Some questions

Ron Daniel RDaniel at
Wed Dec 1 20:23:04 GMT 1999

Tim Bray said:
At 10:34 AM 12/1/99 -0500, Sherriff, Joel wrote:
>Can somebody explain, in a nutshell, the purpose of RDF.  After reading the
>spec, I don't feel any more educated than before.

My attempt to explain RDF is at

You may also want to take a look at Tim Berners-Lee's
document on "Describing and Exchanging Data":

but here is my attempt to explain RDF's purpose in a 

   The purpose of RDF is to provide metadata (data about
   other data) in a manner that is very easy to process
   by machines.

and here is my attempt to give an example of why RDF
is useful:

Assume you have a bunch of XML documents from a variety of
sources, many of which contain an <author> element, and your
job is to build a simple card-catalog style database so that
you can search by author and get the documents written by
that person.
Also assume you have an XML-aware version of a
tool like grep that lets you search the documents for the <author> element.
Like grep, this tool prints the filename where a match
was found. Unlike grep it prints the content of the matched element
rather than a line. (This seems like a reasonable minimum
functionality for an XML-aware grep-like tool).

That tool should make the job easy. Search for <author> elements,
pipe the output to 'cut', and you can make a text file
ready for import into your database. But there is one little
hitch - you can't assume that the person identified in the <author>
element of file X is the author of file X. Maybe file X is
saying that they are really the author of document Y. Without
knowledge of the convention followed in each file you can't tell.
And since the files came from a lot of sources, you are talking
a lot of work to see what conventions are being followed.

RDF does not leave this important information implicit. Each
RDF statement has exactly three parts:
   Subject - the thing being talked about (the documents in
     the example above).
   Predicate - the type of statement being made about the subject
     (author in this example)
   Object - the value portion of the statement (the author name
     in this example).

If the data was expressed in RDF, an RDF-aware grep-like
tool would let you select all the RDF properties labeled
"author", get the URIs of the resource and the name of the
author, and plop that info into the database. There would
be no ambiguity about the thing which was authored.

This regularity in the form of expression is key to making
the metadata easy to process by machines.


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