Resource Description Framework Model, Syntax, and Schema Specifications

1
ResourceDescriptionFrameworkModel, Syntax,
and Schema Specifications
Rohit Khare Adam Rifkin 4K Associates
2
Metadata about the RDF Spec

• ltrdfDescription about""
  xmlnsrdf"http//www.w3.org/TR/WD-rdf-syntax"
  xmlnsdc"http//purl.org/metadata/dublin_cor
  e" xmlnsddc"http//purl.org/net/ddc"
  dcTitle"Resource Description Framework
  (RDF) Model and Syntax Specification
  dcDescription"The Resource Description
  Framework (RDF) is a foundation for processing
  metadata it provides interoperability between
  applications that exchange machine-understandable
  information on the Web. RDF emphasizes
  facilities to enable automated processing of
  Web resources." dcPublisher"World Wide
  Web Consortium" dcDate"1998-08-19"
  dcFormat"text/html" dcType"technical
  specification" dcLanguage"eng"gt
  ltdcSubject resource"http//purl.org/net/ddc/025.
  30285" ddcClass"025.30285"
  ddcHeading"data processing computer
  applications" /gt ltdcSubject
  resource"http//purl.org/net/ddc/025.316"
  ddcClass"025.316" ddcHeading"Machine-readab
  le catalog record formats" /gt ltdcSubject
  ddcClass"025.302855741" ddcHeading"Applicat
  ions of computer file organization and access
  methods" /gt ltdcCreatorgt ltrdfBag
  rdf_1"Ora Lassila"
  rdf_2"Ralph Swick" /gt lt/dcCreatorgt
  lt/rdfDescriptiongt

3
Dissecting the label 1/3

• A set of statements about this object (the spec)
• ltrdfDescription about""
• Introducing three vocabularies to describe it
• Basic RDF
• xmlnsrdf"http//www.w3.org/TR/WD-rdf-syntax"
• Dublin Core
• xmlnsdc"http//purl.org/metadata/dublin_core"
• Dewey Decimal Code
• xmlnsddc"http//purl.org/net/ddc"
• (xmlns prefixes are used to define new XML
  attributes and tags)

4
Dissecting the label 2/3

• Using Dublin Core, as attributes of Description
• dcTitle"Resource Description Framework..."
• dcDescription"(RDF) is a foundation for..."
• dcPublisher"World Wide Web Consortium"
• dcDate"1998-08-19"
• dcFormat"text/html"
• dcType"technical specification"
• dcLanguage"eng"gt
• ...and as an element, using an RDF bag
• ltdcCreatorgt ltrdfBag rdf_1"Ora
  Lassila" rdf_2"Ralph Swick"
  /gtlt/dcCreatorgt

5
Dissecting the label 3/3

• ... and as a repeated Dublin Core element, with
  Dewey Decimal Code attribtes
• ltdcSubject resource"http//purl.org/net/ddc/025.
  30285" ddcClass"025.30285" ddcHeading"data
  processing computer applications"/gt
• ltdcSubject resource"http//purl.org/net/ddc/025.
  316" ddcClass"025.316" ddcHeading"Machine-re
  adable catalog record formats"/gt
• ltdcSubject ddcClass"025.302855741"
  ddcHeading"Applications of computer file
  organization and access methods"/gt
• Finally, returning to the original HTML head
• lt/rdfDescriptiongt

6
Introduction to RDF

• The label we just dissected is critical to the
  Web's future
• Beyond machine-readable to machine-understandable
• The RDF effort unites a wide array of players
• Digital librarians, content-raters, privacy
  advocates, ...
• Significant industrial momentum, led by W3C
• 1. The Data Model
• Resources, properties, and statements
• 2. The Syntax
• Rendering into XML with Namespaces
• 3. The RDF Schema
• Using RDF to describe new vocabularies
• Implications Applications

