Resource Description Framework Model Syntax and Schema Specifications

Resource Description Framework Model, Syntax, and Schema Specifications Rohit Khare Adam Rifkin CSCW'98 Introduction to Resource Description Framework 4 K Associates

Metadataabout the RDF Spec z CSCW'98 Introduction to Resource Description Framework 2

Dissecting the label 1/3 z A set of statements about this object (the spec): y

Dissecting the label 2/3 z Using Dublin Core, as attributes of Description y dc: Title="Resource Description Framework. . . " y dc: Description="(RDF) is a foundation for. . . " y dc: Publisher="World Wide Web Consortium" y dc: Date="1998 -08 -19" y dc: Format="text/html" y dc: Type="technical specification" y dc: Language="eng"> z. . . and as an element, using an RDF bag: y

Dissecting the label 3/3 z. . . and as a repeated Dublin Core element, with Dewey Decimal Code attribtes: y y y z Finally, returning to the original HTML head: CSCW'98 Introduction to Resource Description Framework 5

Introduction to RDF z The label we just dissected is critical to the Web's future y Beyond machine-readable to machine-understandable z The RDF effort unites a wide array of players y Digital librarians, content-raters, privacy advocates, . . . y Significant industrial momentum, led by W 3 C z 1. The Data Model y Resources, properties, and statements z 2. The Syntax y Rendering into XML with Namespaces z 3. The RDF Schema y Using RDF to describe new vocabularies z Implications & Applications CSCW'98 Introduction to Resource Description Framework 6

Why metadatamatters. . . z Automated processing of Web resources: y y y Resource discovery, cataloging Content rating PICS Collections of pages Sitemaps Security & Privacy P 3 P, DSIG Intelligent software agents z Sharing data between multiple applications and organizations requires explicit definitions z XML enables processing; RDF enables understanding CSCW'98 Introduction to Resource Description Framework 7

The RDF Working Group y Co-chaired by Ora Lassila & Ralph Swick y Chartered in the Technology & Society Domain y First draft was published in August 1997 z Represents many communities: y y Web Standardization HTML Meta, PICS Library Dublin Core, Warwick Framework Structured Documents SGML, XML Knowledge Representation KIF z Significant Industrial Momentum y Ex: “What’s Related” button in Netscape Navigator. . . CSCW'98 Introduction to Resource Description Framework 8

1. The Data Model z Resources y Any URI reference, from a fragment to a site. z Property Types y Named type defines meaning, permitted values, and relationship to other types. y (Types are also resources) z Statements (a. k. a. Properties) y “Resource has Property with Value” y (Values can be resources or atomic XML data) CSCW'98 Introduction to Resource Description Framework 9

A Trivial Example z Sentence y “Ora Lassila is the creator of the resource http: //www. w 3. org/Home/Lassila” z Structure y Resource http: //www. w 3. org/Home/Lassila y Property type. Creator y Value "Ora Lassila" z Directedacyclicgraph CSCW'98 Introduction to Resource Description Framework 10

An Indirect Example z To add properties to Creator, point through a (possibly anonymous) intermediate Resource. CSCW'98 Introduction to Resource Description Framework 11

Collection Containers z Multiple occurrences of the same Property. Type doesn ’t establish a relation between the values y The Millers own a boat, a bike, and a TV set y The Millers need (a car or a truck) y (Sarah and Bob) bought a new car z RDF defines three special Resources: y Bag unordered values rdf: Bag y Sequence ordered values rdf: Seq y Alternative single value rdf: Alt CSCW'98 x Core RDF does not enforce ‘set’ semantics amongst values Introduction to Resource Description Framework 12

Example: Bag z The students in course 6. 001 are Amy, Tim, John, Mary, and Sue CSCW'98 Introduction to Resource Description Framework 13

Example: Alternative z The source code for X 11 may be found at ftp. x. org, ftp. cs. purdue. edu, or ftp. eu. net CSCW'98 Introduction to Resource Description Framework 14

Reification z Making statements about statements requires a process for transforming them into Resources y y prop. Obj the original referent prop. Name the original property type value the original value instance. Of the type of this resource x Reified statements are themselves RDF: Property x Collections are also built-in RDF types z Distributive Referents y Referring to a resource vs. its members (about. Each) CSCW'98 Introduction to Resource Description Framework 15

Example: Reification z Ralph Swick says that y Ora Lassila is the creator of the resource http: //www. w 3. org/Home/Lassila CSCW'98 Introduction to Resource Description Framework 16

Recap: A Formal Model of RDF z RDF itself is mathematically straightforward: y Definitions y Typing y Reification y Collections z. . . though the mapping onto XML syntax (and XML’s formal model) is less so. . . CSCW'98 Introduction to Resource Description Framework 17

