MySQL锁之二：锁相关的配置参数

锁相关的配置参数：

mysql> SHOW VARIABLES LIKE '%timeout%';

+-----------------------------+----------+

| Variable_name               | Value    |

+-----------------------------+----------+

| connect_timeout             | 10       |

| delayed_insert_timeout      | 300      |

| have_statement_timeout      | YES      |

| innodb_flush_log_at_timeout | 1        |

| innodb_lock_wait_timeout    | 50       |

| innodb_rollback_on_timeout  | OFF      |

| interactive_timeout         | 28800    |

| lock_wait_timeout           | 31536000 |

| net_read_timeout            | 30       |

| net_write_timeout           | 60       |

| rpl_stop_slave_timeout      | 31536000 |

| slave_net_timeout           | 60       |

| wait_timeout                | 28800    |

+-----------------------------+----------+

13 rows in set (0.00 sec)

一、innodb_rollback_on_timeout变量

innodb_rollback_on_timeout是mysql锁超时后的回滚机制，如下：

innodb_rollback_on_timeout为OFF：如果事务因为加锁超时，相当于回滚到上一条语句。但是报错后，事务还没有完成，用户可以选择是继续提交，或者回滚之前的操作，由用户选择是否进一步提交或者回滚事务。
innodb_rollback_on_timeout为ON：整个事务都会回滚。

下面是MySQL官方手册关开innodb_rollback_on_timeout变量的说明：

In MySQL 5.0.13 and up, InnoDB rolls back only the last statement on a transaction timeout by default. If --innodb_rollback_on_timeout is specified, a transaction timeout causes InnoDB to abort and roll back the entire transaction (the same behavior as before MySQL 5.0.13). This variable was added in MySQL 5.0.32.

该变量默认值为OFF，如果事务因为加锁超时，会回滚上一条语句执行的操作。如果设置ON，则整个事务都会回滚。

下面通过一个示例来验证上面这段话。

1.1、示例说明

(1) innodb_rollback_on_timeout为OFF

Session 1	Session 2
mysql> create table tt(c1 int primary key, c2 int)engine=innodb; Query OK, 0 rows affected (0.01 sec) mysql> insert into tt values(1, 1); Query OK, 1 row affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where c1=1 lock in share mode; +----+------+ \| c1 \| c2 \| +----+------+ \| 1 \| 1 \| +----+------+ 1 row in set (0.00 sec)
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt values(10,10); Query OK, 1 row affected (0.00 sec) mysql> delete from tt where c1=1; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> select * from tt; +----+------+ \| c1 \| c2 \| +----+------+ \| 1 \| 1 \| \| 10 \| 10 \| +----+------+ 2 rows in set (0.00 sec) mysql> rollback; Query OK, 0 rows affected (0.01 sec) mysql> select * from tt; +----+------+ \| c1 \| c2 \| +----+------+ \| 1 \| 1 \| +----+------+ 1 row in set (0.00 sec)
mysql> select * from tt; +----+------+ \| c1 \| c2 \| +----+------+ \| 1 \| 1 \| +----+------+ 1 row in set (0.00 sec)
	mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt values(10,10); Query OK, 1 row affected (0.00 sec) mysql> delete from tt where c1=1; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> commit; Query OK, 0 rows affected (0.02 sec) mysql> select * from tt; +----+------+ \| c1 \| c2 \| +----+------+ \| 1 \| 1 \| \| 10 \| 10 \| +----+------+ 2 rows in set (0.00 sec)

session2因为加锁超时，事务回退到上一条语句。

(2) innodb_rollback_on_timeout为ON

Session 1

Session 2

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from tt where c1=1 lock in share mode;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

+----+------+

1 row in set (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into tt values(11,11);

Query OK, 1 row affected (0.00 sec)

mysql> delete from tt where c1=1;

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

mysql> commit;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

session2加锁超时，整个事务回滚。

1.2、总结

innodb_rollback_on_timeout为OFF，事务会回滚到上一个保存点，InnoDB在执行每条SQL语句之前，都会创建一个保存点，参见代码：

int

row_insert_for_mysql(

/* out: error code or DB_SUCCESS */

byte* mysql_rec, /* in: row in the MySQL format */

row_prebuilt_t* prebuilt) /* in: prebuilt struct in MySQL

handle */

{

。。。

savept = trx_savept_take(trx);

。。。

如果事务因为加锁超时，相当于回滚到上一条语句。但是报错后，事务还没有完成，用户可以选择是继续提交，或者回滚之前的操作，由用户选择是否进一步提交或者回滚事务。

innodb_rollback_on_timeout为ON，整个事务都会回滚。这可以从row_mysql_handle_errors函数中得到验证。

ibool
row_mysql_handle_errors(
/*====================*/
                /* out: TRUE if it was a lock wait and
                we should continue running the query thread */
    ulint*        new_err,/* out: possible new error encountered in
                lock wait, or if no new error, the value
                of trx->error_state at the entry of this
                function */
    trx_t*        trx,    /* in: transaction */
    que_thr_t*    thr,    /* in: query thread */
    trx_savept_t*    savept)    /* in: savepoint or NULL */
{
...
else if (err == DB_DEADLOCK //发生死锁
           || err == DB_LOCK_TABLE_FULL
           || (err == DB_LOCK_WAIT_TIMEOUT
               && row_rollback_on_timeout)) {
        /* Roll back the whole transaction; this resolution was added
        to version 3.23.43 */

trx_general_rollback_for_mysql(trx, FALSE, NULL); //事务全部回滚

    } else if (err == DB_OUT_OF_FILE_SPACE
           || err == DB_LOCK_WAIT_TIMEOUT) {

ut_ad(!(err == DB_LOCK_WAIT_TIMEOUT
&& row_rollback_on_timeout));

if (savept) { //回滚到上一个保存点
/* Roll back the latest, possibly incomplete
insertion or update */

trx_general_rollback_for_mysql(trx, TRUE, savept);
}
/* MySQL will roll back the latest SQL statement */
...

问题：innodb_rollback_on_timeout为OFF，事务的原子性被破坏了吗？

答：NO，从示例中可以看到，事务只是回退上一条语句的状态，而整个事务实际上没有完成(提交或者回滚)，而作为应用程序在检测这个错误时，应该选择是提交或者回滚事务。如果严格要求事务的原子性，当然是执行ROLLBACK，回滚事务。

转自：http://www.cnblogs.com/hustcat/archive/2012/11/18/2775487.html

二、innodb_lock_wait_timeout参数

InnoDB 表类型的时候，锁超时时间是通过innodb_lock_wait_timeout：设置锁等待的时间，默认值是50s。

innodb_lock_wait_timeout指的是事务等待获取资源等待的最长时间，超过这个时间还未分配到资源则会返回应用失败；参数的时间单位是秒，最小可设置为1s(此时需要考虑应用端的频繁异常处理会消耗性能，不能设置过小)，最大可设置1073741824秒以上(再大就会被截断了，不过这样业务一直死循环等待下去而不能将资源使用来做其他的事情也是很浪费的一件事情)。

当有锁等待超过了这个时间（50），会报错1205 - Lock wait timeout exceeded; try restarting transaction，来中断事务，并释放锁。示例见《mysql事务之三：MySQL锁演示》