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RE: Another attempt at 'cascading DRs'

From: Scheppe, Kai-Dietrich <k.scheppe@telekom.de>
Date: Tue, 12 Feb 2008 15:51:06 +0100
Message-ID: <398533C370C23441981074C456AA3BDD031DAED4@QEO00226.de.t-online.corp>
To: "Phil Archer" <parcher@icra.org>, "Public POWDER" <public-powderwg@w3.org>

Hmmm...come to think of it...I think Andrea could really help us with
this :-)

I am thinking if

- scope A = a large area
- scope B = a small area

And

- B is a subset of A, meaning B is contained within A but through some
property different.

If I transfer this onto URIs, I can talk about paths.

A = *.t-online.de
A is a large scope

B = www.t-online.de/c/01/02/03/01020304.html 
B is a small scope, but is also a subset of A


Irrespective of what precisely is listed in the path, it /*IS*/ more
specific than that used for the larger scope.

Some examples:

www.t-online.de/c/01/02/03/ 
www.t-online.de/c/01/02/
www.t-online.de/c/01/
www.t-online.de/c/

Are all subsets, but

www.someotherdomain.de/c/01/02/03/01020304.html

is not.


To summarize:

In CSS I define a style sheet and override this, on a "local" level,
with an inline style.
Here I would define the scope globally and everything that is within
that scope can override, on a local level, the global definition.

Restating the link-back mechanism, if the "local" DR would link back to
the "global" DR you make content discoverable.


Does that make sense?


Kai



 

