Re: Last-Modified vs. Expires

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On Mon, 21 Jul 1997, Duane Wessels wrote:

> I think Expires would be preferable, and especially
> if you can get long Expires values (e.g. days instead of minutes).

That's good to hear somewhat, Expires is easier to generate on the fly
than Last-Modified.

> To turn the question around a bit, how does Apache handle IMS requests
> for dynamic content? Would it use the same Last-Modified timestamp
> to return 'not modified?'

Yes, although you just caused me to find two flaws in the code. At the
moment if the object is static, or if it's generated by mod_include with
XBitHack Full, then the Last-Modified header is used to respond to an IMS
request.

However if Last-Modified is generated by a CGI, the server passes it
without interpreting it. I also noticed that the apache api doesn't have
an interface for HTTP/1.1 ETag manipulation (also related to caching),
although it generates them and implements the preconditions for static
files. I put both of these on the List.

> I have recently been thinking it would be very good to be able
> to mark a URL as "absolutely static." That is, the origin server
> guarantees that the content will never change. So even if the
> client issued a 'pragma: no-cache' or 'max-stale=0' in the request,
> the cache could ALWAYS immediately return a hit for the object.
> Of course, we would generally have HTML remain dynamic as it is
> now, and apply the static feature to images and such.

I don't think there's anything like this yet. You could experiment with
it easily w/apache without any more code. Use the mod_headers module and
something like this:

<Files ~ "\.gif$">
    Header add Cache-control never-expires
</Files>

> This is one big difference between NNTP and HTTP. Usenet has quite
> efficient distribution because each artcile is only transmitted once
> between a pair of NNTP servers. Its certainly not the case with
> the Web. This was all became very apparent during the Mars pathfinder
> mission. It would have been so nice to be able to have JPL mark
> all their images as static, which I'm quite sure they were anyway.

Were there images marked with Last-Modified but no Expires? In that case
does squid requery on every hit, or does it implement its own staleness
timer?

Dean

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Received on Tue Jul 29 2003 - 13:15:42 MDT