MySQL 事务功能
事务：
1.介绍
★事务：
- 一组原子性的SQL查询、或者是一个或多个SQL语句组成的独立工作单元；
★ACID测试：
☉A：AUTOMICITY，原子性；
- 整个事务中的所有操作要么全部成功执行，要么全部失败后回滚；
☉C：CONSISTENCY，一致性；
- 数据库总是应该从一个一致性状态转为另一个一致性状态；
☉I：ISOLATION，隔离性；
- 一个事务所做出的操作在提交之前，是否能为其它事务可见；出于保证并发操作之目的，隔离有多种级别；
☉D：DURABILITY，持久性；
- 事务一旦提交，其所做出的修改会永久保存；
2.事务日志：
mysql会最大程度的使用缓存机制来提高数据库的访问效率，但是万一数据库发生断电，因为缓存的数据没有写入磁盘，导致缓存在内存中的数据丢失而导致数据不一致怎么办？
★Innodb主要是通过事务日志实现ACID特性；
★事务日志包括：
☉重做日志redo
- Redo记录的是已经全部完成的事务，就是执行了commit（提交）的事务，记录文件是ib_logfile0 ib_logfile1；
☉回滚日志undo
- Undo记录的是已部分完成并且写入硬盘的未完成的事务，默认情况下回滚日志是记录下表空间中的（共享表空间或者独享表空间）；
★执行过程：
- 一般情况下，mysql在崩溃之后，重启服务，innodb通过回滚日志undo将所有已完成并写入磁盘的未完成事务进行rollback，然后redo中的事务全部重新执行一遍（后台启动相关线程自动执行）即可恢复数据，但是随着redo的量增加，每次从redo的第一条开始恢复就会浪费长的时间，所以引入了checkpoint（检查点）机制；
★事务日志相关配置参数：
- innodb_log_files_in_group //一组中有几个文件，默认为2个
- innodb_log_group_home_dir //日志目录的存放位置，默认在数据目录中
- innodb_log_file_size //日志文件大小
- innodb_mirrored_log_groups //镜像日志组
演示：
1.查看事物日志相关的参数
MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE ‘%innodb%log%‘; +-------------------------------------------+---------+ | Variable_name | Value | +-------------------------------------------+---------+ | innodb_flush_log_at_trx_commit | 1 | 要不要在提交时从内存中同步数据到事物日志所在磁盘上，1表示ON开启 | innodb_locks_unsafe_for_binlog | OFF | | innodb_log_block_size | 512 | | innodb_log_buffer_size | 8388608 | | innodb_log_file_size | 5242880 | 日志文件大小，默认为5M | innodb_log_files_in_group | 2 | 一组中有几个文件，默认为2个 | innodb_log_group_home_dir | ./ | 日志目录的存放位置，默认在数据目录中 | innodb_mirrored_log_groups | 1 | 镜像日志组（自己） | innodb_recovery_update_relay_log | OFF | | innodb_use_global_flush_log_at_trx_commit | ON | +-------------------------------------------+---------+ 10 rows in set (0.00 sec)
3.自动提交：单语句事务
★自动提交：单语句事务
- 默认情况下mysql开启的是单语句事物功能（即每一次执行完成后都会自动提交至事物日志中，然后再从日志中同步到数据文件中，非常影响性能），我们要想手动控制事物，就要把自动提交事物的功能关闭。
★查看自动提交命令：
- mysql> SELECT @@autocommit;
- mysql> SHOW GLOBAL|SESSION VARIABLES LIKE ‘autocommit‘
★关闭当前会话单语句功能：
- mysql> SET @@session.autocommit=0；
演示：
1.为了演示效果，支持事物功能，我这里修改了之前hellodb.sql的存储引擎为InnoDB，并重新导入mysql；现在，我们要想手动控制事物，就需要关闭单语句事物功能，如下：
MariaDB [hellodb]> show table status like ‘students‘\G *************************** 1. row *************************** Name: students Engine: InnoDB # 存储引擎为InnoDB Version: 10 Row_format: Compact Rows: 25 Avg_row_length: 655 Data_length: 16384 Max_data_length: 0 Index_length: 0 Data_free: 0 Auto_increment: 26 Create_time: 2016-11-28 11:42:19 Update_time: NULL Check_time: NULL Collation: utf8_general_ci Checksum: NULL Create_options: Comment: 1 row in set (0.02 sec) MariaDB [hellodb]> SHOW VARIABLES LIKE ‘autocommit‘; # 查看当前会话的单语句事物功能为ON +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | ON | +---------------+-------+ 1 row in set (0.00 sec) MariaDB [hellodb]> SET @@session.autocommit=0; Query OK, 0 rows affected (0.01 sec) MariaDB [hellodb]> SHOW VARIABLES LIKE ‘autocommit‘; # 再次查看变为OFF +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | OFF | +---------------+-------+ 1 row in set (0.01 sec)
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4.手动控制事务：
★启动：START TRANSACTION
★提交：COMMIT
★回滚：ROLLBACK
★事务支持savepoints：
- SAVEPOINT identifier
- ROLLBACK [WORK] TO [SAVEPOINT] identifier
- RELEASE SAVEPOINT identifier
演示：
1.在关闭单语句事务的基础上，手动启动事物功能,并演示回滚操作
