并发 是指在一个时间段内有多个进程在执行。
什么是高并发
定义：高并发(High Concurrency)是使用技术手段使系统可以并行处理很多请求。
关键指标：-响应时间(Response Time)-吞吐量(Throughput)-每秒查询率QPS(Query Per Second) -每秒事务处理量TPS(Transaction Per Second)-同时在线用户数量 如何应对高并发？
高并发意味着单位时间内系统能处理的请求数很高，也就是说系统所能承载的HTTP请求很多，那要应对高并发，就要从HTTP请求处理层面开始，如下是我理解的一个完整的HTTP请求所经历的流程：
1、DNS域名解析将请求域名解析为IP地址。
2、与IP地址对应的服务器网卡建立连接，TCP的三次握手，连接建立并占用
3、服务器操作系统通过连接读取和处理请求
3.1 从连接中读取字节流（IO密集）
3.2 将读取到的字节流转换成HTTP请求（CPU密集）
4、服务器操作系统将HTTP请求转发给WEB Server或者Application Server
4.1 Application在开发逻辑架构中一般会分层，分为表现层、业务层和持久层
5、Application进行业务逻辑处理并准备响应Response
6、Response准备完成，Response通过网卡回写到用户的浏览器（IO密集）
7、TCP连接三次挥手，断开连接
从这个过程中，我们对一个HTTP的请求能有一个感性的认识，基于这个感性的认识，我们能知道这里面几个关键的点：域名解析、连接处理、系统分层，从这几个关键点其实就可以提取出一些应对方法。 请求示例
- [Curl]
- [PHP]
- [PHPsdk]
- [JAVA]
- [C#]
- [Python]
-- 请求示例 url 默认请求参数已经URL编码处理
curl -i "https://api-gw.onebound.cn/taobao/item_get/?key=<您自己的apiKey>&secret=<您自己的apiSecret>&num_iid=652874751412&is_promotion=1"响应示例
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"desc": "<p style="text-align:center;"><span style="color:#ff0000;"><span style="font-size:24.0px;"><strong>小店所有产品都支持刻字，如需刻字，拍之前联系客服即可。</strong></span></span></p> <p style="text-align:center;"><span style="font-size:18.0px;"><strong>炎炎夏日来临，一把随身携带便携式的折叠小刀，<span style="color:#ff0000;">带开瓶器功能</span>，喝酒不用愁。（钥匙刀不带开瓶器功能）</strong></span></p> <p style="text-align:center;"><strong style="color:#000000;line-height:1.5;"><span style="font-size:18.0px;"><span style="line-height:18.0px;">可以当吊牌项链装饰，也可当钥匙扣挂饰， 水果刀 开瓶器 户外防身.</span></span></strong></p> <p style="text-align:center;"><strong style="line-height:1.5;"><span style="font-size:24.0px;">部分客户跟我们反映链子质量还不够好，因此小店已重新订购一批质量更好的链子赠送，<span style="color:#ff0000;">加量不加价</span>，只为让你买的更舒心，戴的更放心。</span></strong></p> <p style="text-align:center;"><span style="font-size:36.0px;"><strong>购买就送<span style="color:#ff0000;">工具刀卡和链子</span>一条</strong></span></p> <p style="text-align:center;"><span style="font-size:36.0px;"><strong><img align="absmiddle" src="//img.alicdn.com/imgextra/i3/2596264565/TB2Sci2jXXXXXXFXpXXXXXXXXXX_!!2596264565.png" size="611x442"> </strong></span></p> <p style="text-align:center;"><span style="font-size:18.0px;"><strong>璀璨钻石套餐包含【SK016D钥匙扣+GJ019C折叠刀+GJ017D工具卡】</strong></span><img align="absmiddle" src="//img.alicdn.com/imgextra/i3/2596264565/TB2wWohmXXXXXX8XXXXXXXXXXXX_!!2596264565.jpg" size="750x600" /></p> <p style="text-align:center;"><span style="font-size:18.0px;"><strong>超凡大师套餐包括【304不锈钢钥匙扣+GJ019C折叠刀+GJ017D工具卡】</strong></span><a href="//item.taobao.com/item.htm?id=520798805877" target="_blank"><img align="absmiddle" src="//img.alicdn.com/imgextra/i4/2596264565/TB2_uiXnFXXXXXBXXXXXXXXXXXX_!!2596264565.jpg_q90.jpg" style="float:none;margin:0.0px;" /></a></p> <p style="text-align:center;"> </p> <p style="text-align:center;"><span style="font-size:18.0px;"><strong>最强王者套餐包括【钛钢钥匙扣+GJ019C折叠刀+GJ017D工具卡】<a href="//item.taobao.com/item.htm?id=520801586805" target="_blank"><img align="absmiddle" 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