VCL and Non-Existent Backend Objects

[James Quacinella]

Hey,

I was contemplating a setup such as this:

1) Client requests xml file from varnish cache
2) If it exists, fine; else check backend file server running lighttpd
3) If it exists on the backend, again no problem; however, if it doesn't 
exist on the lighttpd backend, I need varnish to check another (dynamic) 
backend, which will dynamically generate it and save it on the lightttpd 
backend for future purposes.

Does anyone have any ideas on how to implement that via VCL? I would 
imagine I would need to be able to see the response headers from the 
original backend, to see if a 404 is generated (like here 
http://varnish.projects.linpro.no/ticket/88).

Thanks!
-- james