Use backup server on HAProxy when connection is too much or one service is going down

config.txt

frontend frontend_service 127.0.0.1:9200
    mode tcp
    balance roundrobin
    acl max_conn_reached fe_conn gt 20
    acl production_almost_death nbsrv(production) lt 2
    default_backend production
    use_backend production_and_backup if max_conn_reached OR production_almost_death

backend production
    mode tcp
    balance roundrobin
    server production01 127.0.0.1:8001 check maxconn 10
    server production02 127.0.0.1:8002 check maxconn 10

backend production_and_backup
    mode tcp
    balance roundrobin
    server production01 127.0.0.1:8001 check maxconn 10
    server production02 127.0.0.1:8002 check maxconn 10
    server backup01 127.0.0.1:8003 check maxconn 10
    server backup02 127.0.0.1:8004 check maxconn 10

I assume low maxconn, change it to suit with your needs