Formal Model: Definitions z 1. There is a set called Nodes y 2. There is a subset of Nodes called Property. Types z 3. There is a set of 3 -tuples called Triples y {p, r, v} where p is a member of Property. Types, r is a member of Nodes, and v (called value) is either a member of Nodes or an atomic value x “v is the value of p for r ” x “r has a property p with a value v” x “the p of r is v” CSCW'98 Introduction to Resource Description Framework 18

Formal Model: Typing z 4. There is an element of Property. Types known as RDF: instance. Of. z 5. Members of Triples of the form {RDF: instance. Of, r, v} imply r and v are members of Nodes. y [RDFSchema] places additional restrictions on the use of instance. Of. CSCW'98 Introduction to Resource Description Framework 19

Formal Model: Reification y 6. There is an element of Nodes, not contained in Property. Types, known as RDF: Property. y 7. There are three elements in Property. Types known as RDF: prop. Name, RDF: prop. Obj and RDF: value. z 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: y y y CSCW'98 y {RDF: prop. Name, n, p} {RDF: prop. Obj, n, r} {RDF: value, n, v} Introduction to Resource Description Framework {RDF: instance. Of, n, [RDF: Property]} 20

Formal Model: Collections z 9. There are three elements of Nodes, not contained in Property. Types, known as RDF: Seq, RDF: Bag, and RDF: Alt. z 10. There is a subset of Property. Types corresponding to the ordinals called Ord. y Refer to elements of Ord as RDF: _1, RDF: _2, . . . y There must always be one value for RDF: Alt x (RDF: _1 is the default) CSCW'98 Introduction to Resource Description Framework 21

2. The Syntax z z z Why XML alone does not suffice Basic RDF-in-XML Syntax Abbreviated Forms Distributing RDF metadata Collected BNF Grammar Formal mapping to XML CSCW'98 Introduction to Resource Description Framework 22

Why XML alone does not suffice z XML can already handle new Property. Types: y I liked Ora Lassila’s RDF introduction! x Just declare a new element or attribute! z. . . but “raw XML” fails in two ways: y Interchange: Namespaces only identify new tags; DTD semantics do not provide types or composition y Scalability: Processing generic XML requires parsing text (“the entity tax”); and the order of XML elements is considered significant, requiring the whole graph. 23 CSCW'98 Introduction to Resource Description Framework

Basic RDF-in-XML Syntax z A Description block about a Resource z z z [1] RDF : : = [''] description* [''] [2] description : : = '' property* '' [3] id. About. Attr : : = id. Attr | about. Attr [4] about. Attr : : = 'about="' URI-reference '"' [5] id. Attr : : = 'ID="' IDsymbol '"' z contains Property. Name block or empty elements z [6] property : : = '<' prop. Name '>' value '' | '<' prop. Name resource. Attr '/>' z using fully-qualified XML Namespaces on each tag z [7] prop. Name : : = Qname z and allows values to be XML data, RDF, or external links z z [8] value : : = description | string [9] resource. Attr : : = 'resource="' URI-reference '"' CSCW'98 Introduction to Resource Description Framework 24

Example: Basic RDF-in-XML z “Ora Lassila is the Creator of the resource. . . ” y Ora Lassila| y (where s: is a separately-declared namespace) x xmlns: s="http: //description. org/schema/" z So the complete, valid XML document would be: y CSCW'98 Ora Lassila Introduction to Resource Description Framework 25

Abbreviated Forms z XML Namespace defaulting can shorten that: y Ora Lassila y (but such aggressive elision is officially discouraged) z RDF itself offers 3 abbreviations: y String values as XML attributes y Nested descriptions as XML attributes CSCW'98 Introduction to Resource Description Framework y instance. Of: property types as XML element 26

RDF Abbreviation Rules z Non-repeated, string-valued properties can fold into attributes; Description can be empty y z Simple Description-valued properties, too: y z Property. Types can be promoted to elements y CSCW'98 27

Example: Aggregates z z z a document with two authors specified alphabetically, a title specified in two different languages, and with two equivalent locations CSCW'98 z z Mary Andrew Jacky Crystal z z The Coolest Web Page Il Pagio di Web Fuba z Introduction to Resource Description Framework 28

Example: PICS Labels z PICS includes a schema, statements (about pages), and metastatements (about labels) z z until "1995. 12. 31 T 23: 59 -0000" z "http: //w 3. org/Underview. html" z z CSCW'98 Introduction to Resource Description Framework 29 z

