http://blogs.osuosl.org/gchaix/2009/10/12/pressflow-varnish-and-caching/
http://highervisibilitywebsites.com/example-varnish-vcl-drupal-pressflow...
There are actually 8 subroutines that control how Varnish behaves, and that you can change in your Varnish config. They are:
vcl_recv()
Called after a request is received from the browser, but before it is processed.
vcl_pipe()
Called when a request must be forwarded directly to the backend with minimal handling by Varnish (think HTTP CONNECT)
vcl_hash()
Called to determine the hash key used to look up a request in the cache.
vcl_hit()
Called after a cache lookup when the object requested has been found in the cache.
vcl_miss()
Called after a cache lookup when the object requested was not found in the cache.
vcl_pass()
Called when the request is to be passed to the backend without looking it up in the cache.
vcl_fetch()
Called when the request has been sent to the backend and a response has been received from the backend.
vcl_deliver()
Called before a response object (from the cache or the web server) is sent to the requesting client.
