Laravel 之嵌套事务 transactions 实现
写在之前
关于mysql 的事务嵌套可以查看这个地址:
https://dev.mysql.com/doc/refman/5.5/en/im...
里面有这么一句话。
Transactions cannot be nested. This is a consequence of the implicit commit performed for any current transaction when you issue a START TRANSACTION statement or one of its synonyms.
大体意思是db不支持事务嵌套,如果你嵌套执行START TRANSACTION
时会隐式执行commit
我们做个测试:
mysql> BEGIN;
Query OK, 0 rows affected (0.16 sec)
mysql> INSERT INTO T2 VALUES(300);
Query OK, 1 row affected (0.29 sec)
mysql> BEGIN;
Query OK, 0 rows affected (0.04 sec)
mysql> rollback;
Query OK, 0 rows affected (0.04 sec)
mysql> SELECT * FROM T2;
+------+
| ID |
+------+
| 300 |
+------+
2 rows in set (0.04 sec)
果然,我们直接rollback上面的语句,但是还是执行了查询操作。
laravel之嵌套事务transactions实现
为啥官网不支持,但是 laravel
框架却优雅的实现了事务嵌套,我们来看看它的实现原理。
调用示例:
\DB::beginTransaction(); //主事务
try{
\DB::beginTransaction(); //子事务
\DB::insert('insert into T2 set ID=100');
\DB::rollBack(); //子事务回滚
\DB::insert('insert into T2 set ID=200');
\DB::commit();
}catch (\Exception $e) {
\DB::rollBack();
echo $e->getMessage();exit;
}
查看执行结果:
mysql> SELECT * FROM T2;
+------+
| ID |
+------+
| 100 |
+------+
1 row in set (0.05 sec)
说明子事务成功回滚了,下面看下子事务的实现。
代码分析:
laravel/framework/src/Illuminate/Database/Concerns/ManagesTransactions.php 90行
public function beginTransaction()
{
$this->createTransaction();
$this->transactions++;
$this->fireConnectionEvent('beganTransaction');
}
每调一次beginTransaction会使$this->transactions加1
接着看一下$this->createTransaction();的实现
/**
* Create a transaction within the database.
*
* @return void
*/
protected function createTransaction()
{
if ($this->transactions == 0) {
try {
$this->getPdo()->beginTransaction();
} catch (Exception $e) {
$this->handleBeginTransactionException($e);
}
} elseif ($this->transactions >= 1 && $this->queryGrammar->supportsSavepoints()) {
$this->createSavepoint();
}
}
if ($this->transactions == 0)
首先判断是否在事务中。
没有在事务中则执行 $this->getPdo()->beginTransaction()
相当于执行 BEGIN;
在事务中执行 $this->createSavepoint();
下面是createSavepoint方法的实现。
/**
* Create a save point within the database.
*
* @return void
*/
protected function createSavepoint()
{
$this->getPdo()->exec(
$this->queryGrammar->compileSavepoint('trans'.($this->transactions + 1))
);
}
这里相当于在mysql里执行 SAVEPOINT trans1;
下面看下rollback方法实现:
public function rollBack($toLevel = null)
{
$toLevel = is_null($toLevel)
? $this->transactions - 1
: $toLevel;
if ($toLevel < 0 || $toLevel >= $this->transactions) {
return;
}
$this->performRollBack($toLevel);
$this->transactions = $toLevel;
$this->fireConnectionEvent('rollingBack');
}
首先rollback会使$this->transactions减一。
然后调用$this->performRollBack
protected function performRollBack($toLevel)
{
if ($toLevel == 0) {
$this->getPdo()->rollBack();
} elseif ($this->queryGrammar->supportsSavepoints()) {
$this->getPdo()->exec(
$this->queryGrammar->compileSavepointRollBack('trans'.($toLevel + 1))
);
}
}
performRollBack方式实际就是在重新设定savepoint值。
下面看下commit的实现:
/**
* Commit the active database transaction.
*
* @return void
*/
public function commit()
{
if ($this->transactions == 1) {
$this->getPdo()->commit();
}
$this->transactions = max(0, $this->transactions - 1);
$this->fireConnectionEvent('committed');
}
commit方法,只有在最外层时才会真正的提交。
总结:
- 基本实现原理是 savepoint
- 通过$this->transactions对应的数值设定 不同的savepoint实现不同层次嵌套
- 只有在最后一个commit时才会真正提交请求。
SAVEPOINT 使用demo如下:
mysql> CREATE TABLE T2(ID INT);
Query OK, 0 rows affected (0.05 sec)
mysql> select * from T2;
Empty set (0.17 sec)
mysql> BEGIN;
Query OK, 0 rows affected (0.04 sec)
mysql> INSERT INTO T2 VALUES(100);
Query OK, 1 row affected (0.04 sec)
mysql> SAVEPOINT trans1;
Query OK, 0 rows affected (0.04 sec)
mysql> INSERT INTO T2 VALUES(200);
Query OK, 1 row affected (0.04 sec)
mysql> ROLLBACK TO SAVEPOINT trans1;
Query OK, 0 rows affected (0.04 sec)
mysql> RELEASE SAVEPOINT trans1;
Query OK, 0 rows affected (0.03 sec)
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
mysql> SELECT * FROM T2;
+------+
| ID |
+------+
| 100 |
+------+
1 row in set (0.05 sec)
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