ip_hash

ip_hash 可以保证用户访问可以请求到上游服务中的固定的服务器，前提是用户ip没有发生更改。
使用ip_hash的注意点：
不能把后台服务器直接移除，只能标记down.

If one of the servers needs to be temporarily removed, it should be marked with the down parameter in order to preserve the current hashing of client IP addresses.

upstream tomcats {
        ip_hash;
        server 192.168.1.173:8080;
        server 192.168.1.174:8080 down;
        server 192.168.1.175:8080;
}

参考：
http://nginx.org/en/docs/http/ngx_http_upstream_module.html#ip_hash

url_hash

根据每次请求的url地址，hash后访问到固定的服务器节点

upstream tomcats {
    # url hash
    hash $request_uri;
    # 最少连接数
    # least_conn
    server 192.168.1.173:8080;
    server 192.168.1.174:8080;
    server 192.168.1.175:8080;
}
server {
    listen 80;
    server_name www.tomcats.com;
    location / {
        proxy_pass  http://tomcats;
    }
}

least_conn

upstream tomcats {
    # url hash
    # hash $request_uri;
    # 最少连接数
    least_conn
    server 192.168.1.173:8080;
    server 192.168.1.174:8080;
    server 192.168.1.175:8080;
}
server {
    listen 80;
    server_name www.tomcats.com;
    location / {
        proxy_pass  http://tomcats;
    }
}