多次听到“MySQL为什么选择RR为默认隔离级别”的问题,其实这是个历史遗留问题,当前已经解决,但MySQL的各个版本沿用了原有习惯。历史版本中的问题是什么,当前还有没有问题,本次就通过简单的测试来说明一下。
01
准备工作
部署一套主从架构的集群,创建过程较简单,可以参考历史文章部署 MySQL主从复制搭建 部署一主一从即可。
在主库中创建表及测试数据
mysql> create table users(id int primary key auto_increment,user_name varchar(20),c_id tinyint(4),c_note varchar(50),key c_id(c_id)) engine=innodb;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> create table class(c_id int primary key ,c_name varchar(1),c_note varchar(50)) engine=innodb;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
02
RR隔离级别
MySQL默认的隔离级别为 RR(Repeatable Read),在此隔离级别下,对比binlog格式为ROW、STATEMENT是否会造成主从数据不一致
其实不用测试大家也应该对RR级别下ROW格式的binlog有信心,但是,万事皆需实践检验。
步骤说明如下:
具体步骤如下:
步骤 | SESSION A | SESSION B |
---|---|---|
1 | mysql>show variables like '%iso%';+-----------------------+-----------------+| Variable_name | Value |+-----------------------+-----------------+| transaction_isolation | REPEATABLE-READ || tx_isolation | REPEATABLE-READ |+-----------------------+-----------------+2 rows in set (0.00 sec) mysql>show variables like '%binlog_format%';+---------------+-------+| Variable_name | Value |+---------------+-------+| binlog_format | ROW |+---------------+-------+1 row in set (0.00 sec) | mysql>show variables like '%iso%';+-----------------------+-----------------+| Variable_name | Value |+-----------------------+-----------------+| transaction_isolation | REPEATABLE-READ || tx_isolation | REPEATABLE-READ |+-----------------------+-----------------+2 rows in set (0.00 sec) mysql>show variables like '%binlog_format%';+---------------+-------+| Variable_name | Value |+---------------+-------+| binlog_format | ROW |+---------------+-------+1 row in set (0.01 sec) |
2 | mysql>set autocommit=0;mysql>update users set c_note='t1' where c_id in (select c_id from class);Query OK, 5 rows affected (0.00 sec)Rows matched: 5 Changed: 5 Warnings: 0 | |
3 | mysql>set autocommit=0;Query OK, 0 rows affected (0.00 sec) mysql>delete from class where c_id=2;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction | |
4 | mysql>commit;Query OK, 0 rows affected (0.00 sec) | |
5 | mysql>set autocommit=0;Query OK, 0 rows affected (0.00 sec) mysql>delete from class where c_id=2;Query OK, 1 row affected (0.00 sec) mysql>commit;Query OK, 0 rows affected (0.00 sec) | |
6 | mysql>update users set c_note='t2' where c_id in (select c_id from class);Query OK, 3 rows affected (0.00 sec)Rows matched: 3 Changed: 3 Warnings: 0 mysql>commit;Query OK, 0 rows affected (0.00 sec) | |
7 | mysql>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) | mysql>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) |
8 | 在从库查看数据root@testdb:3307 12:02:20>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) |
为了和之前的步骤一致,先初始化数据
root@testdb:3306 12:14:27>truncate table users;
Query OK, 0 rows affected (0.08 sec)
root@testdb:3306 12:14:29>truncate table class;
Query OK, 0 rows affected (0.04 sec)
root@testdb:3306 12:14:50>insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙 权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@testdb:3306 12:15:10>insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
再将binlog日志格式改为STATAMENT格式(全局及会话级都改一下,或者修改全局变量后重新登录也行,当然 只改会话级别的也可以测试),然后 再次进行测试。
步骤说明如下:
步 骤 | SESSION A | SESSION B |
---|---|---|
