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Re: ETags? (was: WG Last call for BIND)

From: Julian Reschke <julian.reschke@gmx.de>
Date: Wed, 19 Jan 2005 02:14:25 +0100
Message-ID: <41EDB471.3090000@gmx.de>
To: ejw@soe.ucsc.edu
CC: 'Brian Korver' <briank@xythos.com>, 'WebDAV' <w3c-dist-auth@w3.org>

Jim Whitehead wrote:
>  
> 
>>  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.
> 
> 
> My recollection is that a server might decide to implement Etags as a simple
> integer that is updated on every state modification. In this case, all of
> the resources on a server would start out with the same Etag value of "1".
> In this case, the fact of all Etags being "1" certainly doesn't imply that
> all URIs are mapped to the same resource. The fact that all Etags have the
> value "1" is still consistence with Etags being defined on resources.

Correct. For instance, subversion could use it's global version counter 
as Etag.

>>   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.
> 
> 
> So, if we were to put in language, I'd add something like:
> 
> Implementors note: Etag values are only required to be unique across all
>   versions of a particular resource, not unique across all versions and
>   all resources. As a result, two separate resources may have the same
>   Etag value, and hence Etag values cannot reliably be used to establish
>   the identity of a resource accessible via multiple URIs.

That's correct, but I really have a hard time understanding why anybody 
would *think* that ETags can be used that way. If they could, we 
wouldn't need DAV:resource-id...

 > ...

Best regards, Julian

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Received on Wednesday, 19 January 2005 01:15:03 GMT

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