Flash sales in the mall to prevent oversold inventory
PHP-MALL
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
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
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
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
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
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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