1 | mysql>show variables like '%iso%';+-----------------------+-----------------+| Variable_name | Value |+-----------------------+-----------------+| transaction_isolation | REPEATABLE-READ || tx_isolation | REPEATABLE-READ |+-----------------------+-----------------+2 rows in set (0.01 sec) mysql>show variables like '%binlog_format%';+---------------+-----------+| Variable_name | Value |+---------------+-----------+| binlog_format | STATEMENT |+---------------+-----------+1 row in set (0.01 sec) | mysql>show variables like '%iso%';+-----------------------+-----------------+| Variable_name | Value |+-----------------------+-----------------+| transaction_isolation | REPEATABLE-READ || tx_isolation | REPEATABLE-READ |+-----------------------+-----------------+2 rows in set (0.01 sec) mysql>show variables like '%binlog_format%';+---------------+-----------+| Variable_name | Value |+---------------+-----------+| binlog_format | STATEMENT |+---------------+-----------+1 row in set (0.01 sec) |
2 | root@testdb:3306 12:37:04>set autocommit=0;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:37:17>update users set c_note='t1' where c_id in (select c_id from class);Query OK, 5 rows affected, 1 warning (0.00 sec)Rows matched: 5 Changed: 5 Warnings: 1 | |
3 | root@testdb:3306 12:28:25>set autocommit=0;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:38:06>delete from class where c_id=2;Query OK, 1 row affected (4.74 sec) | |
4 | root@testdb:3306 12:38:09>commit;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:38:13>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t1 || 3 | 孙 权 | 3 | t1 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t1 |+----+-----------+------+--------+5 rows in set (0.00 sec) root@testdb:3306 12:39:07>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 2 | 蜀 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+4 rows in set (0.00 sec) | |
5 | root@testdb:3306 12:38:13>commit;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:39:56>select * from class ;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.00 sec) | |
6 | root@testdb:3306 12:52:23>update users set c_note='t2' where c_id in (select c_id from class);Query OK, 3 rows affected, 1 warning (0.00 sec)Rows matched: 3 Changed: 3 Warnings: 1 root@testdb:3306 12:52:45>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 2 | 蜀 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+4 rows in set (0.00 sec) root@testdb:3306 12:52:49>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙 权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.01 sec) root@testdb:3306 12:53:03>commit;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:53:06>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙 权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) root@testdb:3306 12:53:11>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.00 sec) | |
7 | 查看从库数据root@testdb:3307 12:44:22>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.01 sec) root@testdb:3307 12:57:07>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙 权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) |
也就是此时主从结果也是一致的,原因在于,binlog里存储的语句顺序如下:
binlog里的顺序 | 语句内容 |
---|---|
1 | update users set c_note='t1' where c_id in (select c_id from class); |
2 | delete from class where c_id=2; |
3 | update users set c_note='t2' where c_id in (select c_id from class); |
与主库执行的顺序是一致的,因此,主从的结果是一致的。
03
RC隔离级别
为了和之前的步骤一致,先初始化数据
root@testdb:3306 12:14:27>truncate table users;
Query OK, 0 rows affected (0.08 sec)
root@testdb:3306 12:14:29>truncate table class;
Query OK, 0 rows affected (0.04 sec)
root@testdb:3306 12:14:50>insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙 权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@testdb:3306 12:15:10>insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
再将binlog日志格式改为STATAMENT格式(全局及会话级都改一下,或者修改全局变量后重新登录也行,当然 只改会话级别的也可以测试),然后 再次进行测试。
步骤说明如下:
步 骤 | SESSION A | SESSION B |
---|---|---|
