MySQL/MariaDB数据库的并发控制
作者：尹正杰
版权声明：原创作品，谢绝转载！否则将追究法律责任。
一.并发控制概述
1>.什么是并发控制
MySQL是一个服务器级别的数据库，它通常放在网络上有很多用户并发访问的,因此并发控制很关键。
2>.锁粒度
表级锁(MyISAM和Innodb都支持)
行级锁(仅Innodb支持,MyISAM不支持)
3>.锁
读锁：
共享锁，只读不可写（包括当前事务） ，多个读互不阻塞
写锁：
独占锁，排它锁，写锁会阻塞其它事务（不包括当前事务）的读和它锁
4>.实现
存储引擎：
自行实现其锁策略和锁粒度
服务器级：实现了锁，表级锁，用户可显式请求
5>.分类
隐式锁：
由存储引擎自动施加锁
显式锁：
用户手动请求
6>.锁策略
在锁粒度及数据安全性寻求的平衡机制
7>.死锁
两个或多个事务在同一资源相互占用，并请求锁定对方占用的资源的状态
二.针对表级别显式使用锁实战案例
1>.查看锁帮助信息
MariaDB [yinzhengjie]> HELP LOCK Name: 'LOCK' Description: Syntax: LOCK TABLES tbl_name [[AS] alias] lock_type [, tbl_name [[AS] alias] lock_type] ... lock_type: READ [LOCAL] | [LOW_PRIORITY] WRITE UNLOCK TABLES MySQL enables client sessions to acquire table locks explicitly for the purpose of cooperating with other sessions for access to tables, or to prevent other sessions from modifying tables during periods when a session requires exclusive access to them. A session can acquire or release locks only for itself. One session cannot acquire locks for another session or release locks held by another session. Locks may be used to emulate transactions or to get more speed when updating tables. This is explained in more detail later in this section. LOCK TABLES explicitly acquires table locks for the current client session. Table locks can be acquired for base tables or views. You must have the LOCK TABLES privilege, and the SELECT privilege for each object to be locked. For view locking, LOCK TABLES adds all base tables used in the view to the set of tables to be locked and locks them automatically. If you lock a table explicitly with LOCK TABLES, any tables used in triggers are also locked implicitly, as described in https://mariadb.com/kb/en/triggers-and-implicit-locks/. UNLOCK TABLES explicitly releases any table locks held by the current session. LOCK TABLES implicitly releases any table locks held by the current session before acquiring new locks. Another use for UNLOCK TABLES is to release the global read lock acquired with the FLUSH TABLES WITH READ LOCK statement, which enables you to lock all tables in all databases. See [HELP FLUSH]. (This is a very convenient way to get backups if you have a file system such as Veritas that can take snapshots in time.) URL: https://mariadb.com/kb/en/transactions-lock/ MariaDB [yinzhengjie]>
2>.添加读锁
MariaDB [yinzhengjie]> LOCK TABLES students READ; #添加读锁后，可读不可写 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 100 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ 25 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UPDATE students SET Age = 120 WHERE Name = '齐天大圣孙悟空'; ERROR 1099 (HY000): Table 'students' was locked with a READ lock and can't be updated MariaDB [yinzhengjie]> MariaDB [yinzhengjie]>
3>.解锁
MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 100 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ 25 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UPDATE students SET Age = 120 WHERE Name = '齐天大圣孙悟空'; ERROR 1099 (HY000): Table 'students' was locked with a READ lock and can't be updated MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UNLOCK TABLES; #对表进行解锁 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UPDATE students SET Age = 120 WHERE Name = '齐天大圣孙悟空'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 120 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ 25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
