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Re: ETags?

From: Julian Reschke <julian.reschke@gmx.de>
Date: Wed, 19 Jan 2005 09:18:38 +0100
Message-ID: <41EE17DE.8090108@gmx.de>
To: Lisa Dusseault <lisa@osafoundation.org>
CC: "Roy T. Fielding" <fielding@gbiv.com>, Brian Korver <briank@xythos.com>, WebDAV <w3c-dist-auth@w3.org>

Lisa Dusseault wrote:
> 
> When I first considered ETags and bindings, I assumed that all bindings 
> to the same resource MUST show the same ETag.  That came from my 
> understanding of what the ETag is used for, which is shaped by the 
> implementations I've been involved in and what they do (authoring, 
> sharing).  Note that I'm not making the naive assumption that an ETag 
> can be used to identify two bindings -- but I did make the assumption 

ETags do not identify resources or bindings. ETags can be used *with* 
URIs (as pair (URI, etag)) to identify one specific entity body you may 
get when applying GET to that URI.

> that a single ETag can be recorded in order to synchronize a set of 
> bindings to a single resource.

If this would be the case, we wouldn't need DAV:resource-id. In 
particular, how would that work with changing content (= changing etags?).

> Then I saw Brian's email, where he put forward an excellent argument for 
> why every binding MUST show its own unique ETag.  I can entirely 

I haven't seen that argument. Please quote, link to or explain again.

> understand a server implementor reading these specs and making that 
> decision.

Well, I can't, but I'm interested. Where is that argument?

> So if I wrote a synchronizing client (which I am), and Brian wrote an 
> authoring server (which he does), if we were guided only by the specs 
> and our interpretations, we would probably have interoperability 
> problems.  My client would probably be confused by synchronizing two 
> URLs which it knew to be bindings to the same resource but which 
> reported different ETags.

Well, that's an entirely different issue compared to: "Then I saw 
Brian's email, where he put forward an excellent argument for why every 
binding MUST show its own unique ETag."

There are good reasons why Etags for the same entity body accessed 
through different URIs *should* be the same, and draft 10 already says 
(<http://greenbytes.de/tech/webdav/draft-ietf-webdav-bind-10.html#rfc.issue.2.6_bindings_vs_properties>) 
that the value of a live property -- and this includes DAV:getetag -- 
SHOULD be the same. I'm not sure we can go any further than that, 
because Entity Tags are defined by RFC2616, not us. Maybe Roy can give 
some guidance here.

> Thus, I support adding text to bindings, either limiting the ways that 
> servers can implement ETags and bindings, or explaining to clients the 
> wide range of possible implementations they might have to deal with.

That being said I do agree with the other comments Geoff made in 
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2005JanMar/0060.html> 
-- I'm just not convinced that BIND needs to decide either way at this 
stage of the standards process. Sometimes, when something is initially 
submitted, being silent on a particular thing can be the right thing to 
do. In particular, this seems to be an issue that actually affects 
RFC2616 itself and possibly should be clarified there.

Best regards, Julian

-- 
<green/>bytes GmbH -- http://www.greenbytes.de -- tel:+492512807760
Received on Wednesday, 19 January 2005 08:19:18 GMT

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