面试-mysql篇-mysql锁解决高并发

利用Mysql的锁来解决高并发的问题，先看没有利用事务的时候并发的后果

创建库存管理表

CREATE TABLE `storage` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `number` int(11) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1

创建订单管理表

CREATE TABLE `order` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `number` int(11) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=34 DEFAULT CHARSET=latin1

测试代码

$pdo = new PDO('mysql:host=127.0.0.1;port=3306; dbname=test','root','123456');
$sql="select `number` from storage where id=1 limit 1";
$res = $pdo->query($sql)->fetch();
$number = $res['number'];

if($number>0)
{
  $sql ="insert into `order` VALUES (null,$number)";

  $order_id = $pdo->query($sql);
  if($order_id)
  {

    $sql="update storage set `number`=`number`-1 WHERE id=1";
    $pdo->query($sql);
  }
}

我们预置库存是十个，然后执行ab测试查看结果

mysql> select * from storage
  -> ;
+----+--------+
| id | number |
+----+--------+
| 1 |   -2 |
+----+--------+
1 row in set (0.00 sec)

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 22 |   10 |
| 23 |   10 |
| 24 |   8 |
| 25 |   8 |
| 26 |   7 |
| 27 |   6 |
| 28 |   4 |
| 29 |   3 |
| 30 |   2 |
| 31 |   2 |
| 32 |   2 |
| 33 |   1 |
+----+--------+
12 rows in set (0.00 sec)

得到了订单共有12个，而库存表的库存也减到了-2，这显然不符合实际逻辑的;

下面我们来看利用数据库行锁来解决这个问题

修改代码如下

$pdo = new PDO('mysql:host=127.0.0.1;port=3306; dbname=test','root','123456');
$pdo->beginTransaction();//开启事务
$sql="select `number` from storage where id=1 for UPDATE ";//利用for update 开启行锁
$res = $pdo->query($sql)->fetch();
$number = $res['number'];

if($number>0)
{
  $sql ="insert into `order` VALUES (null,$number)";

  $order_id = $pdo->query($sql);
  if($order_id)
  {

    $sql="update storage set `number`=`number`-1 WHERE id=1";
    if($pdo->query($sql))
    {
      $pdo->commit();//提交事务
    }
    else
    {
      $pdo->rollBack();//回滚
    }

  }
  else
  {
    $pdo->rollBack();//回滚
  }
}

查看结果

mysql> select * from storage;
+----+--------+
| id | number |
+----+--------+
| 1 |   0 |
+----+------
--+
1 row in set (0.00 sec)

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 1 |   10 |
| 2 |   9 |
| 3 |   8 |
| 4 |   7 |
| 5 |   6 |
| 6 |   5 |
| 7 |   4 |
| 8 |   3 |
| 9 |   2 |
| 10 |   1 |
+----+--------+
10 rows in set (0.00 sec)

很明显在利用了mysql锁之后，对库存进行了有效的控制，很好的解决了第一段代码里面，因为并发引起的一些逻辑性的问题。