Want to know about some of the varnish features

[Rajat]

I am new to varnish and running a heavy education portal , so we want to deploy varnish as cache engine.
But before using it, I want to be cleared about these point, Please help me in understanding these points
a>Is it possible to expire a particular file/content from cache so that if any changes are made to a file/content it can be reloaded into cache from the app server.
I have read about ban (varnish 3.x), but in that case I need to restart the engine to reload the configuration, but that will purge the whole cache. which I do not want
b>Can we instruct varnish not to flush the cache on reboot.
c> What are the rule preferences e.g. if there are two rules applicable for a particular request which rule would get the preference.


Regards
Rajat Toshniwal
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