抢购、秒杀是如今很常见的一个应用场景，主要需要解决的问题有两个：
1 高并发对数据库产生的压力
2 竞争状态下如何解决库存的正确减少（"超卖"问题）
对于第一个问题，已经很容易想到用缓存来处理抢购，避免直接操作数据库，例如使用Redis。
重点在于第二个问题
常规写法：
查询出对应商品的库存，看是否大于0，然后执行生成订单等操作，但是在判断库存是否大于0处，如果在高并发下就会有问题，导致库存量出现负数
<?php$conn=mysql_connect("localhost","big","123456");if(!$conn){ echo "connect failed"; exit;}mysql_select_db("big",$conn);mysql_query("set names utf8"); $price=10;$user_id=1;$goods_id=1;$sku_id=11;$number=1; //生成唯一订单function build_order_no(){return date('ymd').substr(implode(NULL, array_map('ord', str_split(substr(uniqid(), 7, 13), 1))), 0, 8);}//记录日志function insertLog($event,$type=0){global $conn;$sql="insert into ih_log(event,type) values('$event','$type')";mysql_query($sql,$conn);} //模拟下单操作//库存是否大于0$sql="select number from ih_store where goods_id='$goods_id' and sku_id='$sku_id'";//解锁 此时ih_store数据中goods_id='$goods_id' and sku_id='$sku_id' 的数据被锁住(注3)，其它事务必须等待此次事务 提交后才能执行$rs=mysql_query($sql,$conn);$row=mysql_fetch_assoc($rs);if($row['number']>0){//高并发下会导致超卖$order_sn=build_order_no();//生成订单$sql="insert into ih_order(order_sn,user_id,goods_id,sku_id,price) values('$order_sn','$user_id','$goods_id','$sku_id','$price')";$order_rs=mysql_query($sql,$conn);//库存减少$sql="update ih_store set number=number-{$number} where sku_id='$sku_id'";$store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){insertLog('库存减少成功');}else{insertLog('库存减少失败');}}else{insertLog('库存不够');}?>
优化方案1：将库存字段number字段设为unsigned，当库存为0时，因为字段不能为负数，将会返回false
//库存减少$sql="update ih_store set number=number-{$number} where sku_id='$sku_id' and number>0";$store_rs=mysql_query($sql,$conn);if(mysql_affected_rows()){insertLog('库存减少成功');}
优化方案2：使用mysql的事务，锁住操作的行
<?php$conn=mysql_connect("localhost","big","123456");if(!$conn){ echo "connect failed"; exit;}mysql_select_db("big",$conn);mysql_query("set names utf8"); $price=10;$user_id=1;$goods_id=1;$sku_id=11;$number=1; //生成唯一订单号function build_order_no(){return date('ymd').substr(implode(NULL, array_map('ord', str_split(substr(uniqid(), 7, 13), 1))), 0, 8);}//记录日志function insertLog($event,$type=0){global $conn;$sql="insert into ih_log(event,type) values('$event','$type')";mysql_query($sql,$conn);} //模拟下单操作//库存是否大于0mysql_query("BEGIN"); //开始事务$sql="select number from ih_store where goods_id='$goods_id' and sku_id='$sku_id' FOR UPDATE";//此时这条记录被锁住,其它事务必须等待此次事务提交后才能执行$rs=mysql_query($sql,$conn);$row=mysql_fetch_assoc($rs);if($row['number']>0){//生成订单$order_sn=build_order_no();$sql="insert into ih_order(order_sn,user_id,goods_id,sku_id,price) values('$order_sn','$user_id','$goods_id','$sku_id','$price')";$order_rs=mysql_query($sql,$conn);//库存减少$sql="update ih_store set number=number-{$number} where sku_id='$sku_id'";$store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){insertLog('库存减少成功');mysql_query("COMMIT");//事务提交即解锁}else{insertLog('库存减少失败');}}else{insertLog('库存不够');mysql_query("ROLLBACK");}?>
优化方案3：使用非阻塞的文件排他锁
<?php$conn=mysql_connect("localhost","root","123456");if(!$conn){ echo "connect failed"; exit;}mysql_select_db("big-bak",$conn);mysql_query("set names utf8"); $price=10;$user_id=1;$goods_id=1;$sku_id=11;$number=1; //生成唯一订单号function build_order_no(){return date('ymd').substr(implode(NULL, array_map('ord', str_split(substr(uniqid(), 7, 13), 1))), 0, 8);}//记录日志function insertLog($event,$type=0){global $conn;$sql="insert into ih_log(event,type) values('$event','$type')";mysql_query($sql,$conn);} $fp = fopen("lock.txt", "w+");if(!flock($fp,LOCK_EX | LOCK_NB)){echo "系统繁忙，请稍后再试";return;}//下单$sql="select number from ih_store where goods_id='$goods_id' and sku_id='$sku_id'";$rs=mysql_query($sql,$conn);$row=mysql_fetch_assoc($rs);if($row['number']>0){//库存是否大于0//模拟下单操作$order_sn=build_order_no();$sql="insert into ih_order(order_sn,user_id,goods_id,sku_id,price) values('$order_sn','$user_id','$goods_id','$sku_id','$price')";$order_rs=mysql_query($sql,$conn);//库存减少$sql="update ih_store set number=number-{$number} where sku_id='$sku_id'";$store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){insertLog('库存减少成功');flock($fp,LOCK_UN);//释放锁}else{insertLog('库存减少失败');}}else{insertLog('库存不够');}fclose($fp);
