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URI: Name or Network Location?

From: Stephen K. Rhoads <rhoads@thrupoint.net>
Date: Mon, 19 Jan 2004 20:38:18 -0500
Message-ID: <005401c3def6$14df5e90$918afea9@RHOADS03>
To: <www-rdf-interest@w3.org>

Folks,

I am working on an ontology to describe streaming media and find myself
unable to get my head around whether a dereferenced URI of a "typed"
resource should result in a bit of RDF metadata or the data of the resource
itself.  In other words, is the URI specified as the value of the rdf:about
attribute "just a name", or is it to be interpreted as the "network
location" for the data/resource/object itself?

I believe this has implications for semantic-web search engines and their
ability to easily provide hypertext links to users for the objects they
seek -- movies, songs, articles, documents, binaries and any other "typed"
resource under the sun.

I would appreciate any thoughts, insight, examples, or pointers to documents
or previous threads which may enlighten me on this topic.

Consider a simplified example of "The Terminator" -- a resource of type
"mpdo:Movie" -- which is available for streaming from three different
servers for the purposes of fault tolerance and bandwidth.  The three
servers are located in NYC, Seattle, and Houston and (ignoring the potential
for some sort of network or transport-layer solution) each have different
URIs.

I see three different approaches to the design of the ontology which I will
refer to as "Network Object", "Abstract Concept", and "InstanceOf".

(Datatypes and Literals omitted for readability)

1. "Network Object" Approach

<mpdo:Movie rdf:about="rtsp://nyc.example.com/terminator.mp4">
   <mpdo:title>The Terminator</mpdo:title>
   <mpdo:splashSynopsis></mpdo:splashSynopsis>
   <mpdo:synopsis></mpdo:synopsis>
   <mpdo:mpaaRating></mpdo:mpaaRating>
   <mpdo:duration></mpdo:duration>
   <mpdo:releaseDate></mpdo:releaseDate>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>US</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>NY</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US NYC</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>

<mpdo:Movie rdf:about="rtsp://sea.example.com/terminator.mp4">
   <mpdo:title>The Terminator</mpdo:title>

   <mpdo:splashSynopsis></mpdo:splashSynopsis>
   <mpdo:synopsis></mpdo:synopsis>
   <mpdo:mpaaRating></mpdo:mpaaRating>
   <mpdo:duration></mpdo:duration>
   <mpdo:releaseDate></mpdo:releaseDate>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>WA</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>NY</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US SEA</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>

<mpdo:Movie rdf:about="rtsp://hou.example.com/terminator.mp4">
   <mpdo:title>The Terminator</mpdo:title>

   <mpdo:splashSynopsis></mpdo:splashSynopsis>
   <mpdo:synopsis></mpdo:synopsis>
   <mpdo:mpaaRating></mpdo:mpaaRating>
   <mpdo:duration></mpdo:duration>
   <mpdo:releaseDate></mpdo:releaseDate>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>US</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>TX</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US HOU</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>

PROS:

-- The dereferenced URI can be interpreted as the access mechanism for the
Movie itself.  A semantic-web search engine would not need to know what a
"Movie" is or have any intelligence about the attributes of the Movie in
order to present a hypertext link to the user.  As far as the search engine
is concerned, it could be a resource of type "hghghghg" with URI
"oiuoiuoiuoiuoiuoiu".

CONS:

-- Without some type of ISBN-like attribute-value pair, a machine has no way
of knowing that these three Descriptions actually refer to the same Movie.

-- The ability of the URI to provide a sort of globally-unique identifier is
lost.

-- Metadata is repeated for every instance, increasing the potential for
inaccuracy.

