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r past
ver the pa
ouple o y
co p of years
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he mantic
th Sem
Web ba g
andwagon
as eally started o g. ub cat o s a g g o the popular press
has real y sta ted rolling. Publications ranging from t e popu a p ess
op
to the to scientific journals have printed excited articles claiming the
le
this articl we’
perhaps more important, attempt to dispel a pervasive myth hat XML
and the Semmantic Web are somehow incompatible.
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X
ML-based formats are now becoming the dominant street-address-line> and elements. From the pure something complex, such as integrating s lists, calendars, sensors, services, and all the other resources on
way of marking up data on the Web. There are standard XML point of view, there’s nothing but a document-based descrip- with our personnel database, we would have to rely on a human the Web. Done right, such links would allow information in vari-
XML languages for hypertext (XHTML), graphics tion of the names and current arrangements of these elements: programmer. We’d need someone to provide a specific mapping ous forms and formats to be automatically manipulated in a
(SVG), syndication (RSS), multimedia (SMIL), service descrip- that is, that a contains a , which is a from the to the various fields in our database coherent way by our computer programs. XML is clearly an
tion and discovery (WSDL and UDDI), and many others, not string. We could, of course, also have a document with two s in it. An XML Schema for s could go fur- changes in our XML format or in the database – and just forget data model as a feasible way to represent these links and the real-
based languages. ther and state the specific constraints on that structure, for exam- about reusing that mapping with unrelated systems. world semantics they encode.
Furthermore, there are standard ways to query (XPath and ple, that a can contain many elements but only one element. With this s to more general facts about addresses – for Modeling Links
gram with (SAX, and DOM) your XML documents. Finally, schema a validator can conclude that our example, that they identify locations, that mail can be delivered By examining how links work on the current Web, we can get
integration with the many other XML formats can be done in with two s was syntactically incorrect. to them, or that they’re associated with people or businesses. some idea of what it takes to model them. Web links have two key
a reasonably straightforward manner either by inclusion However, the schema doesn’t actually capture much of the And neither the XML model nor XML Schema are going to be aspects: first, things on the Web are consistently identified by
(using XML Namespaces) or transformation (using XSLT). semantics of an address. For example, if we were to take several much help in determining that a mailing address is like a fax URIs (Uniform Resource Identifiers). Second, a Web link has the
different documents and find two addresses that were identical number, at least in the sense that mailing a letter to an address following tripartite structure:
XML as an Interchange Mechanism
XML Labs
except for their zip codes, we wouldn’t be able to realize, on the and faxing it to a number both get the text of the missive to its • The thing you start with (the source of the link). (In XHTML
Given these facts, it’s hard to argue against XML as the inter- basis of the schema, that at least one zip code was wrong. recipient. this is the file containing the “a” tag with an “href” attribute.)
change mechanism for the Web. But the XML model remains pri- Similarly, there’s no automatic way to convert between two very On the Web, however, we really need this semantic informa- • The connecting, or linking, bit between the source and the
marily rooted in documents – in particular, textual documents similar encodings of address (for example, to one where there tion. We want to be able to connect an address that we’ve encod- target of the link.
with hierarchical structure. For example, a well-formed XML doc- could only be one element, which itself had ed in XML in some Web document to people, places, concepts, • The thing you end up with (the target). (In XHTML it’s typi-
ument can have a element that contains s). Further, if we wanted to do letters, other documents, databases, directories, PDA contact cally named named by a URI placed in the href attribute.)