Distributing RDF Metadata z The PICS effort had three goals for labels: y Embedded in documents (primarily HTML META) y Transmitted with documents (in HTTP headers) y Separately , from third parties (HTTP label queries) z Similarly, RDF has embedding mechanisms y Using in , though it’s invalid HTML 4. 0 x The abbreviated form prevents values from rendering y Using the RDF Namespace in any XML document z Remote access is unspecified as yet y. . but XLinks to metadata stores could work neatly CSCW'98 Introduction to Resource Description Framework 30

Collected BNF Grammar 1/2 z [6. 1] RDF : : = '< rdf: RDF>' obj* '' z [6. 2] obj : : = description | container z [6. 3] description : : = '' | '< rdf: Description' id. About. Attr? bag. Id. Attr? prop. Attr* '>' property* '' | typed. Node z [6. 4] container : : = sequence | bag | alternative z [6. 5] id. About. Attr : : = id. Attr | about. Each. Attr z [6. 6] id. Attr : : = 'ID="' IDsymbol '"' z [6. 7] about. Attr : : = 'about="' URI-reference '"' z [6. 8] about. Each. Attr : : = 'about. Each="' URI-reference '"' z [6. 9] bag. Id. Attr : : = 'bag. ID="' IDsymbol '"' z [6. 10] prop. Attr : : = prop. Name '="' string '"' (with embedded quotes escaped) z [6. 11] property : : = '<' prop. Name id. Attr? '>' value '' | '<' prop. Name id. Ref. Attr? bag. Id. Attr? prop. Attr* '/>' z [6. 12] typed. Node : : = '<' type. Name id. About. Attr? bag. Id. Attr? prop. Attr* '/>' | '<' type. Name id. About. Attr? bag. Id. Attr? prop. Attr* '>' property* '' CSCW'98 Introduction to Resource Description Framework 31

Collected BNF Grammar z z z z z 2/2 [6. 13] prop. Name : : = Qname [6. 14] type. Name : : = Qname [6. 15] id. Ref. Attr : : = id. Attr | resource. Attr [6. 16] value : : = obj | string [6. 17] resource. Attr : : = 'resource="' URI-reference '"' [6. 18] Qname : : = [ NSname ': ' ] name [6. 19] URI-reference : : = (see RFC 1738, RFC 1808, [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 "<", ">", and "&" escaped) [6. 24] sequence : : = '< rdf: Seq' id. Attr? '>' member* '' [6. 25] bag : : = '< rdf: Bag' id. Attr? '>' member* '' [6. 26] alternative : : = '' member+ '' [6. 27] member : : = referenced. Item | inline. Item [6. 28] referenced. Item : : = '' [6. 29] inline. Item : : = '' value ' CSCW'98 Introduction to Resource Description Framework 32

Formal mapping to XML 1/2 z Each element in a Description block E defines a {p, r, v} triple: y p is the element name, fully qualified as a URI y r is the about or ID attribute of the Description; or anonymous y v is the string or node contained by E, or the resource attribute of E z The Description block defines a Bag containing the reifications of each included property, named as Bag. ID or anonymous z The about. Each attributeexpands the process for each r in C z The LI elementworks as above, with p assigned in XML order Introduction to Resource Description Framework CSCW'98 33

Formal mapping to XML 2/2 z Each attribute on a Description tag A (other than ID, about. Each or bag. ID) defines a {p, r, v} triple: y p is the attribute name, fully qualified as a URI y r is the about or ID attribute of the Description; or a member of the collection in the about. Each attribute. y v is the (string) value of A z Each attribute on a Property tag A (other than ID, resource, or bag. ID) defines triples: y Linking the node r 2 (ID or resource) to the enclosing element’s resource as {p, r 1, r 2} y On node r 2 for each attribute A, as above CSCW'98 Introduction to Resource Description Framework 34

3. The RDF Schema z Introducing new Property. Types in a machineunderstandable way calls for a schema language y E. g. “a book must have at least one author” z RDFS is a loosely object-oriented solution with: y y y Core Classes Core Property. Types Core Constraints Documentation Hooks Model & Syntax Concepts z Deployment: Dublin Core, DCDs, & Other Issues CSCW'98 Introduction to Resource Description Framework 35

Core Classes z RDF: Resource y All resources (a. k. a. Nodes) are instances of this x Roughly corresponds to Object in OO systems z RDF: Property. Type y All elements of the set Property. Types are instances z RDFS: Class y Loosely corresponds to a type or category y No formal properties of Class itself CSCW'98 Introduction to Resource Description Framework 36

Core Property. Types z RDF: instance. Of y Indicates a resource is a member of a class; the value must be be an instance. Of: Class y A resource may be an instance of several classes z RDFS: sub. Class. Of y Indicates a (strict) subset/superset relationship y Its domain and range is Class y A class may not be a subclass of itself x But there isn't a way to express/enforce this in RDFS CSCW'98 Introduction to Resource Description Framework 37

