https://www.haproxy.com/blog/failover-and-worst-case-management-with-haproxy/
Normal backup servers： In this case, s3 will be used first, until it fails, then s4 will be used. frontent ft_app bind 10.0.0.1:80 default_backend bk_app_main backend bk_app_main server s1 10.0.0.101:80 check server s2 10.0.0.102:80 check server s3 10.0.0.103:80 check backup server s4 10.0.0.104:80 check backup Multiple backup servers： In this case, both s3 and s4 will be used if they are available. option allbackups frontent ft_app bind 10.
https://zhuanlan.zhihu.com/p/41354937
http://haproxy.1wt.eu/
