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Re: A use case for anon nodes - action from telecon

From: Frank Manola <fmanola@mitre.org>
Date: Thu, 19 Jul 2001 17:07:21 -0400
Message-ID: <3B574C09.7E45D844@mitre.org>
To: Brian McBride <bwm@hplb.hpl.hp.com>
CC: rdf core <w3c-rdfcore-wg@w3.org>
Here's some more on the anonymous nodes discussion, helped considerably
by Brian's translation of his original examples [thanks!].  

so here are Brian's examples, with his translation:

> #advert123 :role "buyer"; 
>            :description [:product :roses; 
>                          :quantity [:units :kg; :minValue "100"]].
> 
> translates to:
> 
> <#advert123> <foo:role>        "buyer" .
> <#advert123> <foo:description> _:service .
> _:service    <foo:product>     <foo:roses> .
> _:service    <foo:quantity>    _:q .
> _:q          <foo:units>       <foo:kg> .
> _:q          <foo:minValue>    "100" .
> 
> i.e this is advert placed by a purchaser who wishes to buy roses in
> minimum quantities of 100.
> 
> and
> 
> #advert456 :role "seller"; 
>            :description [:product :roses; 
>                          :quantity [:units :kg; :maxValue "500"]].
> 
> translates to:
> 
> <#advert456> <foo:role>        "seller" .
> <#advert123> <foo:description> _:service .
> _:service    <foo:produce>     <foo:roses> .
> _:service    <foo:quantity>    _:q .
> _:q          <foo:units>       <foo:kg> .
> _:q          <foo:maxValue>    "500" .
> 
> i.e. this is an advert placed by a seller who can sell roses in
> quantities upto 500.

> Neither is a specific service. The "buyer" advert is essentially a template.
> The seller is offering the existence of at least one service that matches a
> template - he may have many actual instances on offer which differ by location, qos
> etc.

> Here is an offer of specific service: 
> 
> #offer890 :inResponseTo #advert123; 
>           :instanceOf   #advert456; 
>           :service #service42
>  
> #service42 :product :roses; 
>            :quantity [:units :kg; :value "200"]]; 
>            :deliveryTime [:units :days; :value "2"]; 
>            :deliveryMethod "TNT"; 
>            etc 
>            . 

I've translated this to:  

<#offer890>  <foo:inResponseTo>  <#advert123>
<#offer890>  <foo:instanceOf>    <#advert456> 
<#offer890>  <foo:service>       <#service42>
     
<#service42> <foo:product>       <foo:roses> 
<#service42> <foo:quantity>      _:q .
_:q          <foo:units>         <foo:kg> 
_:q          <foo:value>         "200"
<#service42> <foo:deliveryTime   _:t .
_:t          <foo:units>         <foo:days> 
_:t          <foo:value>         "2"
<#service42> <foo:deliveryMethod "TNT"
etc.

Brian says: 

> Now. If we don't have anonymous nodes then we have the following problems.
>  
> (1) In the seller advert it would appear that the seller is only advertising
> a single specific (but under-specified) service, #anon12345 or whatever, which
> would be hard to distinguish from an actual service instance like #service42.
>  
> (2) Similarly in the buyer advert instead of describing a template, giving
> the service a URI would make it appear that I am looking for a specific service
> with that URI.


It seems to me that what's going on is this:  There is a distinction in
RDF between anonymous resources and those with URIs, and Brian's folks
use that as a coding scheme for distinguishing references to templates
from references to specific things.  However, I don't think there there
is any built-in support for this specific interpretation in RDF.  So,
while I suppose people could use that coding scheme, they wouldn't be
wise to assume that everyone else was using URIs and anonymous resources
to make the same distinctions.  It seems to me that the distinction in
RDF is mostly "syntactic" in a sense;  that is, anonymous nodes are
provided as a way to, in certain cases, avoid the need to explicitly
specify a URI in the RDF.  It doesn't necessarily mean that there wasn't
one that couldn't have been provided.  The original "Ora Lassila"
example wasn't really describing the creator of the home page as a
template which matched any of a number of people who happened to be
named "Ora Lassila" and had a specified email address (they might have
meant that, and in fact a given description may actually match several
people, but I claim that wasn't the intent here).  The writer of the RDF
was referring to a specific person.  The immediately following example
in the M&S showed how, if a URI for Ora Lassila existed (and if you
wanted to provide it), you could do so without using an anonymous
resource.  