7
Why metadata matters...

• Automated processing of Web resources
• Resource discovery, cataloging
• Content rating PICS
• Collections of pages Sitemaps
• Security Privacy P3P, DSIG
• Intelligent software agents
• Sharing data between multiple applications and
  organizations requires explicit definitions
• XML enables processingRDF enables understanding

8
The RDF Working Group

• Co-chaired by Ora Lassila Ralph Swick
• Chartered in the Technology Society Domain
• First draft was published in August 1997
• Represents many communities
• Web Standardization HTML Meta, PICS
• Library Dublin Core, Warwick Framework
• Structured Documents SGML, XML
• Knowledge Representation KIF
• Significant Industrial Momentum
• Ex Whats Related button in Netscape
  Navigator...

9
1. The Data Model

• Resources
• Any URI reference, from a fragment to a site.
• Property Types
• Named type defines meaning, permitted values, and
  relationship to other types.
• (Types are also resources)
• Statements (a.k.a. Properties)
• Resource has Property with Value
• (Values can be resources or atomic XML data)

10
A Trivial Example

• Sentence
• Ora Lassila is the creator of the resource
  http//www.w3.org/Home/Lassila
• Structure
• Resource http//www.w3.org/Home/Lassila
• Property type Creator
• Value "Ora Lassila"
• Directed acyclic graph

11
An Indirect Example

• To add properties to Creator, point through a
  (possibly anonymous) intermediate Resource.

12
Collection Containers

• Multiple occurrences of the same PropertyType
  doesnt establish a relation between the values
• The Millers own a boat, a bike, and a TV set
• The Millers need (a car or a truck)
• (Sarah and Bob) bought a new car
• RDF defines three special Resources
• Bag unordered values rdfBag
• Sequence ordered values rdfSeq
• Alternative single value rdfAlt
• Core RDF does not enforce set semantics amongst
  values

13
Example Bag

• The students incourse 6.001 are Amy, Tim,John,
  Mary,and Sue

14
Example Alternative

• The source code for X11 may be found at
  ftp.x.org, ftp.cs.purdue.edu, or ftp.eu.net

15
Reification

• Making statements about statements requires a
  process for transforming them into Resources
• propObj the original referent
• propName the original property type
• value the original value
• instanceOf the type of this resource
• Reified statements are themselves RDFProperty
• Collections are also built-in RDF types
• Distributive Referents
• Referring to a resource vs. its members
  (aboutEach)

16
Example Reification

• Ralph Swick says that
• Ora Lassila is the creator of the resource
  http//www.w3.org/Home/Lassila

17
Recap A Formal Model of RDF

• RDF itself is mathematically straightforward
• Definitions
• Typing
• Reification
• Collections
• ... though the mapping onto XML syntax (and
  XMLs formal model) is less so...

18
Formal Model Definitions

• 1. There is a set called Nodes
• 2. There is a subset of Nodes called
  PropertyTypes
• 3. There is a set of 3-tuples called Triples
• p,r,v  where p is a member of PropertyTypes, r
  is a member of Nodes, and v (called value) is
  either a member of Nodes or an atomic value
• v is the value of p for r
• r has a property p with a value v
• the p of r is v

19
Formal Model Typing

• 4. There is an element of PropertyTypes known as
  RDFinstanceOf.
• 5. Members of Triples of the form
  RDFinstanceOf, r, v imply r and v are members
  of Nodes.
• RDFSchema places additional restrictions on the
  use of instanceOf.

20
Formal Model Reification

• 6. There is an element of Nodes, not contained in
  PropertyTypes, known as RDFProperty.
• 7. There are three elements in PropertyTypes
  known as RDFpropName, RDFpropObj and RDFvalue.
• 8. Reification of a triple p,r,v of Triples is
  an element n of Nodes representing the reified
  triple and four new elements of Triples
• RDFpropName, n, p 
• RDFpropObj, n, r 
• RDFvalue, n, v
• RDFinstanceOf, n, RDFProperty

