Implementation principle of acid in MySQL

1. Atomicity （Atomicity）

Atomicity refers to the fact that a transaction is an indivisible unit of work , Either do all of them , Either not .

Realization and utilization of atomicity Innodb Of undo log. undo log Called rollback log , Is the key to atomicity , When the transaction is rolled back, all successfully executed sql sentence , He needs to record the corresponding log information you want to roll back .

for example
(1) When you delete When a piece of data , We need to record the information of this data , When rolling back ,insert This old data
(2) When you update When a piece of data , You need to record the old values before , When rolling back , According to the old value update operation
(3) That year insert When a piece of data , You need the primary key of this record , When rolling back , Execute according to the primary key delete operation

undo log Record the information needed for rollback , When the transaction fails or calls rollback, Causes the transaction to be rolled back , You can use undo log The information in rollback the data to what it looked like before the modification .

2. Isolation, (Isolation)

Isolation refers to the concurrent execution of multiple transactions , Operations within a transaction are isolated from other transactions , Transactions that execute concurrently cannot interfere with each other .

The implementation of isolation uses locks and MVCC Mechanism .MVCC, Multi version concurrent control (Multi Version Concurrency Control), A row record data has multiple versions of snapshot data , The snapshot data is in undo log in . If a row read by a transaction is in progress DELELE perhaps UPDATE operation , The read operation does not wait for the lock on the line to release , Instead, read the snapshot version of the row .

Read committed at transaction isolation level (Read Commited) when , One transaction can read the submitted data of another transaction , Is not satisfied with isolation . But when the transaction isolation level is repeatable (Repeateable Read) in , Is to meet the isolation of .

3. persistence (Durability)

Persistence is when a transaction is committed , Its changes to the database should be permanent . Subsequent operations or failures should not have any impact on it .

Implementation and utilization of persistence Innodb Of redo log（ Redo log ）. As I said before ,Mysql First, load the data on the disk into the memory , Modify data in memory , Swipe it back on the disk . If the machine goes down suddenly , Data in memory will be lost .

3.1 How to solve this problem ？

Simple , Just write the data to disk directly before the transaction is committed .

3.2 What's the problem with this ？

Modify only one byte of a page , Just swipe the entire page into disk , It's a waste of resources . After all, a page 16kb size , You just change a little bit of it , will 16kb The contents of the brush into the disk , It doesn't make sense .
After all, in a business SQL Modification of multiple data pages may be involved , And these data pages may not be adjacent , It's random IO. Obviously the operation is random IO, The speed will be slow .
　　 use redo log Solve the above problems . When data is modified , Not only in memory , Also in the redo log Record this operation . When the transaction is committed , Will redo log Brush the logs (redo log Part of it is in memory , Part of it is on disk ). When the database is down and restarted , Will redo log The content in is restored to the database , According to undo log and binlog Content determines whether to roll back data or submit data .

3.3 use redo log The benefits of ？

redo log Compared with the data page, the efficiency of disk brushing is higher , The specific performance is as follows ：

• redo log Small volume , After all, it only records which page has been modified and what , So it's small , Brush the disc quickly .
• redo log It's going all the way to the end , It belongs to order IO. Efficiency is obviously better than randomness IO Come fast .

4. Uniformity (Consistency)

    Consistency refers to before and after transaction execution , Data is in a legal state , This state is semantic rather than grammatical .  This state is to satisfy a predetermined constraint, which is called a legal state , A little more general , This state is defined by you . Satisfy this state , The data is consistent , Not satisfied with this state , The data is inconsistent ！

Mysql How to ensure consistency ？

• From the database level , Database through atomicity 、 Isolation, 、 Persistence to ensure consistency . in other words ACID Among the four characteristics ,C( Uniformity ) It is purpose ,A( Atomicity )、I( Isolation, )、D( persistence ) It's the means , To ensure consistency , The means provided by the database . Database must be implemented AID The three major characteristics , It's possible to achieve consistency .

• From the application level , Through the code to determine whether the database data is valid , Then decide whether to roll back or commit the data .

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