W3C home > Mailing lists > Public > public-lod@w3.org > March 2010

Re: Conneg representation equivalence

From: John Erickson <olyerickson@gmail.com>
Date: Thu, 11 Mar 2010 19:39:31 -0500
Message-ID: <b813a3fb1003111639k34dce928n1d59de3aa3c8e4de@mail.gmail.com>
To: Linked Data community <public-lod@w3.org>
Nathan <nathan@webr3.org> 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.)

>From http://www.w3.org/TR/webarch/#frag-coneg , the fragment
identifier must be defined consistently by the representations; the
*provider* decides when definitions of fragment identifier semantics
are "sufficiently consistent..."

>> If I recall correctly the URI RFC..., the semantics of fragments identifiers depends on the retrieved content-type. So why would they *have* to identify the same thing?

>From http://www.w3.org/TR/webarch/#frag-coneg , "Individual data
formats may define their own rules for use of the fragment identifier
syntax for specifying different types of subsets, views, or external
references that are identifiable as secondary resources by that media
type."

>> 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?

The interpretation in http://www.w3.org/TR/webarch/#frag-coneg
suggests that it would be correct either for the fragment identifier
to be interpreted by the RFG/XML and Turtle representations
consistently --- refers to a semantically consistent secondary
representation --- or for an interpretation to not be defined at all.
It is not acceptable for representations with an interpretation
defined, to interpret the fragment identifier such that semantically
inconsistent secondary representations are returned. Interpret them
consistently, or don't interpret them, but don't do it inconsistently!

So maybe an answer to Nathan's question needs qualification; IF for
each representation of a resource an interpretation for a given
fragment identifier has been defined, AND we assume that the server is
exhibiting correct behavior, THEN we must accept that the secondary
representations meet the providers definition of "consistent."

Consistency is not the same as equivalence; two representations might
return "consistent" secondary representations that cannot be
considered "equal" because they are of entirely different content
types.

-- 
John S. Erickson, Ph.D.
http://bitwacker.wordpress.com
olyerickson@gmail.com
Twitter: @olyerickson
Received on Friday, 12 March 2010 11:37:24 UTC

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