21
Formal Model Collections

• 9. There are three elements of Nodes, not
  contained in PropertyTypes, known as RDFSeq,
  RDFBag, and RDFAlt.
• 10. There is a subset of PropertyTypes
  corresponding to the ordinals called Ord.
• Refer to elements of Ord as RDF_1, RDF_2, ...
• There must always be one value for RDFAlt
• (RDF_1 is the default)

22
2. The Syntax

• Why XML alone does not suffice
• Basic RDF-in-XML Syntax
• Abbreviated Forms
• Distributing RDF metadata
• Collected BNF Grammar
• Formal mapping to XML

23
Why XML alone does not suffice

• XML can already handle new PropertyTypes
• ltA HREF"RDF-intro"gtI liked ltDCCreatorgtOra
  Lassilalt/DCCreatorgts RDF introductionlt/Agt!
• Just declare a new element or attribute!
• ... but raw XML fails in two ways
• Interchange Namespaces only identify new tags
  DTD semantics do not provide types or composition
  
• Scalability Processing generic XML requires
  parsing text (the entity tax) and the order of
  XML elements is considered significant, requiring
  the whole graph.

24
Basic RDF-in-XML Syntax

• A Description block about a Resource
• 1 RDF 'ltrdfRDFgt' description
  'lt/rdfRDFgt'
• 2 description 'ltrdfDescription'
  idAboutAttr? 'gt' property
• 'lt/rdfDescriptiongt'
• 3 idAboutAttr idAttr  aboutAttr
• 4 aboutAttr 'about"' URI-reference
  '"'
• 5 idAttr 'ID"' IDsymbol '"'
• contains PropertyName block or empty elements
• 6 property 'lt' propName 'gt' value
  'lt/' propName 'gt' 'lt'
  propName resourceAttr '/gt'
• using fully-qualified XML Namespaces on each tag
• 7 propName Qname
• and allows values to be XML data, RDF, or
  external links
• 8 value description  string
• 9 resourceAttr 'resource"' URI-reference
  '"'

25
Example Basic RDF-in-XML

• Ora Lassila is the Creator of the resource...
• ltrdfRDFgt ltrdfDescription about"http//www.w3.o
  rg/Home/Lassila"gt ltsCreatorgtOra
  Lassilalt/sCreatorgt lt/rdfDescriptiongtlt/rdfRDF
  gt
• (where s is a separately-declared namespace)
• xmlnss"http//description.org/schema/"
• So the complete, valid XML document would be
• lt?xml version"1.0"?gtltrdfRDF xmlnsrdf"http//w
  ww.w3.org/TR/WD-rdf-syntax"
  xmlnss"http//description.org/schema/"gt
  ltrdfDescription about"http//www.w3.org/Home/Las
  sila"gt ltsCreatorgtOra Lassilalt/sCreatorgt
  lt/rdfDescriptiongtlt/rdfRDFgt

26
Abbreviated Forms

• XML Namespace defaulting can shorten that
• lt?xml version"1.0"?gtltRDF xmlns"http//www.w3.or
  g/TR/WD-rdf-syntax"gt ltDescription
  about"http//www.w3.org/Home/Lassila"gt
  ltCreator xmlns"http//description.org/schema/"gt
  Ora Lassilalt/Creatorgt lt/Descriptiongtlt/RDFgt
• (but such aggressive elision is officially
  discouraged)
• RDF itself offers 3 abbreviations
• String values as XML attributes
• Nested descriptions as XML attributes
• instanceOf property types as XML element names

27
RDF Abbreviation Rules

• Non-repeated, string-valued properties can fold
  into attributes Description can be empty
• ltrdfDescription about"http//www.w3.org/Home/Las
  sila" sCreator"Ora Lassila" /gt
• Simple Description-valued properties, too
• ltrdfDescription about"http//www.w3.org/Home/Las
  sila"gt ltsCreator resource"http//www.w3.org/s
  taffId/85740" vName"Ora Lassila"
  vEmail"lassila_at_w3.org"
  /gtlt/rdfDescriptiongt
• PropertyTypes can be promoted to elements
• ltrdfDescription about"http//www.w3.org/staffId/
  85740"gt ltrdfinstanceOf resource"sPerson" /gt
  ...
• ltsPerson about"http//www.w3.org/staffId/85740"gt

