RDF: A Frame System for the Web
Ora Lassila
(with help from Henry S. Thompson)
Agent Technology Group
Nokia Research Center, Boston
March 2000
ora.lassila@nokia.com
1
© NOKIA
3/19/2016 - 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…
2
© NOKIA
3/19/2016 - 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 objectoriented 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
•
•
3
© NOKIA
it might reside, for example, in an RDB
XML relieves us of syntactic details when transporting RDF
3/19/2016 - Ora Lassila
RDF model
• RDF “statements” consist of
resources (= nodes)
which have properties
which have values (= nodes, strings)
resource
property
value
= subject
= predicate
= object
predicate(subject, object)
“http://www.w3.org/TR/REC-rdf-syntax/ has the author Ora Lassila”
http://www.w3.org/TR/REC-rdf-syntax/
author
“Ora Lassila”
4
© NOKIA
3/19/2016 - 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” }
•
•
5
© NOKIA
“author” replaced by a label which is uniquely qualified (using the XML
namespace mechanism)
Drawn from a consensually pre-established vocabulary
3/19/2016 - Ora Lassila
RDF model (example)
“W3C”
dc:Publisher
http://www.w3.org/TR/REC-rdf-syntax/
dc:Creator
dc:Date
“1999-02-22”
6
© NOKIA
3/19/2016 - Ora Lassila
“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)
http://www.w3.org/TR/REC-rdf-syntax/
dc:Creator
• Example:
p:Name
p:EMail
“ora.lassila@nokia.com”
7
© NOKIA
3/19/2016 - Ora Lassila
“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, “ora.lassila@nokia.com” }
http://www.w3.org/TR/REC-rdf-syntax/
dc:Creator
p:Name
p:EMail
“ora.lassila@nokia.com”
8
© NOKIA
3/19/2016 - Ora Lassila
“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
•
9
© NOKIA
there is no mechanism to enforce unique value constraints
3/19/2016 - Ora Lassila
Containers (continued)
http://www.w3.org/TR/REC-rdf-syntax
dc:Creator
rdf:Type
rdf:_1
“Ora Lassila”
10
© NOKIA
3/19/2016 - Ora Lassila
rdf:_2
“Ralph Swick”
rdf:Seq
Containers (continued)
http://www.w3.org/TR/REC-rdf-syntax
11
© NOKIA
dc:Creator
dc:Creator
“Ora Lassila”
“Ralph Swick”
3/19/2016 - 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)
12
© NOKIA
3/19/2016 - 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
•
•
•
•
13
© NOKIA
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
3/19/2016 - 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
dc:Creator
“Ora Lassila”
dc:Creator
“Library of Congress”
• The dotted box corresponds to the following
statements
•
•
•
•
14
© NOKIA
{ 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” }
3/19/2016 - 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
•
15
© NOKIA
specification contains a “meta-circular” definition of the RDF core classes
3/19/2016 - 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
16
© NOKIA
3/19/2016 - 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
17
© NOKIA
3/19/2016 - 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
18
OOP
RDF
Frame Systems
• instance
• resource
• frame
• instance variable • property
• slot
• class, type
• class
• class, type, frame
• ?
• qualified property • facet
© NOKIA
3/19/2016 - Ora Lassila
My Frame Systems
Q: Why do I keep talking about frame
systems? A: It seems I keep doing the
same stuff over and over :-)
• 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
19
© NOKIA
3/19/2016 - 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”
•
•
20
© NOKIA
this is probably a good starting point for DAML as well
(details to be worked out by this workshop…)
3/19/2016 - 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?
•
•
21
© NOKIA
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.
3/19/2016 - 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
•
22
© NOKIA
Is this resource really about Pamela Anderson
3/19/2016 - 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;
•
•
Providing a standard way of defining standard vocabularies (but not
actually defining any).
•
23
© NOKIA
RDF Model and Syntax
RDF Schema
3/19/2016 - 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.
24
© NOKIA
3/19/2016 - Ora Lassila
The Semantic Web
• W3C's vision of the future
• New syntax for RDF
• Logic on top
25
© NOKIA
3/19/2016 - Ora Lassila
Questions?
• mailto: ora.lassila@nokia.com
• mailto: ht@cogsci.ed.ac.uk
26
© NOKIA
3/19/2016 - Ora Lassila