> -----Original Message-----
> From: public-powderwg-request@w3.org 
> [mailto:public-powderwg-request@w3.org] On Behalf Of Phil Archer
> Sent: Tuesday, February 12, 2008 3:14 PM
> To: Public POWDER
> Subject: Re: Another attempt at 'cascading DRs'
> 
> 
> The problem is that your DR_2 directly contradicts DR_1 and 
> both are created by the same person, meaning "I think 
> http://www.t-online.de/c/01/02/03/01020304.html is text AND I 
> think it's an image." Something has to change. Yes, we can 
> can have conflicting DRs but these would generally be created 
> by different people, so it comes down to whom you trust. If 
> the same person creates two conflicting DRs, you can't use 
> trust as a means to separate them.
> 
> In my proposal today I've effectively removed the scope 
> entirely from the DR so it becomes impossible to go from the 
> DR to the resource - i.e. 
> eliminates the first problem you identify.
> 
> On the upside, no machine can find the two descriptions and 
> infer that they both apply to the same thing and therefore 
> end up with a logical paradox. The downside is that you then 
> can't use the DR as a discovery mechanism, only to provide 
> information about what you already have.
> 
> That's perhaps a bit of an over-statement. If you publish a 
> POWDER doc that has an aboutHosts attribute of t-online.de 
> and you have a description that says 'text' and one that says 
> 'image' then it would be reasonable to infer that t-online.de 
> includes both text and images - you just wouldn't know 
> exactly where they were. You would, however, be able to crawl 
> the site looking for links to "...#text" and "...#image" and 
> create a catalogue/site map with relative ease.
> 
> Taking your last paragraph: I'm not sure how you would define 
> 'locally based' - it sounds very much like the linkFrom 
> attribute already posited in the published doc [1] - i.e. you 
> make it local by linking to it, yes? 
> So the operation here is "use the cached DR that covers all 
> of t-online.de" and the image would then have an HTTP header 
> that linked to
> DR_2 that says "I can override DR_1." You still need the link 
> to create the property of being local. OK, I can see that 
> working operationally but it seems to have a real danger of 
> creating data that, when taken in isolation, can be contradictory.
> 
> Another option might be to make more use of the issue date 
> with the idea that if you have two conflicting DRs then the 
> most recently issued one wins. What worries me here is the 
> caching. If I get a DR that says t-online.de is all text, I 
> might cache that. If there is no validUntil date, in theory 
> it's valid until the entropic heat death of the universe. If 
> there is a validUntil date, OK, I should only cache it up 
> until that date. Either way, the new, more recent DR can 
> legitimately be ignored by an optimised system until the 
> original expires. Again, this problem disappears if there is no scope.
> 
> P
> 
> [1] http://www.w3.org/TR/2007/WD-powder-dr-20070925/#noPattern
> 
> 
> Scheppe, Kai-Dietrich wrote:
> > Hi,
> > 
> > I have a question:
> > 
> > The problem really doesn't exist when going from the 
> resource to the DR.
> > It does exist when going from the DR to the resource.
> > 
> > But since we say at some point that conflicting DRs can 
> > exist...afterall different CA could have different opinions about 
> > content, it is up to the user to decide which DR he believes.
> > 
> > Can't this principle apply here as well?
> > 
> > Or better, if it doesn't apply here, why does it apply in general?
> > And if it does and if this is a problem, how do we solve 
> it...with the 
> > knowledge that solving that problem would also solve this problem?
> > 
> > 
> > Either way, I think if we just say that
> > 
> > DR_1 says all content on t-online.de is text based
> > DR_2 says that 
> http://www.t-online.de/c/01/02/03/01020304.html is an 
> > image
> > 
> > then it is up to the peruser to decide whether to download this 
> > resource.
> > 
> > We could defuse the problem somewhat by requiring the more locally 
> > based DR to refer to the more globally based DR.  This way an 
> > application could create its own set of exceptions.
> > So in the example above DR_2 would contain a link to DR_1.
> > 
> > 
> > However, the problem centers on dealing with unknown DRs.
> > 
> > 
> > -- Kai
> > 
> > 
> > 
> > 
> > 
> > 
> >> -----Original Message-----
> >> From: public-powderwg-request@w3.org 
> >> [mailto:public-powderwg-request@w3.org] On Behalf Of Phil Archer
> >> Sent: Tuesday, February 12, 2008 1:17 PM
> >> To: Public POWDER
> >> Subject: Another attempt at 'cascading DRs'
> >>
> >>
> >> The basic POWDER model has a resource that describes a lot 
> of other 
> >> resources. A processor may start at the descriptive resource (the 
> >> POWDER
> >> document) and discover the resources it describes. To aid 
> discovery 
> >> of the description, a resource may link to to the POWDER document 
> >> that describes it.
> >>
> >> In some important circumstances however this doesn't work. 
> >> POWDER's Grouping mechanism [1] (currently under revision 
> with a new 
> >> draft due for publication v. soon) assumes that by 
> examining a URI, 
> >> one can deduce which description of a collection applies to it. If 
> >> URIs don't follow a particular pattern, such as numerical URIs 
> >> generated by some content management systems, we need a different 
> >> mechanism: we must rely on the link from the described resource to 
