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Re: for telecon: ISSUE-25 @profile Document Retrieval Failure

From: Ivan Herman <ivan@w3.org>
Date: Thu, 27 May 2010 09:29:02 +0200
Cc: public-rdfa-wg@w3.org
Message-Id: <43013170-3B0B-4FD3-9A84-62448FB6F78F@w3.org>
To: Toby Inkster <tai@g5n.co.uk>
Thanks Toby.

In general, I agree with the approach proposed by Jeni. Looking at the interpretation of Toby below, I wonder whether we cannot make it even a little bit simpler. We can simply say that if a @profile cannot be interpreted (the way Toby describes it below) then the processing stops as if neither on the element being processed nor on any elements down the tree were there any RDFa attributes. The difference, as I see, between Toby's interpretation and this is that even incomplete triples on that particular element would not be completed, ie, somewhat less triples would be generated. So be it...

Actually, Toby, isn't it correct that if we follow the approach below would yield _different_ triples whether the @profile is accessible or not? That might be a problem; a more radical solution of not generating triples at all might be preferable if this is the case...

'See' you this afternoon!

Ivan

On May 26, 2010, at 10:41 , Toby Inkster wrote:

> The issue: assume that profiles 'a' and 'b' both define the term
> 'Person' as tokens for different URIs. Additionally, assume that 'b'
> defines the prefix 'dc' to point to 'http://purl.org/dc/terms/'. Then:
> 
> 	<html profile="http://example.com/a"
> 	  prefix="dc: http://purl.org/dc/elements/1.1/
> 	    foaf: http://xmlns.com/foaf/0.1/">
> 	  ...
> 	  <body profile="http://example.net/b">
> 	    <address>
> 	      Last modified by <a typeof="Person"
> 	      rel="foaf:homepage" href="/joe"
> 	      property="foaf:name">Joe Bloggs</a>
> 	      on <span property="dc:modified">2010-05-26</span>.
> 	    </address>
> 	  </body>
> 	</html>
> 
> If profile 'b' is inaccessible (e.g. the host example.net is down
> temporarily or permanently) then an RDFa processor will create
> erroneous triples. typeof="Person" will be mapped to its definition
> from profile 'a'; and the 'dc' in 'dc:modified' will be mapped to the
> older Dublin Core 1.1 URI, which does not define a property called
> 'modified'.
> 
> The suggested solution, originally from Jeni Tennison is that when
> parsers encounter a profile URI that cannot be retrieved, they should
> not process that subtree.
> 
> My interpretation/clarification of this is:
> 
> If the profile does not return an HTTP 200 status (or equivalent for any
> non-HTTP profile URIs) possibly after following some redirections, then
> the profile is a "Failed Profile".
> 
> If the profile is delivered in a media type or dialect which the RDFa
> processor is not able to handle (e.g. delivered in Turtle to a processor
> that cannot handle Turtle), it is also deemed a Failed Profile.
> 
> Having processed @profile, if any of the profiles are Failed Profiles,
> then an RDFa processor cannot continue. It completes any incomplete
> triples using the following method to determine the subject of
> incomplete forward triples / object of incomplete reversed triples:
> 
> 	1. if @about is set to a relative URI, use that;
> 	2. otherwise, if @about is set, use a new blank node;
> 	3. otherwise, if @src is set, use that;
> 	4. otherwise, if @resource is set to a relative URI, use that;
> 	5. otherwise, if @resource is set, use a new blank node;
> 	6. otherwise, if @href is set, use that;
> 	7. otherwise, use a new blank node.
> 
> Note that this method will result in any token containing a colon
> resulting in a blank node - this is because it becomes impossible
> to determine whether that token represents a CURIE that would
> have been defined by the Failed Profile, or is an absolute URI.
> 
> Descendant elements are then not processed.
> 
> I'll add that there is actually a safe subset of RDFa to process within
> the subtree of Failed Profiles, however the rules for determining this
> safe subset are complex, so for the sanity of RDFa publishers, it's
> simpler to say that a Failed Profile causes the subtree to be ignored.
> 
> For the record though, a method for continuing parsing safely is:
> 
> * when a parser encounters a Failed Profile, it empties the local lists
>  of prefix mappings and terms.
> * it continues processing (including processing other profiles, as
>  the @profile attribute is a list of profiles) with a small
>  modification in behaviour - when a token is encountered that matches
>  the CURIE syntax but does not match a prefix mapping, instead of
>  assuming that it's an absolute URI, the parser assumes it's
>  an undefined value.
> 
> I don't recommend that method though as it might lead to confusion
> amongst authors.
> 
> -- 
> Toby A Inkster
> <mailto:mail@tobyinkster.co.uk>
> <http://tobyinkster.co.uk>
> 
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
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Received on Thursday, 27 May 2010 07:28:12 UTC

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