28
Example Aggregates

• a document with two authors specified
  alphabetically,
• a title specified in two different languages,
• and with two equivalent locations

• ltrdfRDF xmlnsrdf"http//www.w3.org/TR/WD-r
  df-syntax" xmlnsdc"http//purl.org/metadata
  /dublin_core"gt ltrdfDescription
  about"http//www.foo.com/cool.html"gt
• ltdcCreatorgt ltrdfSeq
  ID"CreatorsAlphabeticalBySurname"gt
  ltrdfligtMary Andrewlt/rdfligt
  ltrdfligtJacky Crystallt/rdfligt
  lt/rdfSeqgt lt/dcCreatorgt
• ltdcIdentifiergt ltrdfBag
  ID"MirroredSites"gt ltrdfli
  rdfresource"http//www.foo.com.au/cool.html"/gt
  ltrdfli rdfresource"http//www.foo.com.
  it/cool.html"/gt lt/rdfbaggt
  lt/dcIdentifiergt
• ltdcTitlegt ltrdfAltgt
  ltrdfli xmllang"en"gt The Coolest
  Web Pagelt/rdfligt ltrdfli
  xmllang"it"gt Il Pagio di Web
  Fubalt/rdfligt lt/rdfAltgt
  lt/dcTitlegt
• lt/rdfDescriptiongt lt/rdfRDFgt

29
Example PICS Labels

• PICS includes a schema, statements (about pages),
  and metastatements (about labels)
• (PICS-1.1 "http//www.gcf.org/v2.5" by "John
  Doe" labels on "1994.11.05T0815-0500"
  until "1995.12.31T2359-0000"
• for "http//w3.org/Overview.html"
  ratings (suds 0.5 density 0
  color/hue 1)
• for "http//w3.org/Underview.html"
  by "Jane Doe" ratings (subject 2
  density 1 color/hue 1))
• hypothetical this is not a standards proposal

• ltrdfRDF xmlnsrdf"http//www.w3.org/TR/199
  8/WD-rdf-syntax" xmlnspics"http//www.w3.or
  g/TR/_at__at_/WD-PICS-labels" xmlnsgcf"http//www
  .gcf.org/v2.5"gt
• ltrdfDescription bagID"L01"
  about"http//w3.org/Overview.html"
  gcfsuds"0.5" gcfdensity"0"
  gcfcolor.hue"1"/gt
• ltrdfDescription bagID"L02"
  about"http//w3.org/Underview.html"
  gcfsubject"2" gcfdensity"1"
  gcfcolor.hue"1"gt
• ltrdfDescription aboutEach"L01"
  picsby"John Doe" picson"1994.11.05T0815
  -0500" picsuntil"1995.12.31T2359-0000"/gt
• ltrdfDescription aboutEach"L02"
  picsby"Jane Doe" picson"1994.11.05T0815
  -0500" picsuntil"1995.12.31T2359-0000"/gt
• lt/rdfRDFgt

30
Distributing RDF Metadata

• The PICS effort had three goals for labels
• Embedded in documents (primarily HTML META)
• Transmitted with documents (in HTTP headers)
• Separately, from third parties (HTTP label
  queries)
• Similarly, RDF has embedding mechanisms
• Using ltRDFgt in ltHEADgt, though its invalid HTML
  4.0
• The abbreviated form prevents values from
  rendering
• Using the RDF Namespace in any XML document
• Remote access is unspecified as yet
• .. but XLinks to metadata stores could work neatly

