MySQL: how to solve data modification conflict

itread01 2021-01-21 20:09:14
mysql solve data modification conflict

Abstract : Recently, there is a problem in the reception demand , Suppose that an order consultation exceeds 3 I can't see you again , But if two doctors consult the order at the same time , When you check the database, you can find the order that meets the conditions , Both doctors can see , The so-called reception can be understood as updating the number of reception , This is where bug( More than 3 Time ).

In fact, this problem seems very clear , But a complete solution requires understanding the concepts of transactions and locks , In the past, the isolation level and lock of transactions were a little hazy , Now we can make it clear through this case .


The smallest unit of work to operate a database , To put it simply, it is to put more than one dml( To add, delete, or alter ) Statement joint completion . To succeed at the same time , To fail at the same time . Seeing this, you may find that adding transactions alone can't solve the above problems , And after adding the transaction , The relationship between multiple transactions is related to the isolation level of transactions , So then look down .

Transaction isolation level

READ UNCOMMITTED( Read uncommitted , Dirty reading )

Changes in transactions , Even if you don't submit , It's also visible to other conversations . Can read uncommitted data —— Dirty reading . Dirty reading can cause a lot of problems , This isolation level is generally not applicable ..


-- ------------------------- read-uncommitted For example -------------------------------- Set the global system isolation level SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;-- Session ASTART TRANSACTION;SELECT * FROM USER;UPDATE USER SET NAME="READ UNCOMMITTED";-- commit;-- Session BSELECT * FROM USER;//SessionB Console You can see Session A Handling of uncommitted things , In the other Session We also see , This is the so-called dirty reading id name2 READ UNCOMMITTED34 READ UNCOMMITTED


READ COMMITTED( Read submitted , Don't read again )

Most databases default to use this isolation level (MySQL No ), This isolation level ensures that if a transaction does not completely succeed (commit End of execution ), Operations in a transaction are not visible to other sessions .

-- ------------------------- read-cmmitted For example -------------------------------- Set the global system isolation level SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;-- Session ASTART TRANSACTION;SELECT * FROM USER;UPDATE USER SET NAME="READ COMMITTED";-- COMMIT;-- Session BSELECT * FROM USER;//Console OUTPUT:id name2 READ UNCOMMITTED34 READ UNCOMMITTED----------------------------------------------------- When Session A Executed commit,Session B The results are as follows :id name2 READ COMMITTED34 READ COMMITTED

REPEATABLE READ ( Repeatable )

Perform unified read multiple times in a transaction SQL, The return result is the same . This isolation level solves the problem of dirty reading , Unreal reading problem . This means innodb Of rr Level ,innodb Use in next-key Lock pair " Now read " Lock it , Lock lines and insert positions where unreal reading may occur , Prevent new data from being inserted to create a magic line .

Conversation T1 A query is executed in a transaction , And then the conversation T2 Insert a new line of record , This line of records just meets T1 The condition of the query used . And then T1 Use the same Search the table again , But then you see the transaction T2 The new line just inserted . This new line is called “ Visions ”, Because yes T1 It's like all of a sudden It's the same .innoDB Of RR Level cannot avoid unreal reading completely .

SERIALIZABLE ( Serializable )

The strongest level of isolation , By locking each row read in a transaction , Write and lock , Make sure there is no unreal reading problem , But it can lead to a lot of timeout and lock contention problems .

mysql The default isolation level is repeatable , You should understand this , Even if the isolation level is repeatable , But because select The operation is not locked , All of them will find the information that meets the requirements , So here we introduce the concept of lock : Row level lock .

Row level lock

  • Shared lock (S)  Shared locks are also called read locks , Read lock allows multiple connections to read the same resource concurrently at the same time , They don't interfere with each other ;

  • Exclusive lock (X)  Exclusive lock is also called write lock , One write lock blocks other write locks or read locks , Ensure that only one connection can write data at the same time , At the same time, it prevents other users from reading and writing this data .

