# 如何设计一个秒杀系统
## 系统的特点
- 高性能:秒杀涉及大量的并发读和并发写,因此支持高并发访问这点非常关键
- 一致性:秒杀商品减库存的实现方式同样关键,有限数量的商品在同一时刻被很多倍的请求同时来减库存,在大并发更新的过程中都要保证数据的准确性。
- 高可用:秒杀时会在一瞬间涌入大量的流量,为了避免系统宕机,保证高可用,需要做好流量限制
## 优化思路
- 后端优化:将请求尽量拦截在系统上游
- 限流:屏蔽掉无用的流量,允许少部分流量走后端。假设现在库存为 10,有 1000 个购买请求,最终只有 10 个可以成功,99% 的请求都是无效请求
- 削峰:秒杀请求在时间上高度集中于某一个时间点,瞬时流量容易压垮系统,因此需要对流量进行削峰处理,缓冲瞬时流量,尽量让服务器对资源进行平缓处理
- 异步:将同步请求转换为异步请求,来提高并发量,本质也是削峰处理
- 利用缓存:创建订单时,每次都需要先查询判断库存,只有少部分成功的请求才会创建订单,因此可以将商品信息放在缓存中,减少数据库查询
- 负载均衡:利用 Nginx 等使用多个服务器并发处理请求,减少单个服务器压力
- 前端优化:
- 限流:前端答题或验证码,来分散用户的请求
- 禁止重复提交:限定每个用户发起一次秒杀后,需等待才可以发起另一次请求,从而减少用户的重复请求
- 本地标记:用户成功秒杀到商品后,将提交按钮置灰,禁止用户再次提交请求
- 动静分离:将前端静态数据直接缓存到离用户最近的地方,比如用户浏览器、CDN 或者服务端的缓存中
- 防作弊优化:
- 隐藏秒杀接口:如果秒杀地址直接暴露,在秒杀开始前可能会被恶意用户来刷接口,因此需要在没到秒杀开始时间不能获取秒杀接口,只有秒杀开始了,才返回秒杀地址 url 和验证 MD5,用户拿到这两个数据才可以进行秒杀
- 同一个账号多次发出请求:在前端优化的禁止重复提交可以进行优化;也可以使用 Redis 标志位,每个用户的所有请求都尝试在 Redis 中插入一个 `userId_secondsKill` 标志位,成功插入的才可以执行后续的秒杀逻辑,其他被过滤掉,执行完秒杀逻辑后,删除标志位
- 多个账号一次性发出多个请求:一般这种请求都来自同一个 IP 地址,可以检测 IP 的请求频率,如果过于频繁则弹出一个验证码
- 多个账号不同 IP 发起不同请求:这种一般都是僵尸账号,检测账号的活跃度或者等级等信息,来进行限制。比如微博抽奖,用 iphone 的年轻女性用户中奖几率更大。通过用户画像限制僵尸号无法参与秒杀或秒杀不能成功
## 代码优化
1. 将 SSM 换成 SpringBoot,开箱即用,替换 Mapper XML 为注解,去掉 Dubbo 和 Zookeeper
2. 原项目中依赖了开发者自己的开源包 [distributed-redis-tool](<https://github.com/crossoverJie/distributed-redis-tool>),本项目将用到的限流部分直接集成到代码中
3. 加入缓存预热,在秒杀开始前,将库存信息读到缓存中,并暴露数据库和缓存重置方法便于服务器部署压测
4. 缓存更新逻辑中加入 Redis 事务,避免脏数据
5. 将 Kafka-client 替换为 spring-kafka,自动配置,通过 KafkaTemplate 和 Listen 进行消息的生产和消费,采用 Gson 进行 Kafka 消息序列化和反序列化,精简大量代码
### Jmeter 压测
![](<https://github.com/gongfukangEE/gongfukangEE.github.io/raw/master/_pic/%E5%88%86%E5%B8%83%E5%BC%8F/Jmeter%20%E5%8E%8B%E5%8A%9B%E6%B5%8B%E8%AF%95%20TPS.png>)
**测试流程如下:**
首先下载 JMeter 安装包 可以去官网下载:http://jmeter.apache.org
windows 环境下载 zip 安装包,然后将下载的文件进行解压,进入 bin 目录运行 jmeter.bat 即可。
接下来是 Jmeter 测试计划设置:
(1)在测试计划上右键新建一个线程组
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/1.png>)
线程组属性内可以修改线程数、Ramp-Up 时间和循环次数。
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/2.png>)
(2)在线程组上右键添加 HTTP 请求
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/3.png>)
其属性包括 WEB 服务器的协议、服务器名称或 IP 和端口号,HTTP 请求的方法和路径。
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/4.png>)
(3)在HTTP请求上右键添加一个监听器,可以根据自己的需求添加汇总报告、查看结果树等等。
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/5.png>)
如下图所示为汇总报告,可以查看异常比例和吞吐量,方便调优。
![](<https://github.com/daydreamdev/MeetingFilm/raw/master/pic/seconds-kill/6.png>)
这样一个简单的 Jmeter 测试计划就算添加完了。一个 HTTP 请求对应一个接口,可以添加多个 HTTP 请求 以达到多个接口同时检测的需求。