31
Collected BNF Grammar 1/2

• 6.1 RDF 'ltrdfRDFgt' obj
  'lt/rdfRDFgt'
• 6.2 obj description container
• 6.3 description 'ltrdfDescription'
  idAboutAttr? bagIdAttr? propAttr '/gt'
  'ltrdfDescription' idAboutAttr?
  bagIdAttr? propAttr 'gt'
  property 'lt/rdfDescriptiongt'
  typedNode
• 6.4 container sequence bag
  alternative
• 6.5 idAboutAttr idAttr aboutAttr
   aboutEachAttr
• 6.6 idAttr 'ID"' IDsymbol '"'
• 6.7 aboutAttr 'about"' URI-reference
  '"'
• 6.8 aboutEachAttr 'aboutEach"'
  URI-reference '"'
• 6.9 bagIdAttr 'bagID"' IDsymbol '"'
• 6.10 propAttr propName '"' string
  '"' (with embedded
  quotes escaped)
• 6.11 property 'lt' propName idAttr?
  'gt' value 'lt/' propName 'gt'
  'lt' propName idRefAttr? bagIdAttr? propAttr
  '/gt'
• 6.12 typedNode 'lt' typeName
  idAboutAttr? bagIdAttr? propAttr '/gt'
  'lt' typeName idAboutAttr?
  bagIdAttr? propAttr 'gt'
  property 'lt/' typeName 'gt'

32
Collected BNF Grammar 2/2

• 6.13 propName Qname
• 6.14 typeName Qname
• 6.15 idRefAttr idAttr resourceAttr
• 6.16 value obj string
• 6.17 resourceAttr 'resource"'
  URI-reference '"'
• 6.18 Qname NSname '' name
• 6.19 URI-reference (see RFC1738, RFC1808,
  URI)
• 6.20 IDsymbol (any legal XML name
  symbol)
• 6.21 name (any legal XML name
  symbol)
• 6.22 NSname (any legal XML
  namespace prefix)
• 6.23 string (any XML text, with
  "lt", "gt", and "" escaped)
• 6.24 sequence 'ltrdfSeq' idAttr? 'gt'
  member 'lt/rdfSeqgt'
• 6.25 bag 'ltrdfBag' idAttr? 'gt'
  member 'lt/rdfBaggt'
• 6.26 alternative 'ltrdfAlt' idAttr? 'gt'
  member 'lt/rdfAltgt'
• 6.27 member referencedItem
   inlineItem
• 6.28 referencedItem 'ltrdfli' resourceAttr
  '/gt'
• 6.29 inlineItem 'ltrdfligt' value
  lt/rdfligt'

33
Formal mapping to XML 1/2

• Each element E in a Description block defines a
  p,r,v triple
• p is the element name, fully qualified as a URI
• r is the about or ID attribute of the
  Description or anonymous
• v is the string or node contained by E, or the
  resource attribute of E
• The Description block defines a Bag containing
  the reifications of each included property, named
  as BagID or anonymous
• The aboutEach attribute expands the process for
  each r in C
• The LI element works as above, with p assigned in
  XML order

34
Formal mapping to XML 2/2

• Each attribute A on a Description tag (other
  than ID, about, aboutEach or bagID) defines a
  p,r,v triple
• p is the attribute name, fully qualified as a URI
• r is the about or ID attribute of the
  Description or a member of the collection in the
  aboutEach attribute.
• v is the (string) value of A
• Each attribute A on a Property tag (other than
  ID, resource, or bagID) defines triples
• Linking the node r2 (ID or resource) to the
  enclosing elements resource as p,r1,r2
• On node r2 for each attribute A, as above

35
3. The RDF Schema

• Introducing new PropertyTypes in a
  machine-understandable way calls for a schema
  language
• E.g. a book must have at least one author
• RDFS is a loosely object-oriented solution with
• Core Classes
• Core PropertyTypes
• Core Constraints
• Documentation Hooks
• Model Syntax Concepts
• Deployment Dublin Core, DCDs, Other Issues

36
Core Classes

• RDFResource
• All resources (a.k.a. Nodes) are instances of
  this
• Roughly corresponds to Object in OO systems
• RDFPropertyType
• All elements of the set PropertyTypes are
  instances
• RDFSClass
• Loosely corresponds to a type or category
• No formal properties of Class itself