Looking at this the other way, it seems to me that the specific offer in
Brian's example could just as easily have been expressed using an
anonymous resource, e.g.:

#offer890 :inResponseTo #advert123; 
          :instanceOf   #advert456; 
          :description  [:product :roses; 
                         :quantity [:units :kg; :value "200"]]; 
                         :deliveryTime [:units :days; :value "2"]; 
                         :deliveryMethod "TNT"; 
                          etc ]

which translated is:

<#offer890>  <foo:inResponseTo>  <#advert123>
<#offer890>  <foo:instanceOf>    <#advert456> 
<#offer890>  <foo:service>       _:service
_:service    <foo:product>       <foo:roses> 
_:service    <foo:quantity>      _:q .
_:q          <foo:units>         <foo:kg> 
_:q          <foo:value>         "200"
_:service    <foo:deliveryTime   _:t .
_:t          <foo:units>         <foo:days> 
_:t          <foo:value>         "2"
_:service    <foo:deliveryMethod "TNT"
etc.

This conveys exactly the same information, with the exception of an
explicit URI for the service.  But the buyer doesn't really need to know
the URI of the service;  all he needs to do is reply with an "accept" to
the specific offer, and the seller would understand exactly which
service was meant.   The point is that the offerer in this case might
reply with an anonymous node for the service, but still be referring to
(or at least, able to provide) a specific service.  

Conversely, when you have a specific URI, you know that it is the unique
identifier that someone has assigned to identify something, but you
don't necessarily know what kind of a "something" that is.  For example,
your advert placed by a seller who can sell roses in quantities upto 500
could just as easily been expressed in RDF using a specific URI, as:

#advert456 :role "seller" ;
           :description  #service456 

#service456 :product :roses; 
            :quantity [:units :kg; :maxValue "500"]].

which translates to:

<#advert456>   <foo:role>        "seller" .
<#advert123>   <foo:description> <#service456> .
<#service456>  <foo:product>     <foo:roses> .
<#service456>  <foo:quantity>    _:q .
_:q            <foo:units>       <foo:kg> .
_:q            <foo:maxValue>    "500" .

The seller in this case has chosen to identify a *service description*
with a URI, but that's perfectly OK as far as RDF is concerned (a URI
can denote anything), and in a given application that might be a
perfectly sensible thing to do.   The seller might also have specific
services with their own URIs.  Each kind of resource (service offers and
services) could be defined (say in RDFS) as being of different classes
(so you'd tell them apart by class, not by whether they were URIs vs.
anonymous).  Seller applications dealing with services and service
offers would know how to deal with the differences appropriately;  and
buyers might not have to worry about it, if the service identities
didn't need to be exported.  Even if they did, it seems to me that, in
general, we'd do well to determine what kind of thing was represented by
a URI (or other identifier) on the basis of explicit metadata (e.g., in
RDFS class definitions), rather than on the basis of anonymous resource
vs. URI (after all, there are lots more distinctions than "specific
resource" vs. "variable" that we have to be able to make).

The bottom line here is that while anonymous nodes can certainly be used
(at least in some cases) to make the distinction Brian's example
illustrates, I don't think it's *necessary* for that purpose, and they
aren't necessarily going to be used uniformly by everybody to make those
sorts of distinctions.  (In fact, I'm not terribly confident about how
general a mechanism anonymous resources are for defining template-like
things;  I think if we want variable and template support, we probably
should look at this explicitly and more exhaustively, rather than
assuming that anonymous resources provide what we need for this).  

--Frank

PS:  note also that the _q anonymous resource is used in both
"templates" and specific service offers, and is interpreted  the same
way in each case (what's different isn't the use of _q to indicate an
anonymous quantity resource, but the distinction between precise values
<foo:value> and constraints on values <foo:maxValue>).  

-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Thursday, 19 July 2001 17:12:27 EDT

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