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Re: Conneg representation equivalence

From: Richard Cyganiak <richard@cyganiak.de>
Date: Fri, 12 Mar 2010 11:50:00 +0000
Cc: Toby Inkster <tai@g5n.co.uk>, "nathan@webr3.org" <nathan@webr3.org>, Linked Data community <public-lod@w3.org>
Message-Id: <2D76EF0B-F641-4102-9316-330ADB801BAC@cyganiak.de>
To: Pierre-Antoine Champin <swlists-040405@champin.net>

On 11 Mar 2010, at 20:34, Pierre-Antoine Champin wrote:
>>> Is it correct that all representations must have consistent fragment
>>> identifiers in order to be considered equivalent?
>>
>> A fragment identifier should not identify different things in  
>> different
>> representations. (Though it may be unrepresented in some or all of  
>> the
>> representations.)
>
> Is that so?
> If I recall correctly the URI RFC (no internet when writing the mail,
> sorry), the semantics of fragments identifiers depends on the  
> retrieved
> content-type.

Correct, see [1]: “The semantics of a fragment identifier are defined  
by the set of representations that might result from a retrieval  
action on the primary resource. The fragment's format and resolution  
is therefore dependent on the media type [RFC2046] of a potentially  
retrieved representation, even though such a retrieval is only  
performed if the URI is dereferenced.”

> So why would they *have* to identify the same thing?

You just have to read one paragraph down:

“If the primary resource has multiple representations, as is often the  
case for resources whose representation is selected based on  
attributes of the retrieval request (a.k.a., content negotiation),  
then whatever is identified by the fragment should be consistent  
across all of those representations. Each representation should either  
define the fragment so that it corresponds to the same secondary  
resource, regardless of how it is represented, or should leave the  
fragment undefined (i.e., not found).”

Best,
Richard

[1] http://www.ietf.org/rfc/rfc3986.txt


> That being said, I agree it sounds like a good practice. Especially if
> you consider an RDF/XML and a Turtle representation of the same RDF
> graph... If their fragment identifier were not consistent, that  
> would be
> a serious headache... But is this rule written somewhere?
>
>  pa
>
>
>
Received on Friday, 12 March 2010 12:04:47 UTC

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