MariaDB [hellodb]> START TRANSACTION; # 启动事物功能 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> DESC courses; +----------+----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+----------------------+------+-----+---------+----------------+ | CourseID | smallint(5) unsigned | NO | PRI | NULL | auto_increment | | Course | varchar(100) | NO | | NULL | | +----------+----------------------+------+-----+---------+----------------+ 2 rows in set (0.01 sec) MariaDB [hellodb]> SELECT * FROM courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | +----------+----------------+ 7 rows in set (0.01 sec) MariaDB [hellodb]> INSERT INTO courses (Course) VALUES (‘Zabbix‘),(‘Puppet‘); # 插入一行数据 Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> SELECT * FROM courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 8 | Zabbix | | 9 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> DELETE FROM courses WHERE CourseID=3; # 删除一行数据 Query OK, 1 row affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 8 | Zabbix | | 9 | Puppet | +----------+----------------+ 8 rows in set (0.00 sec) MariaDB [hellodb]> ROLLBACK; # 如果这时我们后悔了，可以执行回滚操作 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # 查看可以发现又会到原来开始的状态了 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | +----------+----------------+ 7 rows in set (0.00 sec)
2.在操作上添加savepoints（存储点），可以指定回滚到存储点的位置，如下：
MariaDB [hellodb]> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> INSERT INTO courses (Course) VALUES (‘Zabbix‘),(‘Puppet‘); Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> SAVEPOINT first; # 添加第一个存储点 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> DELETE FROM courses WHERE CourseID=3; Query OK, 1 row affected (0.00 sec) MariaDB [hellodb]> SAVEPOINT second; # 添加第二个存储点 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> UPDATE courses SET Course=‘nginx‘ WHERE CourseID=4; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [hellodb]> SELECT * FROM courses; # 查看修改后的表数据 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 4 | nginx | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 10 | Zabbix | | 11 | Puppet | +----------+----------------+ 8 rows in set (0.00 sec) MariaDB [hellodb]> ROLLBACK TO second; # 回滚到第二个存储点 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 10 | Zabbix | | 11 | Puppet | +----------+----------------+ 8 rows in set (0.00 sec) MariaDB [hellodb]> ROLLBACK TO first; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # 回滚到第一个存储点 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 10 | Zabbix | | 11 | Puppet | +----------+----------------+ 9 rows in set (0.01 sec) MariaDB [hellodb]> ROLLBACK; # 回滚到原始状态 Query OK, 0 rows affected (0.01 sec) MariaDB [hellodb]> SELECT * FROM courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | +----------+----------------+ 7 rows in set (0.00 sec)
3.启动事物，执行操作后提交（COMMIT），发现再回滚就无效了，因为这时已经同步到数据中了；
MariaDB [hellodb]> START TRANSACTION; # 启动事物 Query OK, 0 rows affected (0.01 sec) MariaDB [hellodb]> INSERT INTO courses (Course) VALUES (‘Zabbix‘),(‘Puppet‘); Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> COMMIT; # 提交 Query OK, 0 rows affected (0.01 sec) MariaDB [hellodb]> SELECT * FROM courses; # 提交后查看数据 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> ROLLBACK; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # 执行回滚命令后发现无效 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec)
5.事务隔离级别：
★事务隔离级别：
