How to use Redis lock to solve high concurrency problems

The problem of high concurrency is a problem we often encounter, so how to solve the problem of high concurrency? This article introduces the use of Redis locks to solve high concurrency problems. Let’s take a look.

Here we mainly use the setnx command of Redis to handle high concurrency.

setnx has two parameters. The first parameter represents the key. The second parameter represents the value. If the current key does not exist, the current key will be inserted, using the second parameter as the value. Return 1. If the current key exists, 0 will be returned.

Create inventory table

CREATE TABLE `storage` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `number` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1

Set the initial inventory to 10

Create order table

CREATE TABLE `order` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `number` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1

When testing without locks

$pdo = new PDO('mysql:host=127.0.0.1;dbname=test', 'root', 'root');
$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);
    }
}

ab The test simulated concurrency and found that the inventory was correct.

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

Looking at the order table

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

I found that there are several orders operating on the same inventory data, which may cause oversold conditions.

Modify the code and add redis lock for data control

<?php /**
 * Created by PhpStorm.
 * User: daisc
 * Date: 2018/7/23
 * Time: 14:45
 */
class Lock
{
    private static $_instance ;
    private   $_redis;
    private function __construct()
    {
        $this->_redis =  new Redis();
        $this->_redis ->connect('127.0.0.1');
    }
    public static function getInstance()
    {
        if(self::$_instance instanceof self)
        {
            return self::$_instance;
        }
        return self::$_instance = new  self();
    }

    /**
     * @function 加锁
     * @param $key 锁名称
     * @param $expTime 过期时间
      */
    public function set($key,$expTime)
    {
        //初步加锁
        $isLock = $this->_redis->setnx($key,time()+$expTime);
        if($isLock)
        {
            return true;
        }
        else
        {
            //加锁失败的情况下。判断锁是否已经存在，如果锁存在切已经过期，那么删除锁。进行重新加锁
            $val = $this->_redis->get($key);
            if($val&&$val<time>del($key);
            }
            return  $this->_redis->setnx($key,time()+$expTime);
        }
    }


    /**
     * @param $key 解锁
     */
    public function del($key)
    {
        $this->_redis->del($key);
    }

}



$pdo = new PDO('mysql:host=127.0.0.1;dbname=test', 'root', 'root');
$lockObj = Lock::getInstance();
//判断是能加锁成功
if($lock = $lockObj->set('storage',10))
{
    $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);
        }
    }
    //解锁
    $lockObj->del('storage');

}
else
{
    //加锁不成功执行其他操作。
}</time>

Carry out the ab test again and check the test results

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)

It is found that the order table does not operate the same inventory The situation of the data. Therefore, using redis lock can effectively handle high concurrency.

There is actually no need to judge the expiration time when locking. In order to avoid deadlock, we add an expiration time judgment. Actively delete the lock when it expires.

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