Trigger alternate response based on file presence or other URL response

[dhallam]

So i can see that I can do something like the maintenance mode example,
e.g.

if (-f $document_root/under_maintenance.html) {
   return 503;
}

Wondering if there is a way to use the URL endpoint to check.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,281639,281640#msg-281640