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RE: FW: ETags again - basic question

From: Lisa Dusseault <lisa@xythos.com>
Date: Mon, 3 Mar 2003 16:23:53 -0800
To: "'Clemm, Geoff'" <gclemm@rational.com>, "'WebDAV'" <w3c-dist-auth@w3.org>
Message-ID: <051701c2e1e4$56550240$f8cb90c6@xythoslap>

I strongly recommend servers to change Etag only when the entity
changes. After all, it's the "Entity" tag.  Here are two to three
practical reasons.

If the ETag changes when the properties change, then:
1.  Clients who have cached based on the ETag will download the file
again unnecessarily. Perf problem only.
2.  Clients who are editing without a lock (including clients who lost
their lock accidentally due to network conditions) must compare Etag to
see if it's safe to upload their changes.  This edit will fail and
confuse the user if somebody else changed the properties and makes their
upload fail.
3.  Clients who are editing with a lock might issue a PROPPATCH, then
try to issue a PUT with an ETag validation. If the ETag has changed due
to the PROPPATCH, then the client will be very confused whether or not
the file can be overwritten. The lock still appears to be there, yet the
ETag changed -- how did that happen?

Geoff's comments are perfectly correct for client implementors -- it's
hard to predict how servers actually modify ETags. There's no rule
saying the content is actually different just because the ETag is


> -----Original Message-----
> From: w3c-dist-auth-request@w3.org 
> [mailto:w3c-dist-auth-request@w3.org] On Behalf Of Clemm, Geoff
> Sent: Sunday, March 02, 2003 5:48 AM
> To: WebDAV
> Subject: RE: FW: ETags again - basic question
> Note that some file-based implementations store the dead 
> properties as part
> of the file content and use the file content information to 
> compute the
> etag.  So an interoperable client can only make the following 
> assumptions:
> - If the resource content changes, the strong etag MUST change.
> - If the properties change, the strong etag MAY change.
> Cheers,
> Geoff 
> -----Original Message-----
> From: Jason Crawford [mailto:nn683849@smallcue.com]
> Sent: Saturday, March 01, 2003 5:15 PM
> To: Jim Whitehead
> Cc: WebDAV
> Subject: Re: FW: ETags again - basic question
> Okay, I'll take a stab at this one...
> > When does an (strong) etag of a resource change?
> >
> > 1) when the content of a resource has changed (e.g. after a PUT)
> > 2) when one of the properties has changed (e.g. after a PROPPATCH)
> > 3) in both cases 1) and 2)
> I think the answer is whenever the GET response changes.  So for most
> resources the answer is almost always (1), not (2) or (3).   
> If you have a
> live
> resource that is dependent on other info, the answer might vary.
> ------------------------------------------
> Phone: 914-784-7569,   ccjason@us.ibm.com
> I do not check nn621779@smallcue.com
Received on Monday, 3 March 2003 19:23:54 UTC

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