Cache invalidation (not PURGE)

[Antoni Villalonga]

Hi!

I'm looking for a method to invalidate some url.
http://example.com/invalidate.html, for example.

If I PURGE using 'purge()' function and 2 diferent users GET 
"http://example.com/invalidate.html" at time, varnish ask for the url to the 
backend server twice.

When a url cache expires, and 2 diferent users GET the url at time, varnish ask 
only one time to the backend and serve an expired version to the "second" user. 
(yes, we use "grace time").

In short, I also want to use "grace time" when urls are "purged".

Thanks!