letters to the editor
DOI: 10.1145/1467247.1467250
children’s magic Won’t Deliver the semantic Web
To explain the nature of
“Ontologies and the Semantic Web” in his contributed
article (Dec. 2008), Ian Horrocks, a leading figure behind the theory and practice of Description Logics (DLs), employed analogous
characters and language of the fictional Harry Potter children’s novels. Notwithstanding the fact this did not help
readers not already familiar with Potter or even those, as there may exist a
few, who find the novels utterly boring
and repetitive, hearing the same story
over again in a new guise prompts me
to ask: When will such presentations
evolve from toy examples into more
realistic accounts of larger, complex
ontologies? That is, when will the important issue of scalability in the storage, retrieval, and use of large ontologies (millions of concepts, hundreds of
millions of roles/attributes, nontrivial
reasoning) be addressed?
Horrocks wrote, “A key feature of
OWL is its basis in Description Logics, a family of logic-based knowledge-representation formalisms that are
descendants of Semantic Networks
and KL-ONE but that have a formal
semantics based on first-order logic.”
While this may be true, it could also
mislead a neophyte to conclude that
DL is somehow the only formalism for
representing and using ontologies.
This is far from true. There is at least
one alternative formalism, also a direct
descendant of KL-ONE—Order-Sorted
Feature (OSF) constraint logic —that
a
lends itself quite well to the task. Elsewhere, I also covered how various DLs
and OSF constraint logics formally relate to one another.b
The trouble I see in such publications by influential members of the
a Ait-Kaci, H. Data models as constraint systems:
A key to the semantic Web. Constraint Programming Letters 1 (Nov. 2007), 33–88; www.cs.brown.
edu/people/pvh/CPL/Papers/v1/hak.pdf.
b Ait-Kaci, H. Description logic vs. order-sorted
feature logic. In Proceedings of the 20th International Workshop on Description Logics. Lecture
Notes in Computer Science. Springer-Verlag,
2007; sunsite.informatik.rwth-aachen.de/Pub-
lications/CEUR-WS/Vol-250/paper_ 2.pdf.
World Wide Web Consortium (W3C) is
that one particular formalism—DL—
is being confused with the general issue of formal representation and use
of ontologies. It would be like saying
Prolog and SLD-Resolution is the only
way to do Logic Programming. To some
extent, the LP community’s insistence
on clinging to this “exclusive method”
has contributed to the relative disinterest in LP following its development
in the 1980s and 1990s. Similarly, DL
formalists have built a de facto exclusive reasoning method—Analytic
Tableaux—into their formalism so the
same causes always result in the same
consequences.
Whether the various languages proposed by the W3C are able to fly beyond
toy applications has yet to be proved,
especially in light of the huge financial
investment being poured into the semantic Web. To realize this promise, we
must not mistake the tools for the goal.
Indeed, while DLs are admittedly one
tool among several for representing and
using ontologies, the goal is still to make
semantic Web ontology languages work,
no matter which method is used, as long
as it is formal, effective, and efficient on
real data. Otherwise, the semantic Web
might well end up being built on nothing more than children’s magic.
hassan aït-Kaci, Vancouver, Canada
author’s Response:
The Harry Potter example was not
intended to be representative of realistic
application ontologies. As I discussed in
the article, such ontologies are often large
and complex, making them unsuitable for
didactic purposes.
I certainly didn’t mean to suggest that
DL is the only possible formal basis for an
ontology language. However, it is important
to agree on the use of some formalism in
order to facilitate the exchange and reuse of
ontologies and encourage the development
of the tools and infrastructure needed for
large-scale ontology development and
deployment. This is a major success of
RDF and OWL; users now have access
to a previously undreamt of range and
quality of tools and is a major factor in their
popularity.
Finally, the W3C standards relate only
to the languages themselves, leaving
the design and implementation of tools
to developers. The OWL standard does
not specify any particular reasoning
algorithm, and existing OWL/DL reasoners
are based variously on (at least) analytic
tableau, resolution, hyper-resolution, query
rewriting, saturation, and rule-extended
triple stores.
ian horrocks, Oxford, U.K.
Give me the science of
Virtualization, not Buzzwords
The “CTO Roundtable on Virtualization, Parts I and II” moderated by Mache Creeger (Nov. and Dec. 2008) was a
rambling discussion filled with vague
assertions, buzzwords, and brand
names but few clear concepts. The anecdotal discussion touched on cloud
computing, late binding, even the terror attacks of 9/11, without clear logical
sequence or relationship with deeper
(unstated) definitions or principles.
As far as I know, VM is an operating
system concept first implemented by
IBM 40 years ago on its punched-card-era mainframes (360– 67) and commercially available on PCs for at least the
past 10 years. VM was invented for essentially the same reasons it is used today:
run multiple operating systems on one
machine in fully isolated ways. Some of
these operating systems may be less reliable than others or may still be under
test but are unable to interfere with one
another. Even if we are talking about the
same thing, the roundtable highlighted
none of these basic concepts. VM was
widely used within a few years of its earliest implementation. One roundtable
participant (in Part I, Nov. 2008) said: “I
support virtualization.” OK, so I support
transistor radios.
To me, this is further confirmation
of the fact that IT progress is fast on
the surface but slow in terms of basic
concepts.
Luigi Logrippo, Gatineau,
Québec, Canada