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1 © NOKIA 10/12/2014 - Ora Lassila RDF: A Frame System for the Web Ora Lassila (with help from Henry S. Thompson) Agent Technology Group Nokia Research.

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Presentation on theme: "1 © NOKIA 10/12/2014 - Ora Lassila RDF: A Frame System for the Web Ora Lassila (with help from Henry S. Thompson) Agent Technology Group Nokia Research."— Presentation transcript:

1 1 © NOKIA 10/12/ Ora Lassila RDF: A Frame System for the Web Ora Lassila (with help from Henry S. Thompson) Agent Technology Group Nokia Research Center, Boston March 2000

2 2 © NOKIA 10/12/ Ora Lassila Contents Brief RDF primer + "problems" Brief discussion of Frame Systems + interpretation of RDF as a Frame System Future (DAML etc.) My main concern is with the adoption of RDF - this has been hindered by people not understanding what RDF is about…

3 3 © NOKIA 10/12/ Ora Lassila What is RDF, really? RDF is a data model the model is domain-neutral, application-neutral and ready for internationalization (i18n) the model can be viewed as directed, labeled graphs or as an object- oriented model (object/attribute/value) the specification provides an encoding (in XML) of the model important: syntactic details are secondary, they are largely handled by using XML (RDF defines a convention of XML usage) RDF data model is an abstract, conceptual layer independent of XML consequently, XML is a transfer syntax for RDF, not a component of RDF RDF data might never occur in XML form it might reside, for example, in an RDB XML relieves us of syntactic details when transporting RDF

4 4 © NOKIA 10/12/ Ora Lassila RDF model RDF “statements” consist of resources (= nodes) which have properties which have values (= nodes, strings) “Ora Lassila” author = subject = predicate = object predicate(subject, object) “http://www.w3.org/TR/REC-rdf-syntax/ has the author Ora Lassila” resource value property

5 5 © NOKIA 10/12/ Ora Lassila RDF model (continued) Graphs can be represented as relational “triples” { “http://www.w3.org/TR/REC-rdf-syntax/”, author, “Ora Lassila” } Any arc in a graph corresponds to one triple, consisting of the arc label itself and the two endpoints Why is all this useful? For example, when searching, this is useful only if we know what the meaning of “author” is e.g., “find all documents where the author is Ora Lassila” new triple { “http://www.w3.org/TR/REC-rdf-syntax/”, dc:Creator, “Ora Lassila” } “author” replaced by a label which is uniquely qualified (using the XML namespace mechanism) Drawn from a consensually pre-established vocabulary

6 6 © NOKIA 10/12/ Ora Lassila RDF model (example) “Ora Lassila” dc:Creator “ ” dc:Date “W3C” dc:Publisher

7 7 © NOKIA 10/12/ Ora Lassila Complex values So far, values of properties have been strings A graph node (corresponding to a resource) also can be the value of a property arbitrarily complex tree and graph structures are possible syntactically, values can be embedded (i.e. lexically in-line) or referenced (linked) Example: “Ora Lassila” dc:Creator p: p:Name

8 8 © NOKIA 10/12/ Ora Lassila Complex values (continued) Corresponding triples { “http://www.w3.org/TR/PR-rdf-syntax/”, dc:Creator, x } { x, p:Name, “Ora Lassila” } { x, p: , } “Ora Lassila” dc:Creator p: p:Name

9 9 © NOKIA 10/12/ Ora Lassila Containers Containers are collections they allow grouping of resources (or literal values) It is possible to make statements about the container (as a whole) or about its members individually Different types of containers exist bag - unordered collection seq - ordered collection (= “sequence”) alt - represents alternatives It is also possible to create collections based on URI patterns for example, all files in a particular web site Duplicate values are permitted there is no mechanism to enforce unique value constraints

10 10 © NOKIA 10/12/ Ora Lassila Containers (continued) “Ora Lassila” rdf:_1 rdf:Seq dc:Creator rdf:Type “Ralph Swick” rdf:_2

11 11 © NOKIA 10/12/ Ora Lassila Containers (continued) “Ora Lassila” dc:Creator “Ralph Swick” dc:Creator

12 12 © NOKIA 10/12/ Ora Lassila Containers (continued) Triples from the first example: { “http://www.w3.org/TR/REC-rdf-syntax”, dc:Creator, x } { x, rdf:_1, “Ora Lassila” } { x, rdf:_2, “Ralph Swick” } { x, rdf:type, rdf:Seq } Triples from the second example: { “http://www.w3.org/TR/REC-rdf-syntax”, dc:Creator, “Ora Lassila” } { “http://www.w3.org/TR/REC-rdf-syntax”, dc:Creator, “Ralph Swick” } Syntactic shorthand provided (much like HTML lists)

13 13 © NOKIA 10/12/ Ora Lassila Higher-order statements One can make RDF statements about other RDF statements example: “Ralph believes that the web contains one billion documents” Higher-order statements allow us to express beliefs (and other modalities) are important for trust models, digital signatures,etc. also: metadata about metadata are represented by modeling RDF in RDF itself

