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Re: ETags?

From: Julian Reschke <julian.reschke@gmx.de>
Date: Wed, 19 Jan 2005 01:52:39 +0100
Message-ID: <41EDAF57.3070206@gmx.de>
To: Brian Korver <briank@xythos.com>
CC: WebDAV <w3c-dist-auth@w3.org>

Brian Korver wrote:
> The BIND spec doesn't mention ETags at all, but buried in section
> 3.11 of RFC 2616 is the following text:
>    An entity tag MUST be unique across all versions of all entities
>    associated with a particular resource. A given entity tag value MAY
>    be used for entities obtained by requests on different URIs. The use
>    of the same entity tag value in conjunction with entities obtained by
>    requests on different URIs does not imply the equivalence of those
>    entities.
> which (at least to me) seems counter-intuitive in the face of multiple
> bindings -- I would have guessed that the ETag would remain constant.

I think all this says is that if you get the same Etag for "/a" and 
"/b", you can't assume that it's the same entity.

> Any chance we could squeeze some text into the draft, similar to that
> dealing with locking?  Something approximately like:
>   Entity Tags and Bindings
>   It might be thought that ETags would be associated with resources,
>   not URIs, and as such two different URIs with identical ETags
>   would imply that the URIs are bindings to the same resource.
>   This is not the case, however.  Section 3.11 of [RFC2616]
>   states that ETags are on URIs, not resources.

I agree with the conclusion, but I think the motivation for saying this 
is a bit weak. Could you please re-read 3.11 and consider my explanation 

That being said here's something for RFC2518bis to clarify: RFC2616's 
definition of ETag semantics doesn't work well with namespace 
operations. In practice, within the same URL namespace ETags will only 
work well when they are indeed unique not only for all versions for a 
single URI, but for the whole namespace (otherwise a server will have to 
do post-processing after MOVE operations to ensure that ETag semantics 
are preserved, possibly assigning new ETags just because a namespace 
operation occured).

Best regards, Julian

<green/>bytes GmbH -- http://www.greenbytes.de -- tel:+492512807760
Received on Wednesday, 19 January 2005 00:53:20 UTC

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