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 10.0.0.1:80  
 default_backend bk_app_main  
backend bk_app_main  
 option allbackups  
 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