Have Nginx hold requests while app is reloading [duplicate]

I have an nginx reverse proxy server set up to forward requests to an app running on port 5000 via proxy_pass http://localhost:5000;.

This works, however when I restart the app, for about 10 sec any requests immediately return 502 Bad Gateway, until the app reloads again.

Is there any way to set up Nginx to hold those requests while the app is down, retrying every so often? I've tried

upstream backend {                         
  server localhost:5000 fail_timeout=20s; 
  server localhost:5000 backup;          
}
...
    proxy_pass http://backend;

but that doesn't seem to have an effect.