37
Core PropertyTypes

• RDFinstanceOf
• Indicates a resource is a member of a class the
  value must be be an instanceOfClass
• A resource may be an instance of several classes
• RDFSsubClassOf
• Indicates a (strict) subset/superset relationship
• Its domain and range is Class
• A class may not be a subclass of itself
• But there isn't a way to express/enforce this in
  RDFS

38
Core Constraints

• RDFSConstraintPropertyType
• Superclass of Range and Domain
• RDFSRange
• Specify the (at most one) class of property
  values
• Any value allowed if no range specified
• RDFSDomain
• Class(es) on which a propertyType may be used
• Allowed on any class if no domain specified

39
Documentation Model

• RDFScomment
• Natural-language description of a resource
• RDFSlabel
• Human-readable version of a resource name
• RDFSCollection
• The superclass of Bag, Seq, and Alt
• RDFSString
• A resource corresponding to MS definiton of
  string (production 15 in the BNF)

40
Recap RDFS Class Hierarchy
41
Recap RDFS Constraints
42
RDFS in RDF

• ltRDF xmlns"http//www.w3.org/TR/WD-rdf-syntax"
  xmlnsrdf"http//www.w3.org/TR/WD-rdf-syntax"
  xmlnss"http//www.w3.org/TR/WD-rdf-schema"gt
• ltsClass rdfID"Property" scomment"A triple
  consisting of a property type, a node, and a
  value" /gt
• ltsClass rdfID"PropertyType" scomment"A
  name of a property, defining specific meaning for
  the property" /gt
• ltsClass rdfID"Bag" scomment"An unordered
  collection" /gt
• ltsClass rdfID"Seq" scomment"An ordered
  collection" /gt
• ltsClass rdfID"Alt" scomment"A collection
  of alternatives" /gt
• ltPropertyType ID"propName" scomment"Identifie
  s the property type of a property in reified
  form" sdomain"Property" srange"PropertyT
  ype" /gt
• ltPropertyType ID"propObj" scomment"Identifies
  the resource that a property describes in
  reified form" sdomain"Property" /gt
• ltPropertyType ID"value" scomment"Identifies
  the value of a property in reified form" /gt
• ltPropertyType ID"instanceOf"
  scomment"Identifies the Class of a resource"
  srange"Class" /gt
• lt/RDFgt

43
Dublin Core on a Slide

• Content
• Title
• Subject
• Includes keywords
• Description
• Source
• Metadata of predecessor
• Language
• Relation
• e.g. isVersionOf, isFormatOf
• Coverage
• Spatial or temporal range

• Intellectual Property
• Creator
• Contributor
• e.g. editor, translator
• Publisher
• Rights
• Instance
• Date
• Type
• e.g. novel, poem, TR
• Format
• Identifier

44
Deployment Issues

• RDF Schemas vs. XML Schemas
• The problems of controlled vocabulary metadata
  and self-describing DTDs are close enough to
  cause confusion
• XML-Data, by Microsoft Co conflates the two
• DTDs-in-XML is a work item before the rechartered
  XML WG
• URI versioning issues
• As with DTDs, the permanence of the schema
  identifier is a popular red herring (as is
  performance)
• Compatibilty with push product formats
• Channel Definition Format, Open Software
  Description,...
• But products and services are shipping...

45
Document Content Description

• Long-standing goal of XML DTDs expressed in XML
• A schema language for tagsets
• Here, DCD models the DL element from HTML in RDF
  