Summary ( Solutions )

In fact, the above analysis of so many , The final solution is simple . It's in the original read and update Add up the business , Original select Sentence plus exclusive lock , That is to say select Add... After the statement for update. If there's a business A,B, After joining the exclusive lock , What if A Get the lock first , Business B You have to wait until the business A commit Then we can start select, So what I read is the latest revised information . As to why no shared lock is added , Except for the possibility of dirty writing , In this case, it can also cause a deadlock . If two things  A 、 B  All read the same row of records , Then add a shared lock to this line , however  A  and B  You need to modify this line in the transaction , Then you have to wait for the other party to release the shared lock , The result is a deadlock .



  1. 【计算机网络 12(1),尚学堂马士兵Java视频教程
  2. 【程序猿历程,史上最全的Java面试题集锦在这里
  3. 【程序猿历程(1),Javaweb视频教程百度云
  4. Notes on MySQL 45 lectures (1-7)
  5. [computer network 12 (1), Shang Xuetang Ma soldier java video tutorial
  6. The most complete collection of Java interview questions in history is here
  7. [process of program ape (1), JavaWeb video tutorial, baidu cloud
  8. Notes on MySQL 45 lectures (1-7)
  9. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  10. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  11. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  12. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  13. 【递归,Java传智播客笔记
  14. [recursion, Java intelligence podcast notes
  15. [adhere to painting for 386 days] the beginning of spring of 24 solar terms
  16. K8S系列第八篇(Service、EndPoints以及高可用kubeadm部署)
  17. K8s Series Part 8 (service, endpoints and high availability kubeadm deployment)
  18. 【重识 HTML (3),350道Java面试真题分享
  19. 【重识 HTML (2),Java并发编程必会的多线程你竟然还不会
  20. 【重识 HTML (1),二本Java小菜鸟4面字节跳动被秒成渣渣
  21. [re recognize HTML (3) and share 350 real Java interview questions
  22. [re recognize HTML (2). Multithreading is a must for Java Concurrent Programming. How dare you not
  23. [re recognize HTML (1), two Java rookies' 4-sided bytes beat and become slag in seconds
  24. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  25. RPC 1: how to develop RPC framework from scratch
  26. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  27. RPC 1: how to develop RPC framework from scratch
  28. 一次性捋清楚吧,对乱糟糟的,Spring事务扩展机制
  29. 一文彻底弄懂如何选择抽象类还是接口,连续四年百度Java岗必问面试题
  30. Redis常用命令
  31. 一双拖鞋引发的血案,狂神说Java系列笔记
  32. 一、mysql基础安装
  33. 一位程序员的独白:尽管我一生坎坷,Java框架面试基础
  34. Clear it all at once. For the messy, spring transaction extension mechanism
  35. A thorough understanding of how to choose abstract classes or interfaces, baidu Java post must ask interview questions for four consecutive years
  36. Redis common commands
  37. A pair of slippers triggered the murder, crazy God said java series notes
  38. 1、 MySQL basic installation
  39. Monologue of a programmer: despite my ups and downs in my life, Java framework is the foundation of interview
  40. 【大厂面试】三面三问Spring循环依赖,请一定要把这篇看完(建议收藏)
  41. 一线互联网企业中,springboot入门项目
  42. 一篇文带你入门SSM框架Spring开发,帮你快速拿Offer
  43. 【面试资料】Java全集、微服务、大数据、数据结构与算法、机器学习知识最全总结,283页pdf
  44. 【leetcode刷题】24.数组中重复的数字——Java版
  45. 【leetcode刷题】23.对称二叉树——Java版
  46. 【leetcode刷题】22.二叉树的中序遍历——Java版
  47. 【leetcode刷题】21.三数之和——Java版
  48. 【leetcode刷题】20.最长回文子串——Java版
  49. 【leetcode刷题】19.回文链表——Java版
  50. 【leetcode刷题】18.反转链表——Java版
  51. 【leetcode刷题】17.相交链表——Java&python版
  52. 【leetcode刷题】16.环形链表——Java版
  53. 【leetcode刷题】15.汉明距离——Java版
  54. 【leetcode刷题】14.找到所有数组中消失的数字——Java版
  55. 【leetcode刷题】13.比特位计数——Java版
  56. oracle控制用户权限命令
  57. 三年Java开发,继阿里,鲁班二期Java架构师
  58. Oracle必须要启动的服务
  59. 万字长文!深入剖析HashMap,Java基础笔试题大全带答案
  60. 一问Kafka就心慌?我却凭着这份,图灵学院vip课程百度云