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：
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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
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Session 1 |
Session 2 |
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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) |
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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) |
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mysql> select * from tt; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | +----+------+ 1 row in set (0.00 sec) |
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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
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Session 1 |
Session 2 |
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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) |
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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：
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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。
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。
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》
