抢购、秒杀是现在很常见的一个利用场景，次要须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缩小（"超卖"问题）
对于第一个问题，曾经很容易想到用缓存来解决抢购，防止间接操作数据库，例如应用Redis。
重点在于第二个问题
惯例写法：
查问出对应商品的库存，看是否大于0，而后执行生成订单等操作，然而在判断库存是否大于0处，如果在高并发下就会有问题，导致库存量呈现正数
<?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;//生成惟一订单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,$conn); }//模仿下单操作//库存是否大于0$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)，其它事务必须期待此次事务 提交后能力执行$rs=mysql_query($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); //库存缩小 $sql="update ih_store set number=number-{$number} where sku_id='$sku_id'"; $store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){ insertLog('库存缩小胜利'); }else{ insertLog('库存缩小失败'); } }else{ insertLog('库存不够');}?>计划1：将库存字段number字段设为unsigned，当库存为0时，因为字段不能为正数，将会返回false
//库存缩小$sql="update ih_store set number=number-{$number} where sku_id='$sku_id' and number>0";$store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){ insertLog('库存缩小胜利');}计划2：应用mysql的事务，锁住操作的行
<?php$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;$number=1;//生成惟一订单号function build_order_no(){ return date('ymd').substr(implode(NULL, array_map('ord', str_split(substr(uniqid(), 7, 13), 1))), 0, 8);}//记录日志function insertLog($event,$type=0){ global $conn; $sql="insert into ih_log(event,type) values('$event','$type')"; mysql_query($sql,$conn); }//模仿下单操作//库存是否大于0mysql_query("BEGIN"); //开始事务$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,其它事务必须期待此次事务提交后能力执行$rs=mysql_query($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); //库存缩小 $sql="update ih_store set number=number-{$number} where sku_id='$sku_id'"; $store_rs=mysql_query($sql,$conn); if(mysql_affected_rows()){ insertLog('库存缩小胜利'); mysql_query("COMMIT");//事务提交即解锁 }else{ insertLog('库存缩小失败'); }}else{ insertLog('库存不够'); mysql_query("ROLLBACK");}?>