Transaction of MySQL underlying principle (2)

One 、 Basic concepts of business

1、 Transaction features ：

• A（Atomicity Atomicity ）： All success or all failure
•  I（Isolation Isolation, ）： Parallel transactions do not interfere with each other
• D（Durability persistence ）： After the transaction is committed , permanent
• C（Consistency Uniformity ）： adopt AID Guarantee

2、 Concurrency issues ：

• Dirty reading (Drity Read)： Read uncommitted data
• It can't be read repeatedly (Non-repeatable read)： The results of the two readings are different
• Fantasy reading (Phantom Read)：select The data state represented by the result of the operation cannot support the subsequent business operation

3、 Isolation level ：

•  Read Uncommitted（ Read uncommitted content ）： Minimum isolation level , Will read uncommitted data from other transactions , Dirty reading ;
• Read Committed（ Read submissions ）： Data submitted by other transactions can be read during the transaction , It can't be read repeatedly ;
• Repeatable Read（ Repeatable ）： Read the same result set every time , Whether other transactions are committed or not , Fantasy reading ;
• Serializable（ Serialization ）： Transaction queuing , The highest level of isolation , The worst performance ;

Two 、 How things work

1、MVCC

• Multi version concurrency control —— The current reading + Snapshot degree ： Solved the read-write conflict , As long as I read Data of one version , No matter how other transactions are modified , It has nothing to do with the version I read , Because what I read is a snapshot . Combined with the figure below .

         

• Predictability judgment ：

                l The moment you create a snapshot , Uncommitted transactions are not visible ;
                l Transactions created after snapshots are not visible ;

• Read View：

                l Read the snapshot Active transaction list 【10,11,15,17】
                l The smallest transaction in the list ID
                l The largest transaction in the list ID

  

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2、undo log： Multiple versions exist here

• Rollback log
• Ensure transaction atomicity
• Realize multi version of data
• delete undo log： For rollback , Clean up upon submission ;
• update undo log： For rollback , At the same time, realize snapshot reading , You can't delete
• clear undo log: According to the minimum active transactions of the system ID Read view

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3、redo log： Achieve transaction persistence

• Record changes
• For exception recovery
• Loop write file
• Write process

 

• When to brush the disk ：2=> Memory brush of operating system

•   significance ：a、 Small volume , Modification of record page , Less expensive than writing pages
  b、 Append at the end , Write in random order , The changed page is not fixed