add by zhj: 看來我對MySQL的理解還有待深入，水還是挺深的啊，MySQL給記錄加鎖時，可以通過innodb_lock_wait_timeout參數設置超時時間，
如果加鎖等待超過這個時間，就會回滾，但回滾的話有兩種方式：第一種：回滾當前加鎖的這條語句；第二種：回滾整個事務。這兩種方式是通過參數
innodb_rollback_on_timeout來控制的。如果是OFF，表示加鎖超時回滾時，只回滾加鎖超時的那條SQL語句；如果是ON，表示回滾整個事務。默認
是OFF。在《MySQL Admin Cookbook》一書中，作者強烈建議該參數設置為ON，但其實OFF也沒關系，你可以在應用程序中捕獲那個加鎖超時，然
后應用程序去執行ROLLBACK，這樣就可以保證原子性，只要你沒執行COMMIT，就不會破壞原子性。Django應該就是這么做的。我個人拙見是：感覺
設置為ON，依靠數據庫本身自動完成回滾更好一些。
原文：http://www.cnblogs.com/hustcat/archive/2012/11/18/2775487.html
1、innodb_rollback_on_timeout變量
下面是MySQL官方手冊關開innodb_rollback_on_timeout變量的說明：
| 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. |
該變量默認值為OFF，如果事務因為加鎖超時，會回滾上一條語句執行的操作。如果設置ON，則整個事務都會回滾。
下面通過一個示例來驗證上面這段話。
2、示例
(1) innodb_rollback_on_timeout為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因為加鎖超時，事務回退到上一條語句。
(2) innodb_rollback_on_timeout為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加鎖超時，整個事務回滾。
3、總結
innodb_rollback_on_timeout為OFF，事務會回滾到上一個保存點，InnoDB在執行每條SQL語句之前，都會創建一個保存點，參見代碼：
| 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); 。。。 |
如果事務因為加鎖超時，相當於回滾到上一條語句。但是報錯后，事務還沒有完成，用戶可以選擇是繼續提交，或者回滾之前的操作，由用戶選擇是否進一步提交或者回滾事務。
innodb_rollback_on_timeout為ON，整個事務都會回滾。這可以從row_mysql_handle_errors函數中得到驗證。
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 //發生死鎖
|| 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); //事務全部回滾
} 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) { //回滾到上一個保存點
/* 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 */
...
問題：innodb_rollback_on_timeout為OFF，事務的原子性被破壞了嗎？
答：NO，從示例中可以看到，事務只是回退上一條語句的狀態，而整個事務實際上沒有完成(提交或者回滾)，而作為應用程序在檢測這個錯誤時，應該選擇是提交或者回滾事務。如果嚴格要求事務的原子性，當然是執行ROLLBACK，回滾事務。