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Re: SeRQL an RDF rule language: scoping Rules vs Query in W3C work

From: Jeen Broekstra <jeen@aduna.biz>
Date: Tue, 04 Nov 2003 12:07:10 +0100
Message-ID: <3FA7885E.4010307@aduna.biz>
To: Dan Brickley <danbri@w3.org>
Cc: sesame-devel@lists.sourceforge.net, www-rdf-rules@w3.org

Dan Brickley wrote:

> Within W3C, we're looking into phase 2 of the Semantic Web
> activity.
> In terms of possible new technology areas, 'Rules' and 'Query' 
> are two topics for recommendation-track work.
> So I'm looking at 
> http://sesame.aidministrator.nl/publications/users/ch05s06.html
> with some interest. The CONSTRUCT mechanism appears to provide a
> bridge between the world of RDF query systems and RDF-based rule
> systems.
> CONSTRUCT {Artist} <rdf:type> {<art:Painter>}; <art:hasPainted>
> {Painting} FROM {Artist} <rdf:type> {<art:Artist>}; 
> <art:hasCreated> {Painting} <rdf:type> {<art:Painting>}
> In this light, do folks on these lists think it is sustainable to
>  maintain that there's an interesting distinction still to be
> made between work on RDF 'query' languages vs 'rules' languages.

Allow me to make some broad, sweeping statements :)

My gut reaction is that although the technologies are very similar,
the use cases are different. This may reflect on requirements for
ease of use, expressiveness and syntax.

We are ourselves not really fixed on a particular QL syntax, but in
developing SeRQL we have deliberately tried to take away some of the
pains developers were having with the RQL syntax (which was
sometimes difficult for users to understand) and have taken special
care to come up with a syntax that was easy to use, extensible and
expressive (as well as easy to implement a parser for). Syntax is
not a trivial issue from a usability perspective, and this may well
be the main distinguishing factor between a rule language and a
query language.

> Can folks here imagine a workable W3C RDF Query WG constrained
> not to get into Rules WG territory, but to maximise compatibility
> with a (future? parallel) Working Group on Rule languages for
> RDF? Or are the two technology areas too close?

It would be hard to envisage a usable (expressive enough) RDF query
language that does not at least partially intrude on rules
territory: after all, a query is generally speaking just a rule
without a head. In requirements for expressivity, operations, and
tranformation possiblities I would imagine that any W3C-endorsed
query language would support a subset of the operations a rule
language supports. I say subset because, IMHO, things like validity
constraints and negative conclusions ("given X and Y, Z must be
false") are outside the scope of a query language (I'm sure at least 
some of you disagree here, though ;)).

The question to my mind is whether a common syntax format can be
found that satisfies the two user communities (and note that this is
a simplification: there is no single QL user community but many
different agendas - as reflected by the plethora of QL proposals out
there - and I imagine the same is true for rules).

In my opinion, a good place to start investigating the
viability/usefulness of seperate specs is not so much looking at the
technology, but looking at use cases. Wouldn't it be a good idea to
collect use cases for both queries and rules (and document the
requirements they pose on expressivity and syntax), and then see how
great the overlap is from that perspective?

Just my two cents,

Aduna (formerly Aidministrator b.v.) - http://www.aduna.biz
Julianaplein 14b, 3817 CS Amersfoort, The Netherlands
tel. +31-(0)33-4659987, fax. +31-(0)33-4659987
Received on Tuesday, 4 November 2003 06:06:03 UTC

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