Opportunity opens up when we use HTTP to connect our server infrastructure components together. If you use HTTP between your front-end web server and your back-end application server, suddenly you gain the ability to swap Apache for Lighttpd on demand without worrying about the FastCGI bits. Your application server won’t notice the difference. You can replace either Apache or Lighttpd with Pound or Pen. You can even replace them with some sort of hardware load balancer solution if that floats your boat.
Even better, when you use HTTP as the glue, you suddenly can use a whole host of tools to probe the various parts of your application. You can use curl to probe just your application server. Or even point a browser at it, assuming that you have a clear path through your firewalls and what not. And, if you’re really l33t, you can do a manual telnet and make just the request you need—with all the right headers—to simulate exactly a problematic client request.
Exactly. Using a web server as your application server means you can replace your application server with yet another web server.