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Re: rev and the costs of inverses/aliases in SPARQL

From: Ivan Herman <ivan@w3.org>
Date: Mon, 08 Mar 2010 17:41:38 +0100
Message-ID: <4B9528C2.5000708@w3.org>
To: Dan Connolly <connolly@w3.org>
CC: public-sparql-dev <public-sparql-dev@w3.org>, public-rdfa <public-rdfa@w3.org>


On 2010-3-8 16:00 , Dan Connolly wrote:
> I just ran into this message again from an HTML 5 validator:
> 
> "The rev attribute on the a element is obsolete. Use the rel attribute
> instead, with a term having the opposite meaning."
> 
> This seems to encourage the pattern of minting an inverse
> for each property, a la:
> 
>   abridgement
>   abridgementOf
>  -- http://vocab.org/frbr/core.html
> 

From an RDFa point of view, if I am an author, I consider using a
specific vocabulary and I use the attributes as they are defined.
Authors cannot be expected to mint additional predicates on the fly if
those are not defined; this is way too much for many of them anyway.

> Doesn't that just gum up the works when doing SPARQL queries? Which
> do you query for, abridgement or abridgementOf? Or do you use
> a UNION?
> 
> It's one thing to discover, post-hoc, that two properties are
> inverses of each other, and to write down that relationship.
> But to make up these inverse-aliases by choice seems like
> a big waste, to me.
> (see also http://esw.w3.org/topic/HasPropertyOf bit on inverses and
> aliases)
> 
> How are SPARQL users dealing with this in practice?
> 
> 
> Meanwhile, RDF/XML doesn't have syntax for inverting a relationship
> (a la is/of in N3), and there's data that says rev="..." is
> too confusing for HTML authors to use.
> 

I am not sure RDF/XML is relevant. RDF/XML gives you different ways of
expressing triples, so one can encode anything freely, there is no real
constraint. In the case of RDFa there is an additional constraint that
one wants to follow the HTML structure to include the presentation content.

It is of course possible, in RDFa to express everything with @rel only
but, in some cases, the missing @rev makes it very convoluted.

> "The short answer is unfortunately "NO". Use of "rev" SHOULD be
> avoided."
>  -- http://microformats.org/wiki/rel-faq
> "the only <link rev=""> link to appear is rev="made" (to point to the
> author's page) — and the latter is not used that much more than the more
> sensible rel="author". Also, ironically, just off the graph in position
> 21 is rel="made", probably showing that the distinction between rel and
> rev may be too subtle for many authors."
>  -- http://code.google.com/webstats/2005-12/linkrels.html
> 
> 
> Would the RDFa authoring community miss a/@rev if it went away?
> Does anyone have 1st-hand experience to share?
> 

I think Damian has just posted a good use case example.

Another example is

<img rev="foaf:depiction"
     resource="http://www.ivan-herman.net/foaf#me"
     src="http://www.ivan-herman.net/Images/me2003-small.png"/>

That gives me

<http://www.ivan-herman.net/foaf#me>
   foaf:depiction <http://www.ivan-herman.net/Images/me2003-small.png> .

Without a @rev, I have to add a new hierarchy to do the same:

<div about="http://www.ivan-herman.net/foaf#me" rel="foaf:depiction">
   <img src="http://www.ivan-herman.net/Images/me2003-small.png"/>
</div>

Which unnecessarily complicates the structure.

There is another issue. There is already deployed RDFa out there. Quite
a lot, actually. As a consequence, there is a strong requirement of
backward compatibility in the RDFa WG charter. This also means that if
the @rev is removed from the core HTML5 document, RDFa will have it
alongside the RDFa specific attributes like @about or @resource...

Ivan



-- 

Ivan Herman, W3C Semantic Web Activity Lead
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Received on Monday, 8 March 2010 16:41:16 GMT

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