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Semantic Markup Languages: A Gentle Introduction Yolanda Gil USC/Information Sciences Institute gil@isi.edu. Outline. I: The Big Picture The Semantic Web http://www.scientificamerican.com/2001/0501issue/0501berners-lee.html http://www.w3.org/DesignIssues/Semantic.html

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  1. Semantic Markup Languages:A Gentle IntroductionYolanda GilUSC/Information Sciences Institutegil@isi.edu

  2. Outline • I: The Big Picture • The Semantic Web http://www.scientificamerican.com/2001/0501issue/0501berners-lee.html http://www.w3.org/DesignIssues/Semantic.html • II: A Gentle Introduction • XSD, RDFS, DAML http://trellis.semanticweb.org/expect/web/semanticweb/comparison.html • III: The Big Picture Revisited • W3C’s Semantic Web principles http://www.semanticweb.org • How this is changing our research in Knowledge Bases http://www.isi.edu/expect/papers/gil-seweb-book-01.pdf


  4. The Semantic Web W3C’s Tim Berners-Lee: “Weaving the Web”: “I have a dream for the Web… and it has two parts.” • The first Web enables communication between people • The Web shows how computers and networks enable the information space while getting out of the way • The new Web will bring computers into theaction • Step 1 -- Describe: putting data on the Web in machine-understandable form -- aSemantic Web • RDF (based on XML) • Master list of terms used in a document (RDF schema) • Each document mixes global standards and local agreed-upon terms (namespaces) • Step 2 -- Infer and reason: apply logic inference • Operate on partial understanding • Answering why • Heuristics

  5. Semantics and Meaning according to TBL • “In the extreme view, the world can be seen as only connections, nothing else. … I like the idea that a piece of information is really defined only by what it’s related to, and how it’s related. There really is little else to meaning. The structure is everything.” • “What matters is in the connections. It isn’t the letters, it’s the way they are strung together into words. […] into phrases. […] into a document. • For the people, by the people: the right to link “Once [… something…] was made available, it should be accesible to anyone […]. And it should be possible to make a link to that thing.”



  8. The Layer Cake [TBL,XML2000]

  9. The Layer Cake [TBL,XML2000]

  10. URIs: Uniform Resource Identifiers (aka URLs) http://trellis.semanticweb.org/semanticweb/slides/ ftp://www.allinone.org/all.gz • The Web is an information space. URIs are the points in that space. • Short strings that identify resources in the web: documents, images, downloadable files, services, electronic mailboxes, and other resources. • They make resources addressable in the same simple way. They reduce the tedium of "log in to this server, then issue this magic command ..." down to a single click.

  11. Unicode • A character encoding system, like ASCII, designed to help developers who want to create software applications that work in any language in the world • Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language

  12. The Layer Cake [TBL,XML2000]

  13. Why XML (eXtensible Markup Language) Problems with HTML HTML design - HTML is intended for presentation of information as Web pages. - HTML contains a fixed set of markup tags. This design is not appropriate for data: - Tags don’t convey meaning of the data inside the tags. - Tags are not extensible.

  14. The Design of XML • Tags can be used to represent the meaning of data/information • separates syntax (structural representation) from semantics => only syntax is considered in XML • There is no fixed set of markup tags - new tags can be defined • Underlying data model is a tree structure • “XML is the new ASCII” -- Tim Bray http://www.w3.org/TR/2000/REC-xml-20001006

  15. Simple XML Example Make up your own tags <Bookstore> <Book ID=“101”> <Author>John Doe</Author> <Title>Introduction to XML</Title> <Date>12 June 2001</Date> <ISBN>121232323</ISBN> <Publisher>XYZ</Publisher> </Book> <Book ID=“102”> <Author>Foo Bar</Author> <Title>Introduction to XSL</Title> <Date>12 June 2001</Date> <ISBN>12323573</ISBN> <Publisher>ABC</Publisher> </Book> </Bookstore> Sub-elements XML by itself is just hierarchically structured text

  16. XSD: XML Schema Definition • Written in the same syntax as XML documents (unlike XML DTDs!) • Elements and attributes • Enhanced set of primitve datatypes. • Wide range of primitive data types, supporting those found in databases (string, boolean, decimal, integer, date, etc.) • Can create your own datatypes (complexType) • - Can derive new type definitions on the basis of old ones (refinement) • Can have constraints on attributes • Examples: maxlength, precision, enumeration, maxInclusive (upper bound), minInclusive (lower bound), etc.

