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Languages for Semantic Web

Languages for Semantic Web. 葉慶隆 大同大學 資訊工程系所 Email: chingyeh@cse.ttu.edu.tw URL: www.cse.ttu.edu.tw/chingyeh. Sources. Knowledge Markup and Resource Semantics, By Harold Boley, Stefan Decker, and Michael Sintek, IJCAI-01 Tutorial, http://www.ijcai-01.org/

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Languages for Semantic Web

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  1. Languages for Semantic Web 葉慶隆 大同大學 資訊工程系所 Email: chingyeh@cse.ttu.edu.tw URL: www.cse.ttu.edu.tw/chingyeh

  2. Sources • Knowledge Markup and Resource Semantics, By Harold Boley, Stefan Decker, and Michael Sintek, IJCAI-01 Tutorial, http://www.ijcai-01.org/ • XML Fundamentals, http://www.ibiblio.org/xml/slides/sd2001east/fundamentals/XML_Fundamentals.html • Anupriya Ankolenkar, et al., “DAML-S: Semantic Markup For Web Services,”, Proceedings of SWWS’ 01, the First Semantic Web Working Symposium, California, USA, July 30 - August 1, 2001. Languages forSemantic Web

  3. Increasing demand for formalized knowledge on the Web: AI’s chance! XML- & RDF-based markup languages provide a 'universal' storage/interchange format for such Web-distributed knowledge representation In this talk, we focus on Semantic Web languages: XML, RDF(S), DAML. Overview Namespaces CSS DTDs XSLT DAML Stylesheets Agents Transformations Ontobroker XQL XML HornML Rules Queries XQuery RuleML XML-QL SHOE RDF[S] Frames Acquisition TopicMaps Protégé Languages forSemantic Web

  4. Web Languages forKnowledge Capturing • Human knowledge is (partially) captured on the Web as informal texts, semiformal documents, and structured metadata • Each kind of knowledge has its (preferred) markup language Languages forSemantic Web

  5. Web Languages forMachine Interpretation • XML (Extensible Markup Language): Semiformal documents range between non-formatted texts and fully formatted databases • RDF (Resource Description Framework): Structured metadata describe arbitrary heterogeneous Web pages/objects in a homogeneous manner. Machines (e.g. search engines) can analyze XML or RDF markups better than full HTML Languages forSemantic Web

  6. The Semantic Web Activityof the W3C • “The Semantic Web is a vision: the idea of having • data on the Web defined and linked in a way that • it can be used by machines not just for display purposes, • but for • automation, • integration and • reuse of data across various applications.” • (http://www.w3.org/2001/sw/Activity) Languages forSemantic Web

  7. The Semantic Web Layered Architecture Tim Berners-Lee: “Axioms, Architecture and Aspirations” W3C all-working group plenary Meeting 28 February 2001 (http://www.w3.org/2001/Talks/0228-tbl/slide5-0.html) Languages forSemantic Web

  8. XML Fundamentals Source: http://www.ibiblio.org/xml/slides/sd2001east/fundamentals/XML_Fundamentals.html

  9. What is XML? • Extensible Markup Language • A syntax for documents • A Meta-Markup Language • A Structural and Semantic language, not a formatting language • Not just for Web pages Languages forSemantic Web

  10. Extensible Markup Language • Language • It has a grammar • It has a vocabulary (sort of) • It can be parsed by machines • Markup Language • It says what things are; not what they do • It is not a programming language • It is not compiled • Extensible • You can add words to the language Languages forSemantic Web

  11. XML is a Meta Markup Language • Not like HTML, troff, LaTeX • Make up the tags you need as you need them • The tags you create can be documented in a Document Type Definition (DTD) • A meta syntax for domain-specific markup languages like MusicML, MathML, and XHTML Languages forSemantic Web

  12. XML Applications • A specific markup language that uses the XML meta-syntax is called an XML application • Different XML applications have their own more constricted syntaxes and vocabularies within the broader XML syntax • Further syntax can be layered on top of this; e.g. data typing through schemas Languages forSemantic Web

  13. XML describes structure and semantics, not formatting • XML documents form a tree • Document Object Model (DOM) • Element and attribute names reflect the kind of the element • DTD, Schema • Formatting can be added with a style sheet • Cascading Style Sheets (CSS) • Extensible Stylesheet language (XSL) Languages forSemantic Web

  14. XML Hypertext • A Uniform Resource Identifier (URI) names or locates a resource • An XLink defines connections between two or more documents identified by URIs • XPath identifies particular nodes within a document • An XPointer adds an XPath to a URI • XBase defines the URI against which relative URIs are resolved • XInclude embeds a document identified by a URI inside an XML document. Languages forSemantic Web

