But the concurrency efficiency is the lowest .

Implementation of isolation level

1. Read uncommitted （RU：read-uncommitted）： You will read the data that has not been submitted .

   When the isolation level is uncommitted read RU when ：

   • be-all Read without lock , The data read are the latest data , Best performance .
   • All the writing Add line lock , After writing, release .

2. Read submitted （RC：read-committed）： Use MVCC technology , Add hidden fields to each line （DB_TRX_ID： Modify the last transaction of the row id,DB_ROLL_PTR： Point to the current line undo log journal ,DB_ROW_ID： Line identification ,DELETE_BIT： Delete logo ）, It realizes the read operation without lock .

   When isolation level is RC when ：

   Write operations ： Add line lock . When the business begins , Will be in UNDO Write the modification record in the log , Hidden columns in data rows DATA_POLL_PTR Store the UNDO Pointer to record .

   Read operations ： No locks . When reading , If the row is locked by other transactions , Then follow the hidden column DATA_POLL_PTR The pointer , Find the last valid history （ Effective records ： This record is visible to the current transaction , And DELETE_BIT=0）.

3. Repeatable （RR：repeatable-read）： Use MVCC Technology to realize the read operation without lock .

   • Write operations ： Add line lock . When the business begins , Will be in UNDO Write the modification record in the log , Hidden columns in data rows DATA_POLL_PTR Store the UNDO Pointer to record .
   • Read operations ： No locks . When reading , If the row is locked by other transactions , Then follow the hidden column DATA_POLL_PTR The pointer , Find the last valid history （ Effective records ： This record is visible to the current transaction , And DELETE_BIT=0）.

4. Serialization （serializable）： Read and write are locked .

You can see from above ： actually Read submitted and Repeatable The operation of level is basically the same , The difference is ： Row records for the current transaction visibility （ visibility ： That is, which version of the row record is visible to this transaction ）. The visibility of read submitted level to data is The latest record of this data , The visibility of repeatable basic data is At the beginning of the business , Record of this data .