30 October 2002 www . XML-JOURNAL . com October 2002
While, naturally, it’s possible to encode Web links in XML Most notably, RDFS defines class and property relations and
(e.g., with ), note how little in common a Web link provides a mechanism for restricting the domains and ranges
has with the XML data model: Web links use URIs instead of tags of the properties. In RDFS, for example, we can express site-
or QNames; unlike XML, Web links impose no inherent hierar- specific Yahoo-like Web site categories as a hierarchy of class-
chy, no notion of containment, and no sequencing of the things es with sets of named (sometimes inherited) properties. This
to which they relate. In fact, a set of Web links doesn’t look much allows other sites to connect their own information to these
like a DOM tree, but does look an awful lot like an RDF terms, providing better interoperability for use in B2C, B2B, or
HOME
(Resource Description Framework) graph, where other Web transactions.
each link corresponds to an RDF triple. After all, OWL extends RDFS into a more capable language, usable
each part of an RDF triple can be, and most for thesauruses and domain models. OWL is based on
often is, identified by a URI, and the struc- DAML+OIL, a Web language jointly developed by the U.S.
ture of a triple is obviously tripartite. Defense Advanced Research Projects Agency (DARPA) and the
“ “RDFS is a simple language for
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creating Web-based ‘controlled
2000-2002
vocabularies’ and taxonomies”
7
The parts of a triple, by design, correspond to
the parts of a Web link:
• The subject of a triple is where you start.
• The predicate connects the subject and the
object.
• The object corresponds to the target of a Web
link.
European Union’s Information Science and Technology (IST)
program. DAML+OIL has begun to get heavy use in the gov-
ernment, and in November 2001 the W3C chartered a Web
Ontology Working Group to refine the DAML+OIL standard
into a W3C recommendation language, now called OWL.
OWL extends RDFS with many more constructs for defin-
ing the relationships between classes and, more important,
” YEARS
83
ISSUES
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1400+
placing restrictions on how properties (i.e., predicates) can be
Indeed, an RDF triple is a representation of a Web link, used when linking entities. OWL thus allows users to define
where each part of the link is made explicit. Thus a collection of simple models of their domains using these properties and
RDF triples is a way to represent, share, and process chunks of their constraints. A full discussion of the language is beyond
-
the Web itself. Having the Web in a standardized representation
allows us to enrich the semantics not just of information in
this article (see www.w3.org/2001/sw/WebOnt for more
details), but revisiting the “Address” example should make a
ARTICLES
documents on the Web, but of the information expressed by the few things clearer.
Web. Every Web link is an often vague, usually ambiguous, and
almost always underspecified assertion about the things it con- XML and OWL
EVERY ISSUE OF
WSJ & JDJ ONE
nects. RDF lets us eliminate that vagueness and nail down the In XML we were able to say that there was a document field
ambiguity. RDF Schema (RDFS) and the new Web Ontology called a that had subfields of a street address, a city
Language (OWL) allow us to model the meaning of our asser- name, a state name, and a zip code. In OWL we can explicitly name
tion links in precise, machine-processible ways. these objects as classes and properties, and place constraints on
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RDFS is a simple language for creating Web-based “con- how to legally relate these entities to each other or to entities
trolled vocabularies” and taxonomies. The language defines defined in other documents. Thus, for example, we could mention
EVER PUBLISHED
several RDF predicates for making links between concepts. that cities are in states, and that each city is in one, and only one,
state. We could know that a U.S. address is a type of international
address where the state field is restricted to be one of Alabama,
Maine, New York, and so on, and that these addresses have zip
codes that consist of either five or nine numbers. We could also
CD
add the information that, in general, international addresses
have country codes, and that the country code for U.S. address-
THE MOST COMPLETE LIBRARY OF
es always has the value “USA,” and many other such facts. (This
sort of specification of these relationships in a formal language is
EXCLUSIVE WSJ & JDJ ARTICLES ON ONE CD!
called ontology, thus the term ontology language for OWL.)
Ontologies let us more precisely link to other documents
and resources based on shared use of conceptual terms, even "The Secrets of the
where there is only a partial match (a key difference from cur-
Web Services Masters"
XML Labs
rent XML-based approaches). Our addresses could thus be
linked in turn to other vocabularies – for example, knowing that CD is edited by well-known editors-in-chief
an address names a location, we could link to other location- Sean Rhody and Alan Williamson
based Web resources. These could be databases or Web services
and organized into more than 50 chapters
that would compute the location of the nearest airport (another
FIGURE 1 Linking a Web site to ontological information kind of location) to a given address, the weather forecast for the containing more than 1,400 exclusive WSJ & JDJ articles.