  17. An important diversion: Namespaces • What is a Namespace ? • The Namespace of an element, is the scope within which, it (and thus it’s name) is valid. (Ex. A basic block { … } in C) • Why do we need Namespaces ? • If elements were defined within a global scope, it becomes a problem when combining elements from multiple documents.  Name collision is hard to avoid. • Modularity: If a markup vocabulary exists which is well understood and for which there is useful software available, it is better to reuse this rather than make it again. • Namespaces in XML: • An XML namespace is a collection of names, identified by a URI reference. • Names from XML namespaces may appear as qualified names, which contain a single colon, separating the name into a prefix and a local part. The prefix, which is mapped to a URI reference, selects a namespace

  18. XSD (XML Schema) Example <?xml version="1.0"?> <xsd:schema xmlns:xsd=“http://www.w3.org/2001/XMLSchema” targetNamespace="http://www.books.org" xmlns=“http://www.books.org”> <xsd:element name="Bookstore"> <xsd:complexType> <xsd:sequence> <xsd:element ref="Book" minOccurs="1" maxOccurs="unbounded"/> </xsd:sequence> </xsd:complexType> </xsd:element> <xsd:element name="Book"> <xsd:complexType> <xsd:sequence> <xsd:element ref="Title" minOccurs="1" maxOccurs="1"/> <xsd:element ref="Author" minOccurs="1" maxOccurs=“unbounded”/> <xsd:element ref="Date" minOccurs="1" maxOccurs="1"/> <xsd:element ref="ISBN" minOccurs="1" maxOccurs="1"/> <xsd:element ref="Publisher" minOccurs="1" maxOccurs="1"/> </xsd:sequence> </xsd:complexType> </xsd:element> <xsd:element name="Title" type="xsd:string"/> <xsd:element name="Author" type="xsd:string"/> <xsd:element name="Date" type="xsd:Date"/> <xsd:element name="ISBN" type="xsd:integer"/> <xsd:element name="Publisher" type="xsd:string"/> </xsd:schema> Prefix “xsd” refers to the XMLSchema namespace “xmlns” refers to the default namespace Defining the element “Bookstore” as a complex Type Containing a sequence of 1 or more “Book” elements When referring to another Element, use “ref” The Author can be 1 or more Element definitions Notice the use of more meaningful data types

  19. Title Author Date ISBN Introduction to XML John Doe 12 June 2001 121232323 Introduction to XSL Foo Bar 12 June 2001 12323573 XSL [XML Stylesheet Language] An Example: <?xml version='1.0'?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/TR/WD-xsl"> <xsl:template match="/"> <html> <body> <table cellpadding="2" cellspacing="0" border="1" bgcolor="#FFFFD5"> <tr> <th>Title</th> <th>Author</th> <th>Date</th> <th>ISBN</th> </tr> <xsl:for-each select="Bookstore/Book"> <tr><td><xsl:value-of select="Title"/></td> <td><xsl:value-of select="Author"/></td> <td><xsl:value-of select="Date"/></td> <td><xsl:value-of select="ISBN"/></td> </tr> </xsl:for-each> </table> </body> </html> </xsl:template> </xsl:stylesheet> “xsl” namespace Match the Root Element Go through Each “Book” Element (inside a “Bookstore” Element) What you print out when the root element matches And, print out their Title, Author, Date, and ISBN Result: (Notice, that some fields have been filtered out from the XML file)

