Briefly describe the working principle of kept

keepalived The principle is vrrp The principle and lvs Combination of principles

vrrp The software implementation of the protocol , The purpose of native design is high availability ipvs service
Official website ：http://keepalived.org/

function ：

• be based on vrrp The protocol completes the address flow
• by vip The node where the address is located generates ipvs The rules ( Predefined in the configuration file )
• by ipvs Each part of the cluster RS Do health tests （ Check the health by visiting the specified page and returning the results ）
• Call the interface based on the script to complete the functions defined in the script , This in turn affects cluster transactions .
• Support nginx、haproxy Etc

1.VRRP Network layer hardware implementation

Access to the external network within the enterprise is realized through the router , To prevent router problems , So we need two routers , But two routers ip And gateway are different , Virtualize two routers into the same ip（vip） And gateway , Let the gateway of the client point to the new gateway , Only run on one router at a time , Float at different times , To avoid conflict . When one goes wrong ,vip Will immediately float to another machine , Ensure continuous Internet access .
vrrp The working host wants to know the situation of the other host at the first time , Through the establishment of heartbeat line communication , Free for adoption arp The way to inform the other party of their working status , The working host sends free arp,（ You can broadcast , It can also be unicast ）, The purpose is to tell yourself that you have vip, And tell yourself your priorities , The standby node receives the message and compares it with its own priority , Your priority is lower than the working node , I don't work myself , When you can't receive free arp, Or the priority of the working node is lower than itself , Work will start from the node , At this point, get from the node vip Work on .
Firewall strategy can be used to destroy each other's heart jumpers , Because I can't receive free arp, At this point, both hosts will get vip, There will be brain cracking , If there is a brain crack , The first thing is to turn off a host keepalived service , Ensure that the service can work normally , Then find out the reason for maintenance .

2.Keepalived The structure and use of lvs Rules work

Official documents ： https://keepalived.org/doc/

http://keepalived.org/documentation.html

• User space core components ：
  vrrp stack：VIP The announcement
  checkers： monitoring real server
  system call： Realization vrrp The function of calling script during protocol state transition
  SMTP： Mail component
  IPVS wrapper： Generate IPVS The rules Netlink Reflector： Network interface WatchDog： Monitor progress

• Control components ： Provide keepalived.conf The parser , complete Keepalived To configure

• IO Multiplexer ： Its own thread abstraction optimized for network purposes

• Memory management components ： For some general memory management functions （ Such as distribution , Redistribution , Release etc. ） Provide access

keepalived It is divided into kernel space and user space ,ipvs yes lvs Core components , adopt IPVS wrapper Component handle lvs Rule passed keepalived write in lvs Kernel space , control lvs The scheduling strategy of ,lvs Cooperation in production keepalived The use of ,netlink reflector Component implementation vip The strategy of ,VRRP Stack Realization vip float .Checkers Achieve health check , Enriched lvs The missing function of ,Keepalived The emergence of is to make up lvs Deficiency , Achieve high availability in production .SMTP It is a mail alarm service .Keepalive And nginx Cooperate , because nginx The primary and standby relationship between agents cannot be checked for health and shared , adopt Keepalived Realization . High availability of agents depends on keepalived Realization .