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city the address is located in, or other such location-specific the (for them) superfluous intricacies of XML. XML experts
data. Metadata can also be used to link nontext media to shake their heads at the way the RDF/XML serialization abus-
ontologies, expressing, for example, that the photo in a particu-
lar picture is of a house at a particular address or that the place
to complain about the contents of a particular streaming video
es QNames and XML Namespaces and treats certain attributes
and child elements as equivalent. However, these kinds of
complaints are nothing new. In fact, they’re common in the
$ ON
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is in a particular state (allowing you to compare its location to XML community itself: witness the fury that some XML people
yours and see if local content restrictions might apply). express over XSLT’s use of QNames as attribute content (to
SUBS
HOME
Figure 1 provides an example of the linking of a Web site to pick one example). Similarly, the RDF world has plenty of dark E
L VALU NA
ontological information. In this case, from a presentation on and overcomplicated corners. Both sets of languages are also UCATIO
OWL given at the W3C session of the World Wide Web continuing to evolve, and each is also exploring new non-XML BEST ED THE YEAR!
OF
Take Your Career
Conference in May 2002, information about the keynote syntaxes (consider Relax-NG, XQuery, and XPath).
speaker is linked to information about events, photos, and
people. Best of the Best EACH CITY WILL BE
The example of addresses is an extremely basic one, yet we In short, the Semantic Web offers powerful new possibili- SPONSORED BY A LEADING
already see a tremendous number of possible uses. By mapping
the implicit semantics inherent in XML DTDs and schemas into
the explicit relationships expressible in RDFS and OWL, a whole
range of new applications, largely created by the linking of exist-
ties and a revolution in function. These capabilities will arrive
sooner if we stop squabbling and realize that the rift between
XML- and the RDF-based languages is now down to the minor
sorts of technical differences easily ironed out in the standards
to the Next Level! WEB SERVICES COMPANY
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Web Services
new and old protocols. Great insight into the
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Very articulate on the Web services SOAP topic
Web Services
easily implementable. In the the best of all these languages, and their variants, is easily
and well-prepared for many questions. I've
learned a lot from this seminar and I
Applications
Applications BE SURE NOT TO
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appreciate this seminar for my job. Thank you!”
business world this kind of enabled by the combination of the “documents” of the XML
linking to models could be Web with the “links” expressed in RDF. Throw interoperable
– Kenneth Unpingco, Southern Wine
& Spirits of America MISS THESE...
done for SEC filings, supply-chain Web services into the mix and the vision is compelling. The “
I liked the overview of Web services and the
use of specific tools to display ways to
TAUGHT BY THE INNOVATORS
AND THOUGHT LEADERS IN
databases, business services posting WSDL descrip- future of the Web can be even more exciting than its past, and distribute Web services. Good for getting up to …COMING TO A CITY NEAR YOU
tions, and a virtually infinite range of others, allowing pulling all these threads together will get us there. speed on the concepts.”
– B. Ashton, Stopjetlag.com
WEB SERVICES
enterprise integration on a Web-wide scale. Current EXPERT PRACTITIONERS TAKING AN APPLIED 2002
Semantic Web–related research is also exploring the use Useful Links Echoed over and over by
APPROACH WILL PRESENT TOPICS INCLUDING SAN JOSE...........................................................................OCTOBER 3
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Web Services Edge World Tour
and extension of these RDF-based languages to express • W3C Semantic Web activity: www.w3.org/2001/sw Attendees: LOS ANGELES................................................................NOVEMBER 5
BASIC TECHNOLOGIES SUCH AS SOAP, WSDL,
trust and authorization relationships, to do the automated • The RDF Schema Language Specification – Working Draft: “Good balance of theory and demonstration.” NEW YORK.................................................................NOVEMBER 18
discovery and composition of Web services, and to design www.w3.org/TR/rdf-schema UDDI AND XML, PLUS MORE ADVANCED
“Excellent scope and depth for my background SAN FRANCISCO.............................................................DECEMBER 3
new languages to continue to enhance the potentially rev- • “Integrating Applications on the Semantic Web” (paper by J. at this time. Use of examples was good.” ISSUES SUCH AS SECURITY, EXPOSING LEGACY
BOSTON.............................................................................DECEMBER 12
olutionary capabilities of the Semantic Web. Hendler, T. Berners-Lee, and E. Miller on using the Semantic “It was tailored toward my needs as a novice SYSTEMS AND REMOTE REFERENCES.
