Table of Contents
1. Mall rush sale, Flash killing oversold inventory is a headache. Here are three methods to prevent oversold
1.2.mysql optimistic lock
1.3.redis atomic operation
2.Test
3. Question
Home Backend Development PHP Tutorial Flash sales in the mall to prevent oversold inventory

Flash sales in the mall to prevent oversold inventory

Apr 04, 2017 pm 02:45 PM

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1. Mall rush sale, Flash killing oversold inventory is a headache. Here are three methods to prevent oversold

1.mysql锁机制,悲观锁InnoDB行级锁方案,不建议使用,对数据库压力较大,如果出现死锁会导致一直不能更新,除非kill掉进程
2.mysql乐观锁 不使用第三方情况下可以使用此方案
3.redis incrby decrby原子性操作,防止超卖
4.为方便扩展,把库存类抽象出接口,方便以后扩展,也可以使用其它方式实现
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1.1.mysql lock mechanism, pessimistic lock, InnoDB row-level lock scheme, queryneed to useIndex

1.事务级别必须为 SERIALIZABLE 级别
2.查询条件验证库存是否够本次购买,例: id = 1 AND inventory >=1
3.PDO update更新后,不但要验证返回状态是否为!==false,并且同时验证影响行数是否大于0
4.数据库链接一定要使用同一链接,单例或DB链接传入,建议使用单例,由于测试网上找了个db类,没有实现单例,所以使用比较笨的方法,传递数据库链接
5.update件件增加验证购买数量条件 AND inventory >=1
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1.2.mysql optimistic lock

数据库表增加版本字段如version,每次修改时版本号+1
如果更新操作顺序执行,则数据的版本(version)依次递增,不会产生冲突。但是如果发生有不同的业务操作对同一版本的数据进行修改,那么,先提交的操作(图中B)会把数据version更新为2,当A在B之后提交更新时发现数据的version已经被修改了,那么A的更新操作会失败。
PDO update更新后,不但要验证返回状态是否为true,并且同时验证影响行数是否大于0
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1.3.redis atomic operation

incr/decr原子性操作,incr增加,decr减少
//此处不可以取出后放入php变量判断库存,否则会出现幻读,导致超卖
if ($redis_client->decrby($key, $parmas['num']) > -1) {
        //减库存
        $goods = new Goods();
        $parmas['version'] = 1;
        return $goods->subInventory($parmas);
} else {
    //购买多个时,如库存不足,需要把数量加回去,否则会出现库减库存,商品并没有卖出去
    $redis_client->incrby($key, $parmas['num']);
    return false;
}
使用redis测试时,每次修改完库存需要删除KEY:DEL goods_1
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2.Test

配置使用方式
//db 悲观锁(事务级别SERIALIZABLE) db2 mysql乐观锁 redis redis方式
define('INVENTORY_TYPE', 'db2');

并发小时可能不会有问题,如查并发较大会有超卖现像,为了可以重现超卖下面代码加入2秒迟时
services/inventory/db/Inventory.php 31行左右
$data = $goods->getInventory($params);
sleep(2); //停2秒,方便测试出问题
nginx配置站点mall.com
本机hosts绑定域名 10.211.55.100 mall.com(10.211.55.100为实际站点的ip)

测试方法使用ab测试,请求100,并发10
ab -n100 -c10 http://mall.com
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3. Question

注意
由于订单流程中,把验证库存和扣减库存放在了下订单前,可以减少下订单数量和开启的事务数,但如果用户在扣减库存后下订单过程中失败,会出现少卖现象
如果对库存要求不高,可以不用考虑,如果对库存要求较高,需要把扣减成功下单失败对应的订单保存到日志中,然后异步处理恢复库存或给用户补单
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