• ltDCDgt  ltElementDef Type"DL" Model"Elements" Con
  tent"Closed"gt    ltDescriptiongtA simple 'definiti
  on list' construct, which contains paired       '
  DT' (DL Term) and 'DD' (DL Definition) elementslt/D
  escriptiongt    ltGroup Occurs"OneOrMore" RDFOrde
  r"Seq"gt      ltElementgtDTlt/Elementgt      ltGroup 
  Occurs"Optional"gtltElementgtDDlt/Elementgtlt/Groupgt  
    lt/Groupgt  lt/ElementDefgt  ltElementDef Type"DT"
   Model "Data" Content"Closed"gt    ltDescriptiongt
  The term being defined in a DL list itemlt/Descript
  iongt  lt/ElementDefgt  ltElementDef Type"DD" Model
   "Mixed" Content"Open"gt    ltDescriptiongtA term'
  s definition in a DL list itemlt/Descriptiongt    lt
  !-- Open because lots of markup can be in a DL --gt
    lt/ElementDefgt lt/DCDgt

46
DCD Beyond Text Processing

• DCD can describe data types and constraints, too
• ltDCDgt ltElementDef Type"Booking" Model"Elements"
   Content"Closed"gt   ltDescriptiongtDescribes an ai
  rline reservationlt/Descriptiongt   ltGroup RDFOrde
  r"Seq"gt     ltElementgtLastNamelt/Elementgt ltElement
  gtFirstInitiallt/Elementgt     ltElementgtSeatRowlt/Ele
  mentgt ltElementgtSeatLetterlt/Elementgt     ltElementgt
  Departurelt/Elementgt ltElementgtClasslt/Elementgt   lt/
  Groupgt lt/ElementDefgt lt!-- example omits boring f
  ield declarations --gt ltElementDef Type"SeatRow" 
  Model"Data" Datatype"i1" Min"1" Max"72" /gt ltE
  lementDef Type"SeatLetter" Model"Data" Datatype
  "char" Min"A" Max"K"/gt ltElementDef Type"Class"
   Model"Data" Datatype"char" Default"1"/gtlt/DCDgt
  
• Sample airline booking record
• ltBookinggt  ltLastNamegtBraylt/LastNamegtltFirstInitial
  gtTlt/FirstInitialgt  ltSeatRowgt33lt/SeatRowgtltSeatLett
  ergtBlt/SeatLettergt  ltDeparturegt1997-05-24T075500
  1lt/Departuregtlt/Bookinggt

47
DCD Basic Concepts 1/2

• DCDs themselves have descriptive parameters
• Description, (Canonical) Namespace URI
• Open or Closed whether documents must validate
• Elements
• Content Model
• Empty, Any, Data, Elements, or Mixed Root flag
• Attribute or AttributeDef declarations
• Default Fixed element contents
• Groups Order (Seq or Alt) Occurs
• Required, Optional, OneOrMore, or ZeroOrMore

48
DCD Basic Concepts 2/2

• Attributes
• Name, is Global, Required or Optional, has
  ID-Role
• Entities
• Name (Value, PublicID, or SystemID)
• Datatypes
• XML DTD types, numbers (int, fixed, float, 1-8
  bytes), booleans, times (dates intervals),
  binary data
• Min, Max MinExclusive, MaxExclusive
• Picture constraints on string fields (per COBOL)
• Characters, numbers, decimals, symbols

49
Implications Applications

• What happens when one application's metadata is
  another application's data?
• Approaching Tim Berners-Lees next phase of the
  Web Reasoning Engines
• RDF is a simple frame system -- not a reasoning
  system (but one can be built atop it)
• Automating the Web of Trust
• New generation of Internet-scale
  identification, rights management, authorization
  tools need signed RDF assertions for trust
  management

50
For more information...

• The Specifications
• http//www.w3.org/TR/WD-rdf-syntax
  WD-rdf-schema
• W3Cs RDF Metadata home pages
• http//www.w3.org/Metadata/ /RDF/
• Eric Millers introduction
• http//www.dlib.org/dlib/may98/miller/05miller.htm
  l
• http//purl.oclc.org/emiller/talks/www7/tutorial
• Dave Becketts RDF Resources
• http//www.cs.ukc.ac.uk/people/staff/djb1/ res
  earch/metadata/rdf.shtml
• This talk is at http//www.ics.uci.edu/rohit/cscw
  98/rdf/