to SOAP Web services – and they explained WEST
The Semantic Web is being built on models based on Web for business applications): www.w3.org/2002/07/swint everything.” 2003
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the RDF representation of Web links. To achieve their full • W3C Web Ontology Working Group home page: www.
impact, however, the enhanced models enabled by the
Semantic Web crucially need to be tied to the document-pro-
w3.org/2001/sw/WebOnt
• Feature Synopsis for OWL Lite and OWL – Working Draft: WHO SHOULD ATTEND: SPONSOR CHARLOTTE.......................................................................JANUARY 7
MIAMI ...........................................................................JANUARY 14
cessing and data-exchange capabilities enabled by the spread
of XML technologies. If XML- and RDF-based technologies
www.w3.org/TR/owl-features
• “Why RDF model is different from the XML model,” by T.
• Architects
• Developers
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were incompatible, as some people seem to think they are, it Berners-Lee: www.w3.org/DesignIssues/RDF-XML.html • Programmers NEW YORK, NY (Doubletree Guest Suites) SOLD OUT!
would be a true shame. But, in fact, they aren’t. While the • IS/IT Managers
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underlying models are somewhat different, the normative AUTHOR BIOS • C-Level Executives ATLANTA ..................................................................................MAY 13
LD
document exchange format for RDF, RDFS, and OWL is XML. Jim Hendler, a University of Maryland professor, is the director of Semantic Web and agent tech- • i-Technology Professionals to discuss how MINNEAPOLIS .........................................................................JUNE 10
Thus, to those preferring to think of the whole world as XML nology at the Maryland Information and Network Dynamics Laboratory. A Fellow of the American REGISTER WITH A COLLEAGUE AND SAVE 15% OFF THE LOWEST REGISTRATION FEE.
based, RDF, RDFS, and OWL may simply be thought of as yet Association for Artificial Intelligence, Jim was formerly chief scientist for information systems at the
another XML language to be managed and manipulated using U.S. Defense Advanced Research Projects Agency (DARPA) and cochairs the Web Ontology
the standard toolkit. To the RDF purist, the documents and Working Group for the W3C.
datasets being expressed in XML and XML Schema can anchor
their models with interoperable data. To those focused on the Bijan Parsia is a Semantic Web researcher at the Maryland Information and Network Dynamics
XML Labs
world of Web services, SOAP and WSDL can carry, in their XML Laboratory. His research interests include Web logics and rule engines, Semantic Web services,
Developing SOAP Web Services
content, RDF models expressing information that can be easi- fine-grained, reflective annotation systems, and trust-focused reasoning. TOPICS HAVE INCLUDED: Architecting J2EE Web Services
ly found, linked, and discovered. PRESENT YOUR COMPANY’S EXPERTS TO AN EAGER AUDIENCE
HENDLER@CS.UMD.EDUNET
Of course, as is the case with any groups doing overlapping READY TO LEARN FROM YOU! ACT TODAY! The San Francisco tutorial drew a record 601 registrations.
tasks, there is friction between some in the RDF community
and some in the XML world. RDF folks often complain about BPARSIA@ISR.UMD.EDU
REGISTRATION FOR EACH CITY CLOSES THREE BUSINESS DAYS BEFORE EACH TUTORIAL DATE. DON’T DELAY. SEATING IS LIMITED. TO REGISTER:
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