> >> point to the correct description.
> >>
> >> We've discussed this a lot, most recently in Athens, and 
> we know we 
> >> need to solve it. We also know that it's impractical in a 
> commercial 
> >> workflow to need to edit the POWDER document continually, 
> adding in 
> >> lists of exceptions to rules. We need to work more along the CSS 
> >> model where there is a central file that carries the 
> defined styles. 
> >> Which style, indeed, which stylesheet, is applicable, is defined 
> >> within the document for which it contains the styles. HTTP 
> and client 
> >> caching ensures that stylesheets need only be accessed once until 
> >> updated.
> >>
> >> A Package of DRs, as currently defined at [2], has an attribute 
> >> 'aboutHosts'. The structure of packages is going to be modified a 
> >> little in the near future but this feature is a very 
> useful one for 
> >> processing efficiency. The plan now is to make it so that, where 
> >> present, the aboutHosts guarantees that the DRs in the 
> package do not 
> >> cover any resources on domains other than those listed (it doesn't 
> >> guarantee that all resources on those domains are described by the 
> >> way, just that if the aboutHosts property lists 
> example.org then you 
> >> can be sure that it does not describe anything on example.com).
> >>
> >> OK, hold on to that and look at this:
> >>
> >> 1  <?xml version="1.0"?>
> >> 2   <POWDER xmlns="http://www.w3.org/2007/05/powder#"
> >> 3           xmlns:ex="http://example.org/vocab#"
> >> 4           
> xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
> >>
> >> 5    <attribution>
> >> 6      <maker>http://authority.example.org/foaf.rdf#me</maker>
> >> 7      <aboutHosts>example.org</aboutHosts>
> >> 8    </attribution>
> >>
> >> 9    <Descriptors xml:id="red">
> >> 10     <ex:color>red</ex:color>
> >> 11   </Descriptors>
> >>
> >> 12   <Descriptors xml:id="blue">
> >> 13     <ex:color>blue</ex:color>
> >> 14   </Descriptors>
> >>
> >> 15 </POWDER>
> >>
> >> This is POWDER doc with its required attribution. Line 7 
> adds in an 
> >> aboutHosts element.
> >>
> >> But there are no DRs here, just two descriptions (and if 
> there are no 
> >> DRs there is no requirement for a URISet). A 'red page' on 
> >> example.org would include a link element with an href attribute of 
> >> "...powder.xml#red", blue pages would have #blue as the fragment 
> >> identifier. The aboutHosts element prevents other domains 
> pointing to 
> >> this POWDER doc and claiming (quite possibly falsely) that 
> the entity 
> >> described at http://authority.example.org/foaf.rdf#me 
> described them 
> >> - that is, the POWDER doc author has a mechanism for 
> restricting the 
> >> scope of the descriptions without actually having a URISet.
> >>
> >> But... what's the POWDER-S version of this, i.e. the output of the 
> >> GRDDL transform with formal semantics? Well, I guess it 
> ends up just 
> >> being:
> >>
> >> <rdf:Description rdf:about="">
> >>    <foaf:maker
> >> rdf:resource="http://authority.example.org/foaf.rdf#me" /> 
> >> </rdf:Description>
> >>
> >> <owl:Class rdf:nodeID="red">
> >>    <owl:intersectionOf rdf:parseType="Collection">
> >>      <owl:Restriction>
> >>        <owl:onProperty
> >> rdf:resource="http://example.org/vocab#color" />
> >>        <owl:hasValue>red</owl:hasValue>
> >>      </owl:Restriction>
> >>    </owl:intersectionOf>
> >> </owl:Class>
> >>
> >> <owl:Class rdf:nodeID="blue">
> >>    <owl:intersectionOf rdf:parseType="Collection">
> >>      <owl:Restriction>
> >>        <owl:onProperty
> >> rdf:resource="http://example.org/vocab#color" />
> >>        <owl:hasValue>blue</owl:hasValue>
> >>      </owl:Restriction>
> >>    </owl:intersectionOf>
> >> </owl:Class>
> >>
> >> Notice that a) the aboutHosts element is not copied from the 
> >> operational semantics - it's not needed here and, I think 
> I'm right 
> >> in saying, won't be of any value in the ordered list in a POWDER-S 
> >> doc either. It could be included but I'm not sure that it 
> will add a 
> >> great deal.
> >> b) there is no subClassOf relation asserted - which is good because
> >> c) there is no URIset to be a sub class of the descriptors.
> >>
> >>
> >> Three questions for people with the appropriate knowledge:
> >>
> >> So the XSLT here must only assert the sub class 
> relationship if there 
> >> is a URISet. Doable?
> >>
> >> I understand that, formally, creating a blank node in an RDF graph 
> >> means that the universe is so arranged that there is at least one 
> >> resource that has the properties given by those of the blank node. 
> >> Does creating an OWL class in this way get us off this hook?
> >>
> >> How does this look, Kai?
> >>
> >> N.B. I'm trying to avoid having to create server-side 
> software that 
> >> returns triples with the described resource's URI as the subject - 
> >> that's clearly the semantically pure way, but it's impractical.
> >>
> >> I'm asking all this because it obviously affects the rules on what 
> >> MUST and SHOULD and MAY be in a POWDER doc - something Andrea's 
> >> poised to encode in the schema and Kevin is poised to 
> enshrine in the 
> >> XSLT.
> >>
> >> Phil.
> >>
> >>
> >>
> >> [1] http://www.w3.org/TR/2007/WD-powder-grouping-20071031/
> >> [2] 
> >> http://www.w3.org/TR/2007/WD-powder-dr-20070925/#package-structure
> >>
> >>
> 
> 
> 
Received on Tuesday, 12 February 2008 14:51:23 GMT

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