### 0. 基本秒杀逻辑
```java
@Override
public int createWrongOrder(int sid) throws Exception {
// 数据库校验库存
Stock stock = checkStock(sid);
// 扣库存(无锁)
saleStock(stock);
// 生成订单
int res = createOrder(stock);
return res;
}
private Stock checkStock(int sid) throws Exception {
Stock stock = stockService.getStockById(sid);
if (stock.getCount() < 1) {
throw new RuntimeException("库存不足");
}
return stock;
}
private int saleStock(Stock stock) {
stock.setSale(stock.getSale() + 1);
stock.setCount(stock.getCount() - 1);
return stockService.updateStockById(stock);
}
private int createOrder(Stock stock) throws Exception {
StockOrder order = new StockOrder();
order.setSid(stock.getId());
order.setName(stock.getName());
order.setCreateTime(new Date());
int res = orderMapper.insertSelective(order);
if (res == 0) {
throw new RuntimeException("创建订单失败");
}
return res;
}
// 扣库存 Mapper 文件
@Update("UPDATE stock SET count = #{count, jdbcType = INTEGER}, name = #{name, jdbcType = VARCHAR}, " + "sale = #{sale,jdbcType = INTEGER},version = #{version,jdbcType = INTEGER} " + "WHERE id = #{id, jdbcType = INTEGER}")
```
### 1. 乐观锁更新库存,解决超卖问题
超卖问题出现的场景
悲观锁虽然可以解决超卖问题,但是加锁的时间可能会很长,会长时间的限制其他用户的访问,导致很多请求等待锁,卡死在这里,如果这种请求很多就会耗尽连接,系统出现异常。乐观锁默认不加锁,更失败就直接返回抢购失败,可以承受较高并发
```java
@Override
public int createOptimisticOrder(int sid) throws Exception {
// 校验库存
Stock stock = checkStock(sid);
// 乐观锁更新
saleStockOptimstic(stock);
// 创建订单
int id = createOrder(stock);
return id;
}
// 乐观锁 Mapper 文件
@Update("UPDATE stock SET count = count - 1, sale = sale + 1, version = version + 1 WHERE " +
"id = #{id, jdbcType = INTEGER} AND version = #{version, jdbcType = INTEGER}")
```
### 2. Redis 计数限流
根据前面的优化分析,假设现在有 10 个商品,有 1000 个并发秒杀请求,最终只有 10 个订单会成功创建,也就是说有 990 的请求是无效的,这些无效的请求也会给数据库带来压力,因此可以在在请求落到数据库之前就将无效的请求过滤掉,将并发控制在一个可控的范围,这样落到数据库的压力就小很多
关于限流的方法,可以看这篇博客[浅析限流�
没有合适的资源?快使用搜索试试~ 我知道了~
温馨提示
系统的特点 高性能:秒杀涉及大量的并发读和并发写,因此支持高并发访问这点非常关键 一致性:秒杀商品减库存的实现方式同样关键,有限数量的商品在同一时刻被很多倍的请求同时来减库存,在大并发更新的过程中都要保证数据的准确性。 高可用:秒杀时会在一瞬间涌入大量的流量,为了避免系统宕机,保证高可用,需要做好流量限制 优化思路 后端优化:将请求尽量拦截在系统上游 限流:屏蔽掉无用的流量,允许少部分流量走后端。假设现在库存为 10,有 1000 个购买请求,最终只有 10 个可以成功,99% 的请求都是无效请求 削峰:秒杀请求在时间上高度集中于某一个时间点,瞬时流量容易压垮系统,因此需要对流量进行削峰处理,缓冲瞬时流量,尽量让服务器对资源进行平缓处理 异步:将同步请求转换为异步请求,来提高并发量,本质也是削峰处理 利用缓存:创建订单时,每次都需要先查询判断库存,只有少部分成功的请求才会创建订单,因此可以将商品信息放在缓存中,减少数据库查询 负载均衡:利用 Nginx 等使用多个服务器并发处理请求,减少单个服务器压力 前端优化: 限流:前端答题或验证码,来分散用户的请求 禁止重复提交:限定每个用户发起
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