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Re: Improving If-Modified-Since

From: Brian Behlendorf <brian@organic.com>
Date: Mon, 14 Aug 1995 22:14:22 -0700 (PDT)
To: Lou Montulli <montulli@mozilla.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <Pine.SOL.3.91.950814215844.21140A-100000@eat.organic.com>
On Mon, 14 Aug 1995, Lou Montulli wrote:
> >Why would a server pumping out bogus last-modified headers act appropriately
> >to another type of check?  Adding something to the protocol just because
> >another part is not being used properly seems a bit weird.  If I'm
> >understanding the problem correctly. 
> 
> The problems currently encountered are mostly caused by the date comparisons
> done by most HTTP servers when dealing with If-modified-since requests.
> Most servers assume that as long as the If-modified-since date is equal to
> or AFTER the current modification date of the document then it is unchanged.
> 
> This is a problem because people screw up the dates on their files and
> sometimes give them dates far into the future.  When they fix the
> dates of the files to correspond to the current date, caches never
> get updated.

If people "screw up their dates", they're hurting themselves and the people
who view their pages.  This *isn't* accidental, is it?  How do you
accidentally set a last-modified to be some future time?  It's like a 
surgeon accidentally removing a right leg instead of a left leg or a 
disgruntled employee accidentally shooting his boss.  

Or maybe it's intentional...

> In addition to supporting size=SIZE I encourage other server authors to
> do an _equals_ comparison rather than a greater than or equal comparison
> of the two dates.

They can't just do a strcmp() since there are a couple date formats it 
needs to deal with.  Also, consider a situation where there are three 
mirrors for a web site, and all three are hidden behind www.host.com and 
selected through shortest-return-trip calculations (like the CERN 
linemode browser does).  Getting a last-modified on one which was later 
than the last-modified on the other, even though they are the same 
document, certainly makes sense.  

> >Will the "size" be determined from the Content-length header or the size on
> >the cache's disk?  If the former, documents with incorrect content-length
> >headers are essentially uncacheable, as are results from CGI scripts which
> >generally don't have content-length headers.  If the latter, could there
> >be encoding problems?
> 
> The size is determined by taking the current length of the document in the
> cache.  The content-length of the transfer is discarded, so encodings
> should have no effect.

Okay, so what should a server do when it doesn't know the actual
content-length of an object, like a CGI script?  It's totally plausible to
ask a dynamic object "hey, has any data upon which you would answer this
question changed since *blah*" in a way that's very quick to answer, but it's
hard to imagine asking it "hey, will your final output be any size
different than *blah*" without having it do what it normally does.  So, 
if it's in there, it must be optional for the server to use it.

	Brian

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brian@organic.com  brian@hyperreal.com  http://www.[hyperreal,organic].com/
Received on Monday, 14 August 1995 22:14:15 EDT

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