从Laravel,Yii,Thinkphp中学习php 操作数据库的事务嵌套
最近维护历史代码，使用的是phalapi 最初版本开发，数据库操作使用的是notorm。notorm本身不支持事务嵌套，但是在开发过程中，多个操作进行拆分，根据不同业务不同进行调用，必然会设计到多个事务嵌套在一起的问题。举个栗子:
1) 公共模块A,更新用户的账户余额，添加流水记录操作。 2) 模块B,根据用户的操作（消费或充值）根据活动配置赠送相应的优惠券。 基础业务A模块就够用了，但是出现一些业务活动的时候，需要在A成功之后调用B模块，只有两个操作成功之后才完整提交事务。要实现这样的功能，无非两种方式:
1) 模块内部不加事务，事务控制统一交给调用方。谁调用，谁负责事务。内部模块只提供内部模块执行结果。 2) 模块内部控制事务，外部调用只需知道内部执行是否成功。
如果设计的合理，还是比较倾向于使用第一种方案。但是不可否认，你无法确定你的调用模块会不会再被其他人调用，最终结果又演变成第二种方案。因此底层还是需要支持事务嵌套。
嵌套事务的核心思想就是添加一个计数器，第一次开启事务，最后一次提交或回滚执行数据库操作，其他情况只是更新计数器数值。分别看一看几个现有框架如何设计数据库事务嵌套操作:
1) Laravel Laravel与事务相关操作封装在 Illuminate\Database\Concerns\ManagesTransactions当中。
public function beginTransaction()
{
$this->createTransaction();
$this->transactions++;
$this->fireConnectionEvent('beganTransaction');
}
protected function createTransaction()
{
if ($this->transactions == 0) {
try {
$this->getPdo()->beginTransaction();
} catch (Exception $e) {
$this->handleBeginTransactionException($e);
}
} elseif ($this->transactions >= 1 && $this->queryGrammar->supportsSavepoints()) {
$this->createSavepoint();
}
}
public function commit()
{
if ($this->transactions == 1) {
$this->getPdo()->commit();
}
$this->transactions = max(0, $this->transactions - 1);
$this->fireConnectionEvent('committed');
}
public function rollBack($toLevel = null)
{
// We allow developers to rollback to a certain transaction level. We will verify
// that this given transaction level is valid before attempting to rollback to
// that level. If it's not we will just return out and not attempt anything.
$toLevel = is_null($toLevel)
? $this->transactions - 1
: $toLevel;
if ($toLevel < 0 || $toLevel >= $this->transactions) {
return;
}
// Next, we will actually perform this rollback within this database and fire the
// rollback event. We will also set the current transaction level to the given
// level that was passed into this method so it will be right from here out.
$this->performRollBack($toLevel);
$this->transactions = $toLevel;
$this->fireConnectionEvent('rollingBack');
}
2) Yii2
Yii2的事务操作也是有一个单独文件进行封装，yii\db\Transaction。
public function begin($isolationLevel = null)
{
if ($this->db === null) {
throw new InvalidConfigException('Transaction::db must be set.');
}
$this->db->open();
if ($this->_level === 0) {
if ($isolationLevel !== null) {
$this->db->getSchema()->setTransactionIsolationLevel($isolationLevel);
}
Yii::debug('Begin transaction' . ($isolationLevel ? ' with isolation level ' . $isolationLevel : ''), __METHOD__);
$this->db->trigger(Connection::EVENT_BEGIN_TRANSACTION);
$this->db->pdo->beginTransaction();
$this->_level = 1;
return;
}
$schema = $this->db->getSchema();
if ($schema->supportsSavepoint()) {
Yii::debug('Set savepoint ' . $this->_level, __METHOD__);
$schema->createSavepoint('LEVEL' . $this->_level);
} else {
Yii::info('Transaction not started: nested transaction not supported', __METHOD__);
}
$this->_level++;
}
public function commit()
{
if (!$this->getIsActive()) {
throw new Exception('Failed to commit transaction: transaction was inactive.');
}
$this->_level--;
if ($this->_level === 0) {
Yii::debug('Commit transaction', __METHOD__);
$this->db->pdo->commit();
$this->db->trigger(Connection::EVENT_COMMIT_TRANSACTION);
return;
}
$schema = $this->db->getSchema();
if ($schema->supportsSavepoint()) {
Yii::debug('Release savepoint ' . $this->_level, __METHOD__);
$schema->releaseSavepoint('LEVEL' . $this->_level);
} else {
Yii::info('Transaction not committed: nested transaction not supported', __METHOD__);
}
}
public function rollBack()
{
if (!$this->getIsActive()) {
// do nothing if transaction is not active: this could be the transaction is committed
// but the event handler to "commitTransaction" throw an exception
return;
}
$this->_level--;
if ($this->_level === 0) {
Yii::debug('Roll back transaction', __METHOD__);
$this->db->pdo->rollBack();
$this->db->trigger(Connection::EVENT_ROLLBACK_TRANSACTION);
return;
}
$schema = $this->db->getSchema();
if ($schema->supportsSavepoint()) {
Yii::debug('Roll back to savepoint ' . $this->_level, __METHOD__);
$schema->rollBackSavepoint('LEVEL' . $this->_level);
} else {
Yii::info('Transaction not rolled back: nested transaction not supported', __METHOD__);
// throw an exception to fail the outer transaction
throw new Exception('Roll back failed: nested transaction not supported.');
}
}
3) Thinkphp5
Thinkphp5的事务操作并没有独立分开，直接在数据库连接类当中think\db\Connection;
public function startTrans()
{
$this->initConnect(true);
if (!$this->linkID) {
return false;
}
++$this->transTimes;
if (1 == $this->transTimes) {
$this->linkID->beginTransaction();
} elseif ($this->transTimes > 1 && $this->supportSavepoint()) {
$this->linkID->exec(
$this->parseSavepoint('trans' . $this->transTimes)
);
}
}
public function commit()
{
$this->initConnect(true);
if (1 == $this->transTimes) {
$this->linkID->commit();
}
--$this->transTimes;
}
public function rollback()
{
$this->initConnect(true);
if (1 == $this->transTimes) {
$this->linkID->rollBack();
} elseif ($this->transTimes > 1 && $this->supportSavepoint()) {
$this->linkID->exec(
$this->parseSavepointRollBack('trans' . $this->transTimes)
);
}
$this->transTimes = max(0, $this->transTimes - 1);
}
三个框架都是通过计数器以及数据库本身的”部分事务”支持嵌套事务的操作。MYSQL 中通过 savepoint 的方式来实现只提交事务的一部分。操作流程大体分一下三步 1) 开启事务，检查计数器是否是第一次开启，如果是则执行pdo开启事务，不是则修改计数器的值，同时根据是否支持部分事务，执行pdo savepoint操作。 2) 事务提交，检查计数器是否是最外层事务，是则执行pdo事务提交操作，否则计数器减1 3) 事务回滚，检查计算器是否是最外层操作，是则执行pdo事务回滚，否则计数器减1，同时根据是否支持部分事务，执行pdo rollbak to savepoint 操作
虽然整体思路一样，但是三个框架根据自身的特定，代码设计抽象程度不一样。从这个相同的功能，也能够很好的体会三个框架不同的设计方式。
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