- READ-UNCOMMITTED：读未提交 --> 脏读；
- READ-COMMITTED：读提交--> 不可重复读；
- REPEATABLE-READ：可重复读 --> 幻读；（默认级别）
- SERIALIZABLE：串行化；
★查看及设定事物隔离级别：
- mysql> SELECT @@session.tx_isolation;
- mysql> SET @@session.tx_isolation=‘...‘
演示：
1.READ-UNCOMMITTED：读未提交 --> 脏读；即当node1和node2两个会话同时访问修改同一张表时，node1用户的事物还没有提交，node2用户就可以看到，导致的结果就是脏读（看到了别人没能提交的数据），如下：
1）在node1会话中，修改隔离级别为读未提交，然后启动事物，修改一行数据，并回滚
MariaDB [hellodb]> SELECT @@tx_isolation; # 查看隔离级别 +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> SET @@session.tx_isolation=‘READ-UNCOMMITTED‘; # 修改隔离级别为读未提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation; +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> START TRANSACTION; # 启动事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> UPDATE courses SET Course=‘nginx‘ WHERE CourseID=4; # 修改数据 Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [hellodb]> select * from courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | nginx | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> rollback; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> select * from courses; # 回滚后查看数据 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec)
2）在node2会话中可以发现，它没有做任何操作，前后两次查看时数据发生了变化，所以读未提交可以看到别人没能提交的数据
MariaDB [hellodb]> set @@session.tx_isolation=‘READ-UNCOMMITTED‘; # 同样设置运行级别为读未提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation； +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> start transaction; # 同样启动事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> select * from courses; # node1会话修改数据后查看，可以看到修改的数据nginx +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | nginx | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.01 sec) MariaDB [hellodb]> select * from courses; # node会话执行回滚后查看，可以看到又为原来的状态 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec)
2.READ-COMMITTED：读提交--> 不可重复读；即当node1和node2两个会话同时访问修改同一张表时，node1用户的事物只有提交了，node2用户才可以看到，但node2用户并没有修改数据。所以，node2会话如果重复读的话，每次的结果可能是不一样的，如下：
1）在node1会话中，修改隔离级别为读提交，然后启动事物，修改一行数据，并提交；
MariaDB [hellodb]> set @@session.tx_isolation=‘READ-COMMITTED‘; #修改隔离级别为读提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation； +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> start transaction; # 启动事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> UPDATE courses SET Course=‘Nagios‘ WHERE CourseID=12; # 修改数据 Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [hellodb]> select * from courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Nagios | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> COMMIT; # 提交数据 Query OK, 0 rows affected (0.01 sec)
2）在node2会话中，可以发现在node1会话未提交数据之前，node2查看到的数据一直没有发生改变，直到node1会话提交数据之后，node2才发现数据发生了改变，但node2没有做任何操作
MariaDB [hellodb]> set @@session.tx_isolation=‘READ-COMMITTED‘; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> start transaction; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # 在node1会话没有提交数据前，数据为改变 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # node1会话提交数据后，可以看到数据发生改变 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Nagios | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> COMMIT; Query OK, 0 rows affected (0.00 sec)
3.REPEATABLE-READ：可重复读 --> 幻读，也是mysql和mariadb的默认运行级别，即两个会话为各自独立的事务。node1和node2会话只要建立事务，第一次查看数据的结果将贯穿整个事物过程中，期间底层数据的变化是看不到的，只有在结束事物后才可以看到数据的变化，这样直接导致的结果就是幻读，即各自会话看到的数据并非真实的数据，底层数据可能已经发生了改变；
时间： 2024-10-22 15:45:03