Re: Caching Servers Considered Harmful (was: Re: Finger URL)

Roy T. Fielding (fielding@simplon.ICS.UCI.EDU)
Mon, 22 Aug 1994 18:02:59 -0700

Brian ( writes:

> Now, this doesn't mean I'm happy with the way caches work. And like GNN and
> Rob, I am definitely interested in how many people are looking at our pages.
> I've mentioned a couple of times in various forums that I'd be happy if the
> caching server sent as a minimum a HEAD request for the object being
> obtained, If its Last-Modified date were more recent that when the cache
> grabbed it, it'd fetch the whole file (preferably doing this in one HTTP
> connection), other wise it'd just serve the cached version. I can count the
> HEADs (removing the subsequest GETs for the same object from the same server)
> as full accesses, thus satisfying my interest as a provider in how popular my
> pages are, and I'm guaranteed that they are getting the most recent version
> of whatever I put up. If our server is overloaded or the link is ddown and
> the HEAD request doesn't get through, then the cache should serve up its
> local copy - this fulfills my wish that the user sees the page even without
> my knowlege if the alternative is that it's unviewable.

Been there, done that... you just described the If-Modified-Since
(a.k.a. Conditional GET) proposal that is already implemented on all
"good" servers, except that GET is used intead of HEAD.

It is already in the protocol spec (HTTP2) and the Proxy stuff that
Kevin mentioned. A full description (with history) is also provided at


which should have been referenced in Kevin and Ari's paper (hint hint).

....Roy Fielding ICS Grad Student, University of California, Irvine USA
<A HREF="">About Roy</A>