nginx host not found in upstream

Question

I tried to follow the instructions here, but I still can't seem to get it to work. As with the question, I don't expect some of the servers to be running at the time of starting nginx. I would actually prefer if instead a local 404 html were returned if the server was not running.

upstream main {                                                                     
  server my_main:8080;                                                        
}                                                                                                                                            

server {                                                                            
  listen 80;                                                                        
  server_name www.my_site.com my_site.com;                                              

  location / {                                                                      
    resolver 8.8.8.8 valid=30s;                                                     
    set $upstream_main main;                                                         
    proxy_pass http://$upstream_main;                                               
  }                                                                                 
}