LB Cluster:
四层：
lvs, nginx(stream)，haproxy(mode tcp)
七层：
http: nginx(http, ngx_http_upstream_module), haproxy(mode http), httpd, ats, perlbal, pound...
HAProxy：
http://www.haproxy.org
http://www.haproxy.com
文档：
http://cbonte.github.io/haproxy-dconv/
HAProxy is a TCP/HTTP reverse proxy which is particularly suited for high availability environments. Indeed, it can:
: - route HTTP requests depending on statically assigned cookies
: - spread load among several servers while assuring server persistence
: through the use of HTTP cookies
: - switch to backup servers in the event a main server fails
: - accept connections to special ports dedicated to service monitoring
: - stop accepting connections without breaking existing ones
: - add, modify, and delete HTTP headers in both directions
: - block requests matching particular patterns
: - report detailed status to authenticated users from a URI intercepted by the application
版本：1.4, 1.5, 1.6, 1.7
程序环境：
主程序：/usr/sbin/haproxy
主配置文件：/etc/haproxy/haproxy.cfg
Unit file：/usr/lib/systemd/system/haproxy.service
配置段：
global：全局配置段
进程及安全配置相关的参数
性能调整相关参数
Debug参数
proxies：代理配置段
defaults：为frontend, listen, backend提供默认配置；
fronted：前端，相当于nginx, server {}
backend：后端，相当于nginx, upstream {}
listen：同时拥前端和后端，将前端和后端进行绑定