抢购、秒杀是平常很常见的场景，面试的时候面试官也经常会问到，比如问你淘宝中的抢购秒杀是怎么实现的等等。
抢购、秒杀实现很简单，但是有些问题需要解决，主要针对两个问题：
一、高并发对数据库产生的压力
二、竞争状态下如何解决库存的正确减少（"超卖"问题）
第一个问题，对于PHP来说很简单，用缓存技术就可以缓解数据库压力，比如memcache，redis等缓存技术。
第二个问题就比较复杂点：
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$sq l= "select number from ih_store where goods_id=‘$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,$conn); ???if(mysql_affected_rows()){ ???????insertLog(‘库存减少成功‘); ???}else{ ???????insertLog(‘库存减少失败‘); ???}}else{ ???insertLog(‘库存不够‘);}出现这种情况怎么办呢？来看几种优化方法：
优化方案1：将库存字段number字段设为unsigned，当库存为0时，因为字段不能为负数，将会返回false
//库存减少$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‘);}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。
我之间做商城项目的时候，在秒杀这一块我直接用的redis，这段时间看了看上面的几种方法，虽然各有不同，但是实现目的都一样的，各位自己选择，开心就好。
抢购、秒杀是平常很常见的场景，面试的时候面试官也经常会问到，比如问你淘宝中的抢购秒杀是怎么实现的等等。
抢购、秒杀实现很简单，但是有些问题需要解决，主要针对两个问题：
一、高并发对数据库产生的压力
二、竞争状态下如何解决库存的正确减少（"超卖"问题）
第一个问题，对于PHP来说很简单，用缓存技术就可以缓解数据库压力，比如memcache，redis等缓存技术。
第二个问题就比较复杂点：
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,$conn); ???if(mysql_affected_rows()){ ???????insertLog(‘库存减少成功‘); ???}else{ ???????insertLog(‘库存减少失败‘); ???}}else{ ???insertLog(‘库存不够‘);}出现这种情况怎么办呢？来看几种优化方法：
优化方案1：将库存字段number字段设为unsigned，当库存为0时，因为字段不能为负数，将会返回false
//库存减少$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‘);}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。
我之间做商城项目的时候，在秒杀这一块我直接用的redis，这段时间看了看上面的几种方法，虽然各有不同，但是实现目的都一样的，各位自己选择，开心就好。
PHP和Redis实现在高并发下的抢购及秒杀功能示例详解
原文地址：https://www.cnblogs.com/dawuge/p/10480807.html