4>.添加写锁
MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 120 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ 25 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> LOCK TABLES students WRITE; #添加写锁后，当前终端可读可写，其它终端既不可读也不可写 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UPDATE students SET Age = 125 WHERE Name = '齐天大圣孙悟空'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 125 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ 25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
三.针对数据库级别显式使用锁实战案例
1>.添加读锁(注意:对数据库级别并没有写锁)
MariaDB [yinzhengjie]> FLUSH TABLES WITH READ LOCK; #为数据库实例添加读锁,可以对数据库进行读取操作，无法对所有数据库进行写入操作，一般用作数据库温备,并不适合数据库冷备和热备。 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> CREATE TABLE students_backup SELECT * FROM students; #不可对当前数据库进行写入操作 ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> GRANT ALL ON yinzhengjie.* to jason@'172.30.1.%' IDENTIFIED BY 'yinzhengjie'; #很显然其它数据库也不能写入数据了，包括mysql系统数据库 ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SELECT * FROM students; +-------+-----------------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+-----------------------+-----+--------+---------+-----------+ | 1 | Shi Zhongyu | 22 | M | 2 | 3 | | 2 | Shi Potian | 22 | M | 1 | 7 | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 4 | Ding Dian | 32 | M | 4 | 4 | | 5 | Yu Yutong | 26 | M | 3 | 1 | | 6 | Shi Qing | 46 | M | 5 | NULL | | 7 | Xi Ren | 19 | F | 3 | NULL | | 8 | Lin Daiyu | 17 | F | 7 | NULL | | 9 | Ren Yingying | 20 | F | 6 | NULL | | 10 | Yue Lingshan | 19 | F | 3 | NULL | | 11 | Yuan Chengzhi | 23 | M | 6 | NULL | | 12 | Wen Qingqing | 19 | F | 1 | NULL | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 14 | Lu Wushuang | 17 | F | 3 | NULL | | 15 | Duan Yu | 19 | M | 4 | NULL | | 16 | Xu Zhu | 21 | M | 1 | NULL | | 17 | Lin Chong | 25 | M | 4 | NULL | | 18 | Hua Rong | 23 | M | 7 | NULL | | 19 | Xue Baochai | 18 | F | 6 | NULL | | 20 | Diao Chan | 19 | F | 7 | NULL | | 21 | Huang Yueying | 22 | F | 6 | NULL | | 22 | Xiao Qiao | 20 | F | 1 | NULL | | 23 | Ma Chao | 23 | M | 4 | NULL | | 24 | Xu Xian | 27 | M | NULL | NULL | | 25 | 齐天大圣孙悟空 | 120 | M | NULL | NULL | +-------+-----------------------+-----+--------+---------+-----------+ rows in set (0.00 sec) MariaDB [yinzhengjie]>
2>.解锁
MariaDB [yinzhengjie]> FLUSH TABLES WITH READ LOCK; Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> CREATE TABLE students_backup SELECT * FROM students; ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> UNLOCK TABLES; Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> CREATE TABLE students_backup SELECT * FROM students; Query OK, 25 rows affected (0.01 sec) Records: 25 Duplicates: 0 Warnings: 0 MariaDB [yinzhengjie]>
3>.关闭正在打开的表（清除查询缓存）
MariaDB [yinzhengjie]> FLUSH TABLES ; Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]>
四.查询时加写或读锁
SELECT clause [FOR UPDATE | LOCK IN SHARE MODE] #这种方式使用相对较少，感兴趣的小伙伴可自行查阅资料 SELECT ... For Update会把行进行写锁定，这是排它锁。 使用主键明确记录行，那么就对存在的主键的记录使用行级锁。例如id=100 使用主键，但不能确定记录，使用表级锁，例如id <> 3。 条件中不使用主键，会使用表级锁。例如，name='tom' 这是悲观锁，我怕别人在我用的时候抢资源，先锁上，独占。悲观锁往往用在数据库的锁机制中，因为独占，所以会影响并发。所以，For Update非要使用，请一定要保证时间短，且一定利用行级锁。 乐观锁，就是心宽，认为极少冲突，只有写入才加锁。但需要检测数据冲突。