Nginx负载均衡(Load Balance，简称LB)是一种服务器或网络设备的集群技术。负载均衡将特定的业务(网络服务、网络流量等)分担给多个服务器或网络设备，从而提高了业务处理能力，保证了业务的高可用性。

Nginx负载均衡示意图：

Nginx负载均衡原理就是将所有客户端的请求通过proxy_pass代理转发到对应的一组后端服务器upstream server上。

配置语法：

Syntax：upstream name {...}

Default：--

Context：http

配置实例：

后端服务器组upstream server

#server1
server {
    listen 8001;
    server_name localhost;
    access_log /var/logs/nginx/access.log main;

    location / {
        root /opt/app/code;
        index index.html index.htm index.php;
    }
    ...
}

#server2
server {
    listen 8002;
    server_name localhost;
    access_log /var/logs/nginx/access.log main;

    location / {
        root /opt/app/code;
        index index.html index.htm index.php;
    }
    ...
}

#server3
server {
    listen 8003;
    server_name localhost;
    access_log /var/logs/nginx/access.log main;

    location / {
        root /opt/app/code;
        index index.html index.htm index.php;
    }
}

负载均衡服务器main server

#默认使用轮询机制
upstream test {
    server 114.249.225.223:8001 down;
    server 114.249.225.223:8002 backup;
    server 114.249.225.223:8003 max_fails=1 fail_timeout=30s; // 允许失败一次，超过30s则直接访问8002
}
#加权
upstream test {
    server 114.249.225.223:8001;
    server 114.249.225.223:8002;
    server 114.249.225.223:8003 weight=5;
}
#ip_hash
upstream test {
    ip_hash;
    server 114.249.225.223:8001;
    server 114.249.225.223:8002;
    server 114.249.225.223:8003;
}
#url_hash
upstream test {
    hash $request_uri;
    server 114.249.225.223:8001;
    server 114.249.225.223:8002;
    server 114.249.225.223:8003;
}
server {
    listen 80;
    server_name localhost www.mantis.me;

    access_log /var/logs/nginx/access.log main;

    location / {
        proxy_pass http://test;
        include proxy_params;
    }
    ...
}

后端服务器在负载均衡调度中的状态：

调度算法：