  15. A Song Description in HTML <dt>Hot Cop <dd> by Jacques Morali, Henri Belolo, and Victor Willis <ul> <li>Producer: Jacques Morali <li>Publisher: PolyGram Records <li>Length: 6:20 <li>Written: 1978 <li>Artist: Village People </ul> Languages forSemantic Web

  16. A Song Description in XML <SONG> <TITLE>Hot Cop</TITLE> <COMPOSER>Jacques Morali</COMPOSER> <COMPOSER>Henri Belolo</COMPOSER> <COMPOSER>Victor Willis</COMPOSER> <PRODUCER>Jacques Morali</PRODUCER> <PUBLISHER>PolyGram Records</PUBLISHER> <LENGTH>6:20</LENGTH> <YEAR>1978</YEAR> <ARTIST>Village People</ARTIST> </SONG> Languages forSemantic Web

  17. Style Sheets Provide Formatting(CSS) SONG {display: block; font-family: New York, Times New Roman, serif} TITLE {display: block; font-size: 24pt; font-weight: bold; font-family: Helvetica, sans} COMPOSER {display: block} PRODUCER {display: block} YEAR {display: block} PUBLISHER {display: block} LENGTH {display: block} ARTIST {display: block; font-style: italic} Languages forSemantic Web

  18. Attaching Style Sheets to Documents <?xml-stylesheet type="text/css" href="song.css"?> <SONG> <TITLE>Hot Cop</TITLE> <COMPOSER>Jacques Morali</COMPOSER> <COMPOSER>Henri Belolo</COMPOSER> <COMPOSER>Victor Willis</COMPOSER> <PRODUCER>Jacques Morali</PRODUCER> <PUBLISHER>PolyGram Records</PUBLISHER> <LENGTH>6:20</LENGTH> <YEAR>1978</YEAR> <ARTIST>Village People</ARTIST> </SONG> Languages forSemantic Web

  19. An XSLT Stylesheet (Part 1) <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="/"> <html> <head><title>Song</title></head> <body> <xsl:apply-templates select="SONG"/> </body> </html> </xsl:template> Languages forSemantic Web

  20. An XSLT Stylesheet (Part 2) <xsl:template match="SONG"> <h1> <xsl:value-of select="TITLE"/> by the <xsl:value-of select="ARTIST"/> </h1> <ul> <li>Length: <xsl:value-of select="LENGTH"/></li> <li>Producer: <xsl:value-of select="PRODUCER"/></li> <li>Publisher: <xsl:value-of select="PUBLISHER"/></li> <li>Year: <xsl:value-of select="YEAR"/></li> <xsl:apply-templates select="COMPOSER"/> </ul> </xsl:template> <xsl:template match="COMPOSER"> <li>Composer: <xsl:value-of select="."/></li> </xsl:template> </xsl:stylesheet> Languages forSemantic Web

  21. Transforming the Document <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Song</title> </head> <body> <h1>Hot Cop by the Village People </h1> <ul> <li>Length: 6:20</li> <li>Producer: Jacques Morali</li> <li>Publisher: PolyGram Records</li> <li>Year: 1978</li> <li>Composer: Jacques Morali</li> <li>Composer: Henri Belolo</li> <li>Composer: Victor Willis</li> </ul> </body> </html> XSL document (template rules) XML document XSLT Processor (IE 5) Output Languages forSemantic Web

  22. A DTD for Songs <!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, LENGTH?, YEAR?, ARTIST+)> <!ELEMENT TITLE (#PCDATA)> <!ELEMENT COMPOSER (#PCDATA)> <!ELEMENT PRODUCER (#PCDATA)> <!ELEMENT PUBLISHER (#PCDATA)> <!ELEMENT LENGTH (#PCDATA)> <!-- This should be a four digit year like "1999", not a two-digit year like "99" --> <!ELEMENT YEAR (#PCDATA)> <!ELEMENT ARTIST (#PCDATA)> Languages forSemantic Web

  23. Well-formedness • Rules: • Open and close all tags • Empty tags end with /> • There is a unique root element • Elements may not overlap • Attribute values are quoted • < and & are only used to start tags and entities • Only the five predefined entity references are used • Plus more... Languages forSemantic Web

  24. Validity • To be valid an XML document must be • Well-formed • Must have a Document Type Definition (DTD) • Must comply with the constraints specified in the DTD Languages forSemantic Web

  25. What Is XML Used for? • Domain-Specific Markup Languages • XML in industrial applications: http://www.xml.org/xml/industry_industrysectors.jsp • Self-Describing Data • Much data is lost due to format problems. • Interchange of Data Among Applications • Electronic business: RosettaNet, ebXML Languages forSemantic Web

