HAProry 简介

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