Core Constraints z RDFS: Constraint. Property. Type y Superclass of Range and Domain z RDFS: Range y Specify the (at most one) class of property values x Any value allowed if no range specified z RDFS: Domain y Class(es) on which a property. Type may be used x Allowed on any class if no domain specified CSCW'98 Introduction to Resource Description Framework 38

Documentation & Model z RDFS: comment y Natural-language description of a resource z RDFS: label y Human-readable version of a resource name z RDFS: Collection y The superclass of Bag, Seq, and Alt z RDFS: String y A resource corresponding to M&S definiton of string (production 15 in the BNF) CSCW'98 Introduction to Resource Description Framework 39

Recap: RDFS Class Hierarchy CSCW'98 Introduction to Resource Description Framework 40

Recap: RDFS Constraints CSCW'98 Introduction to Resource Description Framework 41

RDFS in RDF z z z CSCW'98 Introduction to Resource Description Framework 42

Dublin Core on a Slide z Content z Intellectual Property y Title y Subject y Creator y Contributor x Includes keywords y Description y Source y Publisher y Rights x Metadata of predecessor y Language y Relation x e. g. is. Version. Of, is. Format. Of y Coverage x Spatial or temporal range CSCW'98 x e. g. editor, translator z Instance y Date y Type x e. g. novel, poem, TR y Format y Identifier Introduction to Resource Description Framework 43

Deployment Issues z RDF Schemas vs. “XML Schemas” y The problems of controlled vocabulary metadata and selfdescribing DTDs are close enough to cause confusion x XML-Data, by Microsoft & Co conflates the two x DTDs-in-XML is a work item before the rechartered XML WG z URI versioning issues y As with DTDs, the permanence of the schema identifier is a popular red herring (as is “performance”) z Compatibilty with ‘push’ product formats y Channel Definition Format, Open Software Description, . . . z But products and services are shipping. . . CSCW'98 Introduction to Resource Description Framework 44

Document Content Description z Long-standing goal of XML DTDs expressed in XML y “A schema language for tagsets ” z Here, DCD models the DL element from HTML in RDF: y CSCW'98 A simple 'definition list' construct, which contain s paired 'DT' (DL Term) and 'DD' (DL Definition) elements DT DD The term being defined in a DL list item A term's definition in a DL list item Introduction to Resource Description Framework 45

DCD: Beyond Text Processing z DCD can describe data types and constraints, too: y Describes an airline reservation Last. Name First. Initial Seat. Row Seat. Letter Departure Class z Sample airline booking record: y CSCW'98 BrayT 33B 1997 -05 -24 T 07: 55: 00+1 Introduction to Resource Description Framework 46

DCD: Basic Concepts 1/2 z DCDs themselves have descriptive parameters y Description, (Canonical) Namespace URI y Open or Closed: whether documents must validate z Elements y Content Model x Empty, Any, Data, Elements, or Mixed ; Root flag y Attribute or Attribute. Def declarations y Default & Fixed element contents y Groups & Order (Seq or Alt); Occurs x Required, Optional, One. Or. More, or Zero. Or. More CSCW'98 Introduction to Resource Description Framework 47

DCD: Basic Concepts 2/2 z Attributes y Name, is Global, Required or Optional, has ID-Role z Entities y Name & (Value, Public. ID, or System. ID) z Datatypes y XML DTD types, numbers (int, fixed, float, 1 -8 bytes), booleans, times (dates & intervals), & binary data y Min, Max & Min. Exclusive, Max. Exclusive y Picture constraints on string fields (per COBOL) x Characters, numbers, decimals, symbols CSCW'98 Introduction to Resource Description Framework 48

Implications & Applications z What happens when “one application's metadata is another application's data”? z Approaching Tim Berners-Lee ’s “next phase of the Web”: Reasoning Engines y RDF is a simple frame system -- not a reasoning system (but one can be built atop it) z Automating the Web of Trust y New generation of “Internet-scale” identification, rights management, authorization tools need signed RDF assertions for trust management CSCW'98 Introduction to Resource Description Framework 49

For more information. . . z The Specifications y http: //www. w 3. org/TR/WD-rdf-syntax & WD-rdf-schema z W 3 C ’s RDF Metadata home pages & y http: //www. w 3. org/Metadata/ & /RDF/ z Eric Miller’s introduction y http: //www. dlib. org/dlib/may 98/miller/05 miller. html y http: //purl. oclc. org/~emiller/talks/www 7/tutorial z Dave Beckett’s RDF Resources y http: //www. cs. ukc. ac. uk/people/staff/djb 1/ research/metadata/rdf. shtml z This talk is at http: //www. ics. uci. edu/~rohit/cscw 98/rdf/ CSCW'98 Introduction to Resource Description Framework 50