优化方案4：使用redis队列，因为pop操作是原子的，即使有很多用户同时到达，也是依次执行，推荐使用（mysql事务在高并发下性能下降很厉害，文件锁的方式也是）
先将商品库存如队列
<?php$store=1000;$redis=new Redis();$result=$redis->connect('127.0.0.1',6379);$res=$redis->llen('goods_store');echo $res;$count=$store-$res;for($i=0;$i<$count;$i++){$redis->lpush('goods_store',1);}echo $redis->llen('goods_store');?>
抢购、描述逻辑
<?php$conn=mysql_connect("localhost","big","123456");if(!$conn){ echo "connect failed"; exit;}mysql_select_db("big",$conn);mysql_query("set names utf8"); $price=10;$user_id=1;$goods_id=1;$sku_id=11;$number=1; //生成唯一订单号function build_order_no(){return date('ymd').substr(implode(NULL, array_map('ord', str_split(substr(uniqid(), 7, 13), 1))), 0, 8);}//记录日志function insertLog($event,$type=0){global $conn;$sql="insert into ih_log(event,type) values('$event','$type')";mysql_query($sql,$conn);} //模拟下单操作//下单前判断redis队列库存量$redis=new Redis();$result=$redis->connect('127.0.0.1',6379);$count=$redis->lpop('goods_store');if(!$count){insertLog('error:no store redis');return;} //生成订单$order_sn=build_order_no();$sql="insert into ih_order(order_sn,user_id,goods_id,sku_id,price)values('$order_sn','$user_id','$goods_id','$sku_id','$price')";$order_rs=mysql_query($sql,$conn); //库存减少$sql="update ih_store set number=number-{$number} where sku_id='$sku_id'";$store_rs=mysql_query($sql,$conn);if(mysql_affected_rows()){insertLog('库存减少成功');}else{insertLog('库存减少失败');}
模拟5000高并发测试
webbench -c 5000 -t 60 http://192.168.1.198/big/index.php
ab -r -n 6000 -c 5000 http://192.168.1.198/big/index.php
上述只是简单模拟高并发下的抢购，真实场景要比这复杂很多，很多注意的地方
如抢购页面做成静态的，通过ajax调用接口
再如上面的会导致一个用户抢多个，思路：
需要一个排队队列和抢购结果队列及库存队列。高并发情况，先将用户进入排队队列，用一个线程循环处理从排队队列取出一个用户，判断用户是否已在抢购结果队列，如果在，则已抢购，否则未抢购，库存减1，写数据库，将用户入结果队列。
测试数据表
---- 数据库: `big`-- -- -------------------------------------------------------- ---- 表的结构 `ih_goods`-- CREATE TABLE IF NOT EXISTS `ih_goods` (`goods_id` int(10) unsigned NOT NULL AUTO_INCREMENT,`cat_id` int(11) NOT NULL,`goods_name` varchar(255) NOT NULL,PRIMARY KEY (`goods_id`)) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=2 ; ---- 转存表中的数据 `ih_goods`-- INSERT INTO `ih_goods` (`goods_id`, `cat_id`, `goods_name`) VALUES(1, 0, '小米手机'); -- -------------------------------------------------------- ---- 表的结构 `ih_log`-- CREATE TABLE IF NOT EXISTS `ih_log` (`id` int(11) NOT NULL AUTO_INCREMENT,`event` varchar(255) NOT NULL,`type` tinyint(4) NOT NULL DEFAULT '0',`addtime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,PRIMARY KEY (`id`)) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ; ---- 转存表中的数据 `ih_log`-- -- -------------------------------------------------------- ---- 表的结构 `ih_order`-- CREATE TABLE IF NOT EXISTS `ih_order` (`id` int(11) NOT NULL AUTO_INCREMENT,`order_sn` char(32) NOT NULL,`user_id` int(11) NOT NULL,`status` int(11) NOT NULL DEFAULT '0',`goods_id` int(11) NOT NULL DEFAULT '0',`sku_id` int(11) NOT NULL DEFAULT '0',`price` float NOT NULL,`addtime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='订单表' AUTO_INCREMENT=1 ; ---- 转存表中的数据 `ih_order`-- -- -------------------------------------------------------- ---- 表的结构 `ih_store`-- CREATE TABLE IF NOT EXISTS `ih_store` (`id` int(11) NOT NULL AUTO_INCREMENT,`goods_id` int(11) NOT NULL,`sku_id` int(10) unsigned NOT NULL DEFAULT '0',`number` int(10) NOT NULL DEFAULT '0',`freez` int(11) NOT NULL DEFAULT '0' COMMENT '虚拟库存',PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='库存' AUTO_INCREMENT=2 ; ---- 转存表中的数据 `ih_store`-- INSERT INTO `ih_store` (`id`, `goods_id`, `sku_id`, `number`, `freez`) VALUES(1, 1, 11, 500, 0);
