Upload
cain
View
39
Download
0
Embed Size (px)
DESCRIPTION
RDF: A Frame System for the Web. Ora Lassila (with h elp 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 - PowerPoint PPT Presentation
Citation preview
1 © NOKIA 04/22/23 - 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
2 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
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
5 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
RDF model (example)
http://www.w3.org/TR/REC-rdf-syntax/
“Ora Lassila”
dc:Creator
“1999-02-22”
dc:Date
“W3C”
dc:Publisher
7 © NOKIA 04/22/23 - 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:
http://www.w3.org/TR/REC-rdf-syntax/
“Ora Lassila”
dc:Creator
p:EMail
p:Name
8 © NOKIA 04/22/23 - 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:EMail, “[email protected]” }
http://www.w3.org/TR/REC-rdf-syntax/
“Ora Lassila”
dc:Creator
p:EMail
p:Name
9 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
Containers (continued)
http://www.w3.org/TR/REC-rdf-syntax
“Ora Lassila”
rdf:_1
rdf:Seq
dc:Creator
rdf:Type
“Ralph Swick”
rdf:_2
11 © NOKIA 04/22/23 - Ora Lassila
Containers (continued)
http://www.w3.org/TR/REC-rdf-syntax
“Ora Lassila”
dc:Creator
“Ralph Swick”
dc:Creator
12 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
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” }
15 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
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 :-)
20 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - 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 © NOKIA 04/22/23 - Ora Lassila
The Semantic Web
• W3C's vision of the future
• New syntax for RDF
• Logic on top
26 © NOKIA 04/22/23 - Ora Lassila
Questions?
• mailto: [email protected]
• mailto: [email protected]