1 © nokia 10/12/2014 - ora lassila rdf: a frame system for the web ora lassila (with help from...

1 © NOKIA 04/11/23 - 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

[email protected]


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…


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


RDF model

• RDF “statements” consist ofresources (= nodes)

which have properties which have values (= nodes, strings)

http://www.w3.org/TR/REC-rdf-syntax/

“Ora Lassila”

author

= subject= predicate= object

predicate(subject, object)

“http://www.w3.org/TR/REC-rdf-syntax/ has the author Ora Lassila”

resource valueproperty


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


RDF model (example)


“Ora Lassila”

dc:Creator

“1999-02-22”

dc:Date

“W3C”

dc:Publisher


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

“[email protected]”

p:EMail

p:Name


Complex values (continued)

• Corresponding triples{ “http://www.w3.org/TR/PR-rdf-syntax/”, dc:Creator, x }

{ x, p:Name, “Ora Lassila” }

{ x, p:EMail, “[email protected]” }


“Ora Lassila”

dc:Creator

“[email protected]”

p:EMail

p:Name


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


Containers (continued)

http://www.w3.org/TR/REC-rdf-syntax

“Ora Lassila”

rdf:_1

rdf:Seq

dc:Creator

rdf:Type

“Ralph Swick”

rdf:_2



http://www.w3.org/TR/REC-rdf-syntax

“Ora Lassila”

dc:Creator

“Ralph Swick”

dc:Creator



• 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)


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


Reification• RDF is not really second-order

• But it does provide a built-in predicate vocabulary for reification

http://www.w3.org/TR/REC-rdf-syntax “Ora 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” }


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


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


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


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


My Frame Systems

• BEEF (1988-1992)• 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 (1993-1996)• 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 (1992-1996)• 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 :-)


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…)


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.


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


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


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.


The Semantic Web

• W3C's vision of the future

• New syntax for RDF

• Logic on top


Questions?

• mailto: [email protected]

• mailto: [email protected]

1 © nokia 10/12/2014 - ora lassila rdf: a frame system for the web ora lassila (with help from...

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