  20. XML: Tools/Software XML Spy By far, the most comprehensive editor. Handles XML files, DTD’s, XSL files, as well as XSD (XML Schema). Unfortunately only a 30 day trial version. http://www.xmlspy.com/download.html XML Notepad Microsoft XML Notepad is a simple application for building and editing small sets of XML-based data. Freeware. http://msdn.microsoft.com/xml/notepad/download.asp XML Pro XML Pro is a top-notch XML editor but it doesn’t include as many features as XML Spy. Shareware. http://www.vervet.com/demo.html You can also validate your XML files by just opening them with IE5.0 or above. It checks if the XML file is well-formed or not, and also validates against a DTD (if specified on the DOCTYPE declaration Some nice & short Tutorials on XML/XSL/DTD/XML Schemas can be found at: www.w3schools.com

  21. Summary of the XML+ NS +XSD LayerThe Power of Simplicity • Keeps the principles of SGML in place but its spec is thin enough to wave  • “When I designed HTML, I chose to avoid giving it more power than it absolutely needed – a “principle of least power”, which I have stuck to ever since. I could have used a language like Knuth’s Tex but…” -- TBL • To say you are “Using XML” is sort of like saying you are using ASCII • Using XSD (XML Schema) makes a lot more sense

  22. The Layer Cake [TBL,XML2000]

  23. Where XML & XML Schemas Fail • No semantics! • Will XML scale in the metadata world? • The order in which elements appear in an XML document is often meaningful. This seems highly unnatural in the metadata world. • Furthermore, maintaining the correct order of millions of data items is impractical. • XML allows constructions that mix up some text along with child elements, which are hard to handle. • Ex. <book> <title> … </title> <author> … </author> <isbn> … </isbn> </book> <bookstore> <book> … </book> <book> … </book> </bookstore> <topelem>This is some character string data <elem> this is a child <subelem>this is another child</subelem> </elem> </topelem>

  24. RDF (Resource Description Framework) • RDF provides a way of describing resources via metadata (data about data) • It restricts the description of resources to triplets (subject,predicate,object) • It provides interoperability between applications that exchange machine • understandable information on the Web. • The broad goal of RDF is to define a mechanism for describing resources • that makes no assumptions about a particular application domain, • nor defines (a priori) the semantics of any application domain. • Uses XML as the interchange syntax. • Provides a lightweight ontology system. • The formal specification of RDF is available at: • http://www.w3.org/TR/REC-rdf-syntax/

  25. RDF Syntax • Subject, Predicate and Object Triplets (Tuples) • Subject: The resource being described. • Predicate: A property of the resource • Object: The value of the property • A combination of them is said to be a Statement (or a rule) John Doe http://foo.bar.org/index.html Author A property of the web page (author) [Predicate] A web page being described [Subject] The value of the predicate (here the author) [Object]

  26. RDF Example <?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/TR/WD-rdf-syntax#" xmlns:s="http://description.org/schema/"> <rdf:Description about="http://foo.bar.org/index.html"> <s:Author>John Doe</s:Author> </rdf:Description> </rdf:RDF> Namespace for the RDF spec Namespace ‘s’, a custom namespace Subject Author (property of the subject) (Also a resource) Object. Can also point to a resource The above statement says : The Author of http://foo.bar.org/index.html is “John Doe” In this way, we can have different objects (resources) pointing to other objects (resources) , thus forming a DLG (Directed Line Graph) You can also make statements about statements – reification Ex: ‘xyz’ says that ‘ The Author of http://foo.bar.org/index.html is John Doe’

  27. RDF Schema • A schema defines the terms that will be used in the RDF statements and gives specific meanings to them. • http://www.w3.org/TR/rdf-schema/ • Example: <rdf:RDF xml:lang="en" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"> <rdf:Description ID="MotorVehicle"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="http://www.w3.org/2000/01/rdf-schema#Resource"/> </rdf:Description> <rdf:Description ID="PassengerVehicle"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description> <rdf:Description ID="Truck"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description> RDF Schema Namespace An “ID” attribute actually defines a new resource “Resource” is the top level class PassengerVehicle is a subclass of MotorVehicle