1 | root@testdb:3306 01:25:24>show variables like '%iso%';+-----------------------+----------------+| Variable_name | Value |+-----------------------+----------------+| transaction_isolation | READ-COMMITTED || tx_isolation | READ-COMMITTED |+-----------------------+----------------+2 rows in set (0.01 sec) root@testdb:3306 01:25:36>show variables like '%binlog_format%';+---------------+-------+| Variable_name | Value |+---------------+-------+| binlog_format | ROW |+---------------+-------+1 row in set (0.01 sec) | root@testdb:3306 01:24:57>show variables like '%iso%';+-----------------------+----------------+| Variable_name | Value |+-----------------------+----------------+| transaction_isolation | READ-COMMITTED || tx_isolation | READ-COMMITTED |+-----------------------+----------------+2 rows in set (0.01 sec) root@testdb:3306 01:25:39>show variables like '%binlog_format%';+---------------+-------+| Variable_name | Value |+---------------+-------+| binlog_format | ROW |+---------------+-------+1 row in set (0.00 sec) |
2 | root@testdb:3306 01:27:55>set autocommit=0;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 01:28:27>update users set c_note='t1' where c_id in (select c_id from class);Query OK, 5 rows affected (0.00 sec)Rows matched: 5 Changed: 5 Warnings: 0 | |
3 | root@testdb:3306 01:26:07>set autocommit=0;Query OK, 0 rows affected (0.00 sec) root@testdb:3306 01:28:37>delete from class where c_id=2;Query OK, 1 row affected (0.00 sec) | |
4 | root@testdb:3306 01:28:27>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 2 | 蜀 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+4 rows in set (0.00 sec) | |
5 | root@testdb:3306 01:28:41>commit;Query OK, 0 rows affected (0.00 sec) | |
6 | root@testdb:3306 01:28:59>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.01 sec) root@testdb:3306 01:29:13>update users set c_note='t2' where c_id in (select c_id from class);Query OK, 3 rows affected (0.00 sec)Rows matched: 3 Changed: 3 Warnings: 0 root@testdb:3306 01:29:26>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.00 sec) root@testdb:3306 01:29:31>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙 权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) root@testdb:3306 01:29:38>commit; | |
7 | 查看从库数据 root@testdb:3307 01:40:32>select * from users;+----+-----------+------+--------+| id | user_name | c_id | c_note |+----+-----------+------+--------+| 1 | 刘备 | 2 | t1 || 2 | 曹操 | 1 | t2 || 3 | 孙 权 | 3 | t2 || 4 | 关羽 | 2 | t1 || 5 | 司马懿 | 1 | t2 |+----+-----------+------+--------+5 rows in set (0.00 sec) root@testdb:3307 01:40:35>select * from class;+------+--------+--------+| c_id | c_name | c_note |+------+--------+--------+| 1 | 魏 | NULL || 3 | 吴 | NULL || 4 | 晋 | |+------+--------+--------+3 rows in set (0.00 sec) |
也就是此时主从结果也是一致的。
因为当前版本已经不支持RC+STATEMENT组合下数据的操作,否则将报如下错误:
Cannot execute statement: impossible to write to binary log since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine limited to row-based logging. InnoDB is limited to row-logging when transaction isolation level is READ COMMITTED or READ UNCOMMITTED.
因此单纯根据步骤讲解
步骤 | SESSION A | SESSION B |
---|---|---|
1 | mysql>set autocommit=0; mysql>update users set c_note='t1' where c_id in (select c_id from class); | |
2 | mysql>set autocommit=0;mysql>delete from class where c_id=2;mysql>commit; | |
3 | mysql>update users set c_note='t2' where c_id in (select c_id from class); | |
4 | commit; |
因为binlog是按照commit时间的顺序保存,因此上述步骤在binlog里会以如下顺序存储:
binlog里的顺序 | 语句内容 |
---|---|
1 | delete from class where c_id=2; |
2 | update users set c_note='t1' where c_id in (select c_id from class); |
3 | update users set c_note='t2' where c_id in (select c_id from class); |
从库通过binlog应用后,最终的结果将导致主库的数据不一样(具体案例后续安装低版本后演示)。
因而,此种场景下很容易导致数据不一样。
04
结语
通过上述的实践,可以发现在RR级别下,binlog为任何格式均不会造成主从数据不一致的情况出现,但是当低版本MySQL使用RC+STATEMENT组合时(MySQL5.1.5前只有statement格式)将会导致主从数据不一致。当前这个历史遗漏问题以及解决,大家可以将其设置为RC+ROW组合的方式(例如ORACLE等数据库隔离级别就是RC),而不是必须使用RR(会带来更多的锁等待),具体可以视情况选择。