Nginx:--反向代理之（负载均衡-upstream模块）
一、复杂均衡概述
负载均衡也是Nginx常用的一个功能，负载均衡其意思就是分摊到多个操作单元上进行执行，例如Web服务器、FTP 服务器、企业关键应用服务器和其它关键任务服务器等，从而共同完成工作任务
简单而言就是当有2台或以上服务器时，根据规则随机的将请求分发到 指定的服务器上处理，负载均衡配置一般都需要同时配置反向代理，通过反向代理跳转到负载均衡
二、upstream模块
在前一篇文章中我们介绍了反向代理，其中使用了proxy_pass指令指定了反向代理，但是proxy_pass只能指定一个反向代理服务器
而upstream模块会启用一个新的配置区段，在该区段定义了一组上游服务器
这些服务器可以被设置不同的权重（权重越高的上游服务器会被NgINX传递更多的连接）
这些服务器也可以是不同的类型（TCP、UNIX域）
也可能处于对服务器进行维护的原因，其中某些被标记为down
演示案例：下面的location块，其代理了三台服务器
http {
//上游服务器，其中有三台
upstream app {
server 127.0.0.1:9000;
server 127.0.0.1:9001;
server 127.0.0.1:9002;
}
server {
location / {
//反向代理三台上游服务器
proxy_pass http://app;
}
}
}
三、负载均衡的实现策略
Nginx目前支持6种负载均衡策略：
RR（Nginx自带的）
权重（Nginx自带的）
ip_hash（Nginx自带的）
least_conn（Nginx自带的）
fair（第三方策略）
url_hash（第三方策略）
6种实现策略适用情况不同，其中fair和url_hash策略需要安装第三方模块才能使用
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①轮询（RR，默认策略）
?概念：每个请求按时间顺序逐一分配到不同的后端服务器
?默认模式的特点： ?如果某个后端服务器down掉，会自动剔除