引言

前面一直讲各种具体的负载均衡软件（LVS，HAProxy, Nginx等），它们一般同时来实现集群的高可用，
但这个负载均衡软件本身还存在单点故障，所以我们也要实现负载均衡软件的高可用，一般我们使用 Keepalived 来实现的。

Keepalived 简介

What is Keepalived ?

Keepalived is a routing software written in C. 
The main goal of this project is to provide simple and robust facilities for loadbalancing and 
high-availability to Linux system and Linux based infrastructures. 
Loadbalancing framework relies on well-known and widely used Linux Virtual Server (IPVS) kernel module 
providing Layer4 loadbalancing. 
Keepalived implements a set of checkers to dynamically and adaptively maintain and 
manage loadbalanced server pool according their health. 
On the other hand high-availability is achieved by VRRP protocol. 
VRRP is a fundamental brick for router failover. 
In addition, Keepalived implements a set of hooks to the VRRP finite state machine 
providing low-level and high-speed protocol interactions. 
Keepalived frameworks can be used independently or all together to provide resilient infrastructures.

从这段描述中，我们可以得到几个有用的信息：

1、 Keepalived 是 LVS 的扩展项目，因此它们之间具备良好的兼容性。这点应该是 Keepalived 部署比其他类似工具能更简洁的原因吧！
2、 通过对服务器池对象的健康检查，实现对失效机器/服务的故障隔离。
3、 负载均衡器之间的失败切换 failover，是通过 VRRPv2（Virtual Router Redundancy Protocol）stack 实现的。

几个推荐的学习资料参考 Ref。

Ref

Keepalived 官网
LVS + Keepalived Chinese application doc
Keepalived 权威指南