  28. Example (cont..) <rdf:Description ID="Van"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description> <rdf:Description ID="MiniVan"> <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#Van"/> <rdfs:subClassOf rdf:resource="#PassengerVehicle"/> </rdf:Description> <rdf:Description ID="registeredTo"> <rdf:type resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/> <rdfs:domain rdf:resource="#MotorVehicle"/> <rdfs:range rdf:resource="#Person"/> </rdf:Description> <rdf:Description ID="rearSeatLegRoom"> <rdf:type resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/> <rdfs:domain rdf:resource="#PassengerVehicle"/> <rdfs:domain rdf:resource="#Minivan"/> <rdfs:range rdf:resource="http://www.w3.org/2000/03/example/classes#Number"/> </rdf:Description> </rdf:RDF> Multiple Inheritance Domain of a property Range of a property

  29. RDF: Tools/Resources SirPAC A Simple RDF Parser & Compiler. It parses the RDF, and validates it. It also generates the tuples and even draws a graph of the data model. www.w3.org/RDF/Implementations/SiRPAC/ Reggie A Nice Metadata Editor. Java based simple user interface to describe a web resource. Can mail the metadata file to yourself after finished editing. http://metadata.net/dstc/ Protégé Editor of ontologies in practically any language you care about. Open source. http://www.smi.stanford.edu/projects/protege/

  30. Summary: RDF & RDF Schema layer • Minimalist model - (thing), Class, Property • Subproperty, Subclass • Domain & Range • Still not a W3C recommendation • Continues to change • Other languages are being built on XML substrate: XQUERY, XTM

  31. The Layer Cake [TBL,XML2000]

  32. Limitations of RDF • - Cannot define properties of properties (unique, transitive) • - No equivalence, disjointness, etc. • - No mechanism of specifying necessary and sufficient conditions • for class membership. • Example: • If it is given that ‘XYZ’ has a ‘car’ which is ‘7ft high’, has • ‘wide wheels’ and ‘loading space is 4 cub.m’, then we • should be able to reason that ‘XYZ’ has an ‘SUV’, as given by • the necessary and sufficient conditions for being an ‘SUV’ : • height > 4ft & wide wheels & loading space > 2 cub.m

  33. DAML+OIL’s History • W3C’s Semantic Web Activity: • - RDF and metadata markup efforts to represent data in • a machine understandable form. • DARPA started the DARPA Agent Markup Language (DAML) • program. • possibly with “ARPANET -> Internet” in mind • EC (European Commission) funding programs • - Ontology Interchange Language (OIL) • - logic based language. • - brings logic and inference to the Semantic Web • www.daml.org • DAML+OIL: http://www.daml.org/2001/03/daml+oil-index.html

  34. DAML+OIL (www.daml.org) • It builds on earlier W3C standards such as RDF and RDF Schema. • DAML extends RDF and RDFS with richer modelling primitives. • disjointWith, intersectionOf, oneOf, cardinality • Able to provide properties of properties • uniqueness, transitivity, etc. • Current version DAML+OIL provides a semantic interpretation (model-theoretic semantics) http://www.daml.org/2001/03/daml+oil-index.html