14 14 © NOKIA 10/12/ Ora Lassila Reification RDF is not really second-order But it does provide a built-in predicate vocabulary for reification Lassila” dc:Creator “Library of Congress” dc:Creator The dotted box corresponds to the following statements { x, rdf:predicate, “dc:creator” } { x, rdf:subject, “http://www.w3.org/TR/RED-rdf-syntax } { x, rdf:object, “Ora Lassila” } { x, rdf:type, “rdf:statement” }

15 15 © NOKIA 10/12/ Ora Lassila RDF schema: type facilities RDF was designed with an object-oriented mindset important: in RDF, object-orientation is a modeling vehicle RDF schema provides an extensible, object-oriented type system RDF Schema definitions consist of classes (= “types”) and properties a new class can be defined by extending an existing class extension reuses (and thus shares) existing definition(s) a class can have multiple superclasses schema definitions can include constraints which express validation conditions domain constraints link properties with classes range constraints limit property values “hook” for new constraints Schema definitions are expressed in RDF itself specification contains a “meta-circular” definition of the RDF core classes

16 16 © NOKIA 10/12/ Ora Lassila RDF Syntax RDF Model and Syntax specification defines a convention of XML usage to “serialize” RDF most of the trouble people have had understanding RDF is probably caused by the syntax (this syntax was “designed by a committee”) redesign of the syntax is possible at some point… From the standpoint of this presentation, the concrete syntax is irrelevant

17 17 © NOKIA 10/12/ Ora Lassila Problems with RDF Syntax is hard we can fix this with a new syntax Modeling is difficult this is already harder to fix Harder than I [ht] expected for this audience, should not be a problem My belief is that thinking of RDF as a frame system or an object-oriented modeling system makes things easier to grasp

18 18 © NOKIA 10/12/ Ora Lassila Frame Systems Simple, structural representation tools conceptually easy to understand by “ordinary people” Object-oriented in nature easy to understand (and accept) by software people construction of ontologies, concept taxonomies is possible Terminology needs to be translated into RDF OOP instance instance variable class, type ? RDF resource property class qualified property Frame Systems frame slot class, type, frame facet

19 19 © NOKIA 10/12/ Ora Lassila My Frame Systems BEEF ( ) teaching tool at Helsinki University of Technology used in building several large prototype systems (e.g., a scheduling system for the plate rolling line of Europe's largest steel mill) PORK ( ) CLOS metaobject extension core representation layer of CMU's Ozone/DITOPS family of reconfigurable scheduling systems (some versions are currently being transitioned to US Air Force use) SCAM ( ) lightweight BEEF derivative, compatible with CRL on board NASA's Deep Space 1 probe (somewhere outside the Asteroid Belt at the moment) These systems are rich, understandable, and efficient Q: Why do I keep talking about frame systems? A: It seems I keep doing the same stuff over and over :-)

20 20 © NOKIA 10/12/ Ora Lassila Future: We Need More! Structural modeling obviously not enough we need a “logic layer” on top of RDF some type of description logic is a possibility (after all, we are talking about frame systems) Exposing a wide variety of data sources as RDF is useful, particularly if we have logic/rules which allow us to draw inference from this data My proposal: RDF + DL = “Frame System for WWW” this is probably a good starting point for DAML as well (details to be worked out by this workshop…)

21 21 © NOKIA 10/12/ Ora Lassila The solution(s)? There's been a lot of talk about metadata. What is metadata? It's just data. But it's data about other data. What could metadata do for us? Give search engines something to work with that is designed for their needs. Give us all a place to record what a document is for or about.

22 22 © NOKIA 10/12/ Ora Lassila Requirements for metadata What would we need to make this work? A standard syntax, so metadata can be recognised as such; One or more standard vocabularies, so search engines, authors and users all speak the same language; Lots of documents with metadata attached; Attribution and trust Is this resource really about Pamela Anderson

23 23 © NOKIA 10/12/ Ora Lassila What is RDF? RDF is actually two standardisation efforts, under the aegis of the W3C. It stands for Resource Description Framework (in other words, data about data). The two efforts are: Standardising the syntax and abstract semantics; RDF Model and Syntax Providing a standard way of defining standard vocabularies (but not actually defining any). RDF Schema

24 24 © NOKIA 10/12/ Ora Lassila An aside: what is the W3C? The World Wide Web Consortium. A voluntary association of companies and non-profit organisations. Membership costs serious money, confers voting rights. Complex procedures, with the Chairman (Tim Berners-Lee) holding all the high cards The big vendors (e.g. Microsoft, Adobe, Netscape) have a lot of power. How do standards get drafted and approved? W3C Draft Recommendations come from Working Groups with little (XML) or a lot of input from W3C staff (CSS1,2). They are approved by the Chairman.

25 25 © NOKIA 10/12/ Ora Lassila The Semantic Web W3C's vision of the future New syntax for RDF Logic on top

26 26 © NOKIA 10/12/ Ora Lassila Questions? mailto: mailto:


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