Conditional caching question

[David Pratt]

Hi. In most cases, I want a request to be passed to a backend where it 
will be handled by server. If frequency is high, however; I want to add 
the object to varnish cache and have varnish handle it. I am not worried 
about a mechanism to keeping track of frequency of requests. Question is 
what is available to me to add an object/path to the varnish cache if it 
was originally passed? Many thanks.

Regards,
David