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Re: what RDF is not (was ...)

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 02 Jan 2002 14:14:55 -0500
To: sandro@w3.org
Cc: peter.crowther@networkinference.com, jjc@hplb.hpl.hp.com, www-rdf-interest@w3.org
Message-Id: <20020102141455R.pfps@research.bell-labs.com>
From: Sandro Hawke <sandro@w3.org>
Subject: Re: what RDF is not (was ...) 
Date: Wed, 02 Jan 2002 12:18:42 -0500

> Here's my little rant on what RDF is.   Not directed at anyone in
> particular. 
> 
> RDF is a language for transmitting pieces of collaborative databases.
> It started as a way to categorize web pages, but since the subject
> matter of the web is arbitrary, RDF ended up as a way to express
> arbitrary information, just like one might store in a relational DBMS.
> The pieces of RDF are pieces of a web-wide database of information,
> not just about web pages but about anything.

Well sort of.  RDF cannot express arbitrary information, of course, and
neither can a DBMS.    You indicate that this is the case below,
contradicting your statements in this paragraph.

[You may be thinking that information is different from knowledge.  If so,
I would like to hear how you make the distinction.]

> While SQL is a database manipulation and query language, RDF is just a
> data format, equivalent to the tables that result from a SQL query or
> to an on-disk database file format.  (RDF still needs a SQL-equivalent
> language.)  RDF's database model is different from SQL's in being
> "webized" to support distributed collaboration: tables/columns and
> datatypes are named in a global namespace (URIs) so they can be
> automatically linked.
> 
> There is a temptation to think a mass of RDF fragments can store all
> of human knowledge.  The truth is that RDF is only marginally better
> than a typical SQL database for storing "knowledge".  It works well
> for a catalog of the CDs you own, or the products you sell, or the
> configurations of software installed on your computers, but the only
> thing it does for "knowledge representation" and "machine reasoning"
> is provide a standard underlying format.

I would like to know how RDF can provide a ``standard underlying format''
for knowledge representation, in a way that is different from the way that
sequences of bits can.

> (If RDF sounds a lot like XML, well, it is.  The difference is that an
> XML database fragment is less self-describing than an RDF one.
> Whether this difference is critical is a subject of debate.  Whether
> either of them is better than a comma-separated-values file is also
> subject to debate.  The basic question is whether self-description is
> important.)
> 
> So how do you encode some knowledge like "All men are mortal" or "Only
> 3 Sale-Items Per Customer" in RDF?  The same way you do in SQL: you
> don't.  You need another mechanism - some logic somewhere else in the
> system.  It may, however, be a standard logic, driven by information
> also in the database.  That is, the database can hold software written
> in some programming or constraint language (Perl, Python, i386 machine
> code, first-order predicate logic, DAML, RDFS, etc), and there can be
> conventions about how apply that knowledge to other knowledge in the
> database (eg for database validation or inference).
> 
> Putting other-language elements into a database like this is a common
> design style for complex database applications.  Additionally,
> database systems which do validation or inference (as many of them now
> do) often make available data views of the logic-language expressions.
> It's a fairly obvious technique.  RDF may cloud the issue by
> encouraging a different encoding style, where you encode each
> logic-language token (instead of each whole ASCII expression) in
> separate RDF objects.  This somehow makes the language look more like
> it's "in" RDF or extending RDF; the truth is, for RDF, it's only data
> in the database.
> 
>      -- sandro                 http://www.w3.org/People/Sandro/

peter
Received on Wednesday, 2 January 2002 14:15:59 GMT

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