  26. XML Namespaces • XML namespaces are akin to namespaces, packages, and modules in programming languages • Disambiguation of tag–and attribute–names from different XML applications (“spaces”) through different prefixes • A prefix is separated from the local name by a “:”, obtaining prefix:name tags • Namespaces constitute a layer on top of XML 1.0, since prefix:name is again a valid tag name and namespace bindings are ignored by some tools Languages forSemantic Web

  27. Namespace Bindings • Prefixes are bound to namespace URIs by attaching an xmlns:prefix attribute to the prefixed element or one of its ancestors, prefix:name1 ,...,prefix:namen • The value of the xmlns:prefix attribute is a URI, which may or (unlike for DTDs!) may not point to a description of the namespace’s syntax • An element can use bindings for multiple name-spaces via attributes xmlns:prefix1 ,...,xmlns:prefixm Languages forSemantic Web

  28. Two-Namespace Example: Snail-Mail and Telecoms Address Parts <mail:address xmlns:mail="http://www.deutschepost.de/" xmlns:tele="http://www.telekom.de/"> <mail:name>Xaver M. Linde</mail:name> <mail:street>Wikingerufer 7</mail:street> <mail:town>10555 Berlin</mail:town> <mail:bill>12.50</mail:bill> <tele:phone>030/1234567</tele:phone> <tele:phone>030/1234568</tele:phone> <tele:fax>030/1234569</tele:fax> <tele:bill>76.20</tele:bill> </ mail:address> bill disambiguation through mail and tele prefixes Languages forSemantic Web

  29. Resource Description FrameworkRDF Source:Knowledge Markup and Resource Semantics, By Harold Boley, Stefan Decker, and Michael Sintek, IJCAI-01 Tutorial, http://www.ijcai-01.org/

  30. Outline • Motivation: Why XML is not enough • Introduction to RDF • Requirements for KR on the Web • The RDF Data Model • RDF Schema • Extensions of RDF(S) • Tools for RDF and RDF Schema • Parser, Query, and Inference Engines Languages forSemantic Web

  31. Why The Shift Towards More Semantics? • Information Overload • Information on the Web currently aiming at Human Consumption • Information Consumption is too time consuming • Search Engines fail more and more • combined coverage is less than 42% of the HTML-Web • Data Interchange growing (e.g. B2B) • needs a common semantics Languages forSemantic Web

  32. Extensible Markup Language (XML) Revisited • Key idea: separate structure from presentation • XML DTDs or Schemas define document structure • Replace HTML with two things • A domain specific markup language (defined in XML) • A map from that markup language to HTML (defined using XSLT) • DTD enables document recipients to tell whether they’ve received a grammar-conforming document • Gives a minimal level of validation Languages forSemantic Web

  33. Why XML is Not Enough • Main advantage of using XML is reusing the parserand document validation • Many different possibilities to encode a domain of discourse • Leads to difficulties when understanding of foreign documents is required ==> Next step: separate content from structure! Languages forSemantic Web

  34. Encoding of Knowledge: Example “The Creator of the Resource “http://www.w3.org/Home/Lassila” is Ora Lassila Ora Lassila http://www.w3.org/Home/Lassila Creator Endless encoding possibilities in XML: <Creator> <uri>http://www.w3.org/Home/Lassila</uri> <name>Ora Lassila</name> </Creator> <Document uri=“http://www.w3.org/Home/Lassila” <Creator>Ora Lassila</Creator> </Document> <Document uri=“http://www.w3.org/Home/Lassila” Creator=“Ora Lassila”/> Languages forSemantic Web

  35. XML-based Communication using DTD A Parse Tree XML-Parser Sender using DTD A Point to Point Communicationfor Machine-Understandable Data Conceptual Domain Model(Objects and Relations) Person is_a Mammal Student is_a Person ---- Translation Step DTD or XML Schema <xsd:schema xmlns:xsd="http://..."> <xsd:annotation> A-Schema </xsd:...</xsd:schema> Deployment Recipient using DTD A Common Semantics Languages forSemantic Web

  36. Many Previously Unknown Communication Partners Languages forSemantic Web

  37. New Partners Don’t Understand Each Other ? Communication Partner using DTD B Communication Partner using DTD C ? ? XML-based Communication using DTD A Parse Tree XML-Parser Recipient using DTD A Sender using DTD A Languages forSemantic Web

  38. Merging Steps Between Models DTD A Steps DTD B <xsd:schema xmlns:xsd="http://..."> <xsd:annotation>B-Schema </xsd:...</xsd:schema> <xsd:schema xmlns:xsd="http://..."> <xsd:annotation>A-Schema </xsd:...</xsd:schema> Reengineering of the conceptual model Matching Matching XML Document Translation Generation(e.g. in XSLT) <xsl:stylesheet version="1.0” xmlns:xsl="http://....Transform" <xsl:template match="/"> .... </xsl:template> </xsl:stylesheet> <xsl:stylesheet version="1.0” xmlns:xsl="http://....Transform" <xsl:template match="/"> .... </xsl:template> </xsl:stylesheet> XML Document Translation from DTD A to DTD B (and B to A) Languages forSemantic Web