2. "Abstract Concept" Approach

<mpdo:Movie rdf:ID="Terminator">
   <mpdo:title>The Terminator</mpdo:title>

   <mpdo:splashSynopsis></mpdo:splashSynopsis>
   <mpdo:synopsis></mpdo:synopsis>
   <mpdo:mpaaRating></mpdo:mpaaRating>
   <mpdo:duration></mpdo:duration>
   <mpdo:releaseDate></mpdo:releaseDate>
   <mpdo:source>
      <mpdo:Source rdf:about="rtsp://nyc.example.com/terminator.mp4">
         <mpdo:mediaType>video/mp4</mpdo:mediaType>
         <mpdo:bitrate>512</mpdo:bitrate>
         <mpdo:origin>
            <mpdo:GeographicalLocation>
               <mpdo:countryCode>US</mpdo:countryCode>
               <mpdo:countrySubdivisionCode>NY</mpdo:countrySubdivisionCode>
               <mpdo:cityCode>US NYC</mpdo:cityCode>
            </mpdo:GeographicalLocation>
         </mpdo:origin>
      </mpdo:Source>
   </mpdo:source>
   <mpdo:source>
      <mpdo:Source rdf:about="rtsp://sea.example.com/terminator.mp4">
         <mpdo:mediaType>video/mp4</mpdo:mediaType>
         <mpdo:bitrate>512</mpdo:bitrate>
         <mpdo:origin>
            <mpdo:GeographicalLocation>
               <mpdo:countryCode>US</mpdo:countryCode>
               <mpdo:countrySubdivisionCode>WA</mpdo:countrySubdivisionCode>
               <mpdo:cityCode>US SEA</mpdo:cityCode>
            </mpdo:GeographicalLocation>
         </mpdo:origin>
      </mpdo:Source>
   </mpdo:source>
   <mpdo:source>
      <mpdo:Source rdf:about="rtsp://hou.example.com/terminator.mp4">
         <mpdo:mediaType>video/mp4</mpdo:mediaType>
         <mpdo:bitrate>512</mpdo:bitrate>
         <mpdo:origin>
            <mpdo:GeographicalLocation>
               <mpdo:countryCode>US</mpdo:countryCode>
               <mpdo:countrySubdivisionCode>TX</mpdo:countrySubdivisionCode>
               <mpdo:cityCode>US HOU</mpdo:cityCode>
            </mpdo:GeographicalLocation>
         </mpdo:origin>
      </mpdo:Source>
   </mpdo:source>
</mpdo:Movie>

PROS:

-- It is clear to both machine and human that this a single Movie with
several "Sources".

-- The URI of the Movie serves as a unique identifier.

-- Metadata about the movie appears only once.

CONS:

-- The dereferenced URI results in a bit of RDF metadata about the Movie -- 
not the Movie itself.  No assumptions can be made about the URI of the Movie
or its usefulness as a hypertext link.

3. "InstanceOf" Approach

<mpdo:Movie rdf:ID="Terminator">
   <mpdo:title>The Terminator</mpdo:title>

   <mpdo:splashSynopsis></mpdo:splashSynopsis>
   <mpdo:synopsis></mpdo:synopsis>
   <mpdo:mpaaRating></mpdo:mpaaRating>
   <mpdo:duration></mpdo:duration>
   <mpdo:releaseDate></mpdo:releaseDate>
</mpdo:Movie>

<mpdo:Movie rdf:about="rtsp://nyc.example.com/terminator.mp4">
   <mpdo:instanceOf rdf:resource="#Terminator"/>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>US</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>NY</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US NYC</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>

<mpdo:Movie rdf:about="rtsp://sea.example.com/terminator.mp4">
   <mpdo:instanceOf rdf:resource="#Terminator"/>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>US</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>WA</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US SEA</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>

<mpdo:Movie rdf:about="rtsp://hou.example.com/terminator.mp4">
   <mpdo:instanceOf rdf:resource="#Terminator"/>
   <mpdo:mediaType>video/mp4</mpdo:mediaType>
   <mpdo:bitrate>512</mpdo:bitrate>
   <mpdo:origin>
      <mpdo:GeographicalLocation>
         <mpdo:countryCode>US</mpdo:countryCode>
         <mpdo:countrySubdivisionCode>TX</mpdo:countrySubdivisionCode>
         <mpdo:cityCode>US HOU</mpdo:cityCode>
      </mpdo:GeographicalLocation>
   </mpdo:origin>
</mpdo:Movie>


PROS:

-- The dereferenced URI of the three "instances" can be interpreted as the
access mechanism for the resources specified.

-- The URI of the "primary" Movie (the one which is not declared to be an
"instance") serves as a unique identifier.

-- Those AV pairs which are constant appear only once.

-- It is clear to both machine and human that this a single Movie with
several "Instances".

CONS:

-- The dereferencing of the "primary" Movie object results in a bit of RDF,
not the data itself.
Received on Monday, 19 January 2004 20:55:20 GMT

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