Re: Draft 01 of HTTP/1.0

lilley (
Tue, 20 Dec 1994 16:33:21 +0000 (GMT)

Paul Burchard writes:

> A couple of questions and comments on the new HTTP/1.0 spec...

> (2) The semantics of Expires should discourage clients from
> _indiscriminately_ trying to refetch objects, just because they have
> expired. Dynamically-created pages can be ephemeral, without having
> time-dependent content that needs to be updated.

I think this is really a confusion about what 'Expires' means. Some people seem
to use it to mean that the information is no longer valid. So, for example, a
price list or commercial quote might have a 30 day expiry - after that, the
prices will be different.

Some people use it to mean the information on the server has changed.

I prefer the first, user-perspective view of expires.

For example, a gif of todays weather might expire at midnight. It is not valid
after midnight, because it is then yesterdays weather! Now, it could well be
that the server will have a new gif after midnight. Then again, it might not -
but the old gif is still invalid irrespective of what the server does.

> For example, the Geometry Center interactive gallery generates
> dynamic graphical HTML pages customized to each user's request. The
> user's images are deleted at the same moment they are fetched, and so
> the document ceases to exist on the server the moment it is
> retrieved.

But it is still valid for the user that fetched it.

> Therefore, the gallery expires all documents immediately:
> Expires: Sun, 01 Jan 0000 00:00:00 GMT

I am not sure that this is wise. It is using the server-view of expires
rather than the user view.

> Nevertheless, it would not make sense to try to fetch an "updated"
> version of such a document.

As you say, this view causes problems. I suggest thinking of the Expires:
header as meaning "a cache can validly serve this file on request until:"

Chris Lilley
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