MySQL事务详解

1. 事务： 一个事务是一个连续的一组数据库操作，就好像它是一个单一的工作单元进行。换言之，永远不会是完整的事务，除非该组内的每个单独的操作是成功的。如果在事务的任何操作失败，则整个事务将失败。也是一组原子性的SQL查询、或者是一个或多个SQL语句组成的独立工作单元。

2. 事务日志：

MariaDB [test]> SHOW GLOBAL VARIABLES LIKE 'Innodb%log%';
+-------------------------------------------+---------+
| Variable_name                             | Value   |
+-------------------------------------------+---------+
| innodb_flush_log_at_trx_commit            | 1       |
| innodb_locks_unsafe_for_binlog            | OFF     |
| innodb_log_block_size                     | 512     |
| innodb_log_buffer_size                    | 8388608 |
| innodb_log_file_size                      | 5242880 |
| innodb_log_files_in_group                 | 2       |
| innodb_log_group_home_dir                 | ./      |
| innodb_mirrored_log_groups                | 1       |
| innodb_recovery_update_relay_log          | OFF     |
| innodb_use_global_flush_log_at_trx_commit | ON      |
+-------------------------------------------+---------+

3. 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。

4. 自动提交：单语句事务

MariaDB [test]> SHOW VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
MariaDB [test]> SET @@session.autocommit=0;
MariaDB [test]> SHOW VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+

5. 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
示例：

MariaDB [test]> CREATE TABLE tbl1 (id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, name CHAR(30) NOT NULL);
MariaDB [test]> SHOW TABLE STATUS LIKE 'tbl1';
MariaDB [test]> START TRANSACTION;
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('tom'),('jerry');
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
+----+-------+
MariaDB [test]> ROLLBACK;
MariaDB [test]> SELECT * FROM tbl1;
Empty set (0.00 sec)
MariaDB [test]> START TRANSACTION;
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('tom'),('jerry');
MariaDB [test]> SAVEPOINT firsttag;
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
+----+-------+
MariaDB [test]> DELETE  FROM tbl1 WHERE id=4;
MariaDB [test]> SAVEPOINT secondtag;
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('obama');
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  3 | obama |
+----+-------+
MariaDB [test]> ROLLBACK TO secondtag;
MariaDB [test]> SELECT * FROM tbl1;
+----+------+
| id | name |
+----+------+
|  1 | tom  |
+----+------+
MariaDB [test]> ROLLBACK TO firsttag;
MariaDB [test]> COMMIT;

6. 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;
 SERIALIZABLE：串行化;
示例(两个连接节点)：

MariaDB [test]> SHOW PROCESSLIST;
+----+------+-----------+------+---------+------+-------+------------------+----------+
| Id | User | Host      | db   | Command | Time | State | Info             | Progress |
+----+------+-----------+------+---------+------+-------+------------------+----------+
|  1 | root | localhost | test | Query   |    0 | NULL  | SHOW PROCESSLIST |    0.000 |
|  2 | root | localhost | test | Sleep   |  393 |       | NULL             |    0.000 |
+----+------+-----------+------+---------+------+-------+------------------+----------+
MariaDB [test]> SELECT @@session.tx_isolation;                                       #默认
+------------------------+
| @@session.tx_isolation |
+------------------------+
| REPEATABLE-READ        |
+------------------------+
MariaDB [test]> SET @@session.autocommit=0;                                          #关闭自动提交

读未提交 --> 脏读：

节点1：
MariaDB [test]> SET @@session.tx_isolation='READ-UNCOMMITTED';
节点2：
MariaDB [test]> SET @@session.tx_isolation='READ-UNCOMMITTED';
节点1：
MariaDB [test]> START TRANSACTION;
节点2：
MariaDB [test]> START TRANSACTION;
节点1：
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('trump');
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
|  4 | trump |
+----+-------+
节点1：
MariaDB [test]> ROLLBACK;
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
+----+-------+
MariaDB [test]> COMMIT;

读提交 --> 不可重复读:

节点1：
MariaDB [test]> SET @@session.tx_isolation='READ-COMMITTED';
节点2：
MariaDB [test]> SET @@session.tx_isolation='READ-COMMITTED';
节点1：
MariaDB [test]> START TRANSACTION;
节点2：
MariaDB [test]> START TRANSACTION;
节点1：
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('jack;');
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
+----+-------+
节点1：
MariaDB [test]> COMMIT;
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
|  5 | jack; |
+----+-------+
MariaDB [test]> COMMIT;

可重复读 --> 幻读:

节点1：
MariaDB [test]> SET @@session.tx_isolation='REPEATABLE-READ';
节点2：
MariaDB [test]> SET @@session.tx_isolation='REPEATABLE-READ';
节点1：
MariaDB [test]> START TRANSACTION;
节点2：
MariaDB [test]> START TRANSACTION;
节点1:
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
|  5 | jack; |
+----+-------+
MariaDB [test]> DELETE FROM tbl1 WHERE name='tom';
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
|  5 | jack; |
+----+-------+
节点1：
MariaDB [test]> COMMIT;
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  1 | tom   |
|  2 | jerry |
|  5 | jack; |
+----+-------+
MariaDB [test]> ROLLBACK;
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  2 | jerry |
|  5 | jack; |
+----+-------+

串行化：

节点1：
MariaDB [test]> SET @@session.tx_isolation='SERIALIZABLE';
节点2：
MariaDB [test]> SET @@session.tx_isolation='SERIALIZABLE';
节点1：
MariaDB [test]> START TRANSACTION;
节点2：
MariaDB [test]> START TRANSACTION;
节点1：
MariaDB [test]> INSERT INTO tbl1 (name) VALUES ('tom');
节点2：
MariaDB [test]> SELECT * FROM tbl1;(被阻塞)
节点1：
MariaDB [test]> COMMIT;
节点2：
MariaDB [test]> SELECT * FROM tbl1;
+----+-------+
| id | name  |
+----+-------+
|  2 | jerry |
|  5 | jack; |
|  6 | tom   |
+----+-------+