support standby module when there is only master 

upstream services{
    server 127.0.0.1:80;
    server 127.0.0.1:81 weight=5 backup;   #standby
    server 127.0.0.1:82 weight=3 backup;   #standby
    server 127.0.0.1:83 weight=1 backup;   #standby
}


upstream services{
    server 127.0.0.1:80;
    server 127.0.0.1:81;
    server 127.0.0.1:82;
    server 127.0.0.1:83 weight=1 backup;
}