  39. Merging/Aligning Models • Reengineering step is costly and unnecessary, when a conceptual language is in use • Generation document translation procedures is again complicated and unnecessary ==> use a level on top of XML • What are requirements for such a level? Languages forSemantic Web

  40. Postulates: Fundamental Requirements for KR on the Web 1. Knowledge on the Web is distributed (link Knowledge on the Web) 2. Knowledge on the Web is biased - there is no universal truth it must be possible to dispute statements 3. Many different user communities: Extensibility and Simplicity ==> Resource Description Framework (RDF) Languages forSemantic Web

  41. Introduction to RDF • RDF (Resource Description Framework) • Beyond Machine readable to Machine understandable • RDF unites a wide variety of stakeholders: • Digital librarians, content-raters, privacy advocates, B2B industries, AI... • Significant (but less than XML) industrial momentum, lead by W3C • RDF consists of two parts • RDF Model (a set of triples) • RDF Syntax (different XML serialization syntaxes) • RDF Schema for definition of Vocabularies (simple Ontologies) for RDF (and in RDF) Languages forSemantic Web

  42. RDF Data Model • Resources • A resource is a thing you talk about (can reference) • Resources have URI’s • RDF definitions are themselves Resources (linkage, see requirement 1) • Properties • slots, define relationships to other resources or atomic values • Statements • “Resource has Property with Value” • (Values can be resources or atomic XML data) • Similar to Frame Systems Languages forSemantic Web

  43. Ora Lassila A Simple Example • Statement • “Ora Lassila is the creator of the resource http://www.w3.org/Home/Lassila” • Structure • Resource (subject) http://www.w3.org/Home/Lassila • Property (predicate) http://www.schema.org/#Creator • Value (object) "Ora Lassila” • Directed graph s:Creator http://www.w3.org/Home/Lassila Languages forSemantic Web

  44. Another Example • To add properties to Creator, point through an intermediate Resource. http://www.w3.org/Home/Lassila s:Creator Person://fi/654645635 Email Name Ora Lassila lassila@w3.org Languages forSemantic Web

  45. Collection Containers • Multiple occurrences of the same PropertyType don’t 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 valuesrdf:Bag • Sequence ordered values rdf:Seq • Alternative single valuerdf:Alt • Core RDF does not enforce ‘set’ semantics amongst values Languages forSemantic Web

  46. /courses/6.001 Example: Bag • The students incourse 6.001 are Amy, Tim,John, Mary,and Sue Rdf:Bag rdf:type /Students/Amy students rdf:_1 rdf:_2 /Students/Tim bagid1 rdf:_3 /Students/John rdf:_4 /Students/Mary rdf:_5 /Students/Sue Languages forSemantic Web

  47. http://x.org/package/X11 Example: Alternative • The source code for X11 may be found at ftp.x.org, ftp.cs.purdue.edu, or ftp.eu.net rdf:Alt rdf:type source rdf:_1 altid ftp.x.org rdf:_2 ftp.cs.purdue.edu rdf:_3 ftp.eu.net Languages forSemantic Web

  48. Statements About Statements (Requirement 2: Dispute Statements) • Making statements about statements requires a process for transforming them into Resources • subject the original resource • predicate the original property • object the original value • type rdf:Statement Languages forSemantic Web

  49. A Formal Model of RDF • RDF itself is mathematically straightforward: • Basic Definitions • Resources. • PropertiesResources • Literals • Statements = Properties  Resources  {Resources  Literals} • Typing • rdf:type  Properties • {RDF:type, sub, obj}  Statements  obj  Resources • for triples like {p,r1,r2} the RDF spec should use some different bracketing, like (p,r1,r2) Languages forSemantic Web

  50. Formal Model of RDF II • Reification • rdf:Statement  Resource-Properties • {rdf:predicate, rdf:subject, rdf:object }  Properties • Reification of a triple {pred, sub, obj} of Statements is an element r of Resources representing the reified triple and the elements s1, s2, s3, and s4 of Statements such that • s1: {RDF:predicate, r, pred} • s2: {RDF:subject, r, sub} • s3: {RDF:object, r, obj} • s4: {RDF:type, r, [RDF:Statement]} • Collections • { RDF:Seq, RDF:Bag, and RDF:Alt }  Resources-Properties • There is a subset of Properties corresponding to the ordinals (1, 2, 3, ...) called Ord. We refer to • elements of Ord as RDF:_1, RDF:_2, RDF:_3, ... Languages forSemantic Web

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