  35. An Example (from www.daml.org) <rdf:RDF xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:daml="http://www.daml.org/2000/12/daml+oil#" xmlns ="http://www.daml.org/2000/12/daml+oil-ex#" > <daml:Ontology about=“”> <daml:versionInfo>An example ontology</daml:versionInfo> <rdfs:Class rdf:ID="Animal"> <rdfs:label>Animal</rdfs:label> <rdfs:comment> This class of animals is illustrative of a number of ontological idioms. </rdfs:comment> </rdfs:Class> <rdfs:Class rdf:ID="Male"> <rdfs:subClassOf rdf:resource="#Animal"/> </rdfs:Class> <rdfs:Class rdf:ID="Female"> <rdfs:subClassOf rdf:resource="#Animal"/> <daml:disjointWith rdf:resource="#Male"/> </rdfs:Class> <rdfs:Class rdf:ID="Man"> <rdfs:subClassOf rdf:resource="#Person"/> <rdfs:subClassOf rdf:resource="#Male"/> </rdfs:Class> Start of an ontology (about = “” implies ‘this’ document) The label is not used for logical interpretation Can explicitly specify the set of Females to be disjoint with the set of Males The Person class is defined later To be read conjunctively. A man is a sub-class of ‘Person’ and a ‘Male’

  36. Example (contd..) <rdfs:Class rdf:ID="Woman"> <rdfs:subClassOf rdf:resource="#Person"/> <rdfs:subClassOf rdf:resource="#Female"/> </rdfs:Class> <rdf:ObjectProperty rdf:ID="hasParent"> <rdfs:domain rdf:resource="#Animal"/> <rdfs:range rdf:resource="#Animal"/> </rdf:ObjectProperty> <rdf:ObjectProperty rdf:ID="hasFather"> <rdfs:subPropertyOf rdf:resource="#hasParent"/> <rdfs:range rdf:resource="#Male"/> </rdf:ObjectProperty> <daml:DatatypeProperty rdf:ID="age"> <rdfs:range rdf:resource="http://www.w3.org/2000/10/XMLSchema#nonNegativeInteger"/> </daml:DatatypeProperty <rdfs:Class rdf:ID="Person"> <rdfs:subClassOf rdf:resource="#Animal"/> <rdfs:subClassOf> <daml:Restriction> <daml:onProperty rdf:resource="#hasParent"/> <daml:toClass rdf:resource="#Person"/> </daml:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <daml:Restriction daml:cardinality="1"> <daml:onProperty rdf:resource="#hasFather"/> </daml:Restriction> </rdfs:subClassOf> </rdfs:Class> An objectProperty relates objects to objects Describes the element which encloses this Property Describes the value of the Property Note: Contrary to RDF, DAML takes the ‘intersection’ of the domains/ranges if multiple domains/ranges are specified A datatype property relates an object to a primitive datatype value The XML Schema datatype is referenced here The Restriction defines an anonymous class of all things that satisfy the restriction. Restrictions on the property hasParent (only for the Person class – Local scope, as opposed to rdfs:range) A person can have only another Person as it’s parent A Person can have only 1 Father

  37. Example (contd..) Addition to the Animal Class without modifying it -- “about” <rdfs:Class rdf:about="#Animal"> <rdfs:subClassOf> <daml:Restriction daml:cardinality="2"> <daml:onProperty rdf:resource="#hasParent"/> </daml:Restriction> </rdfs:subClassOf> </rdfs:Class> <rdfs:Class rdf:about="#Person"> <rdfs:subClassOf> <daml:Restriction daml:maxcardinality="1"> <daml:onProperty rdf:resource="#hasSpouse"/> </daml:Restriction> </rdfs:subClassOf> </rdfs:Class> Restrictions on the property hasParent An animal can have exactly 2 parents Restrictions on the property hasSpouse A person can have only 1 spouse Further constructs that the example doesn’t use : Properties: TransitiveProperty (hasAncestor), UniqueProperty (hasMother), inverseOf(hasChild -> hasParent), etc. Classes: intersectionOf (a daml:collection), unionOf (a daml:collection), sameClassAs, complementOf, etc.

  38. DAML References/Tools DAML Viewer: It provides a means to view the instances found in a DAML document. http://www.daml.org/viewer/applet.html DAML Crawler Results: A list of .daml files on the internet http://www.daml.org/crawler/pages.html A DAML Validator http://www.daml.org/validator/ A DAML example explained: It has the same example as in the slides, discussed in detail. http://www.daml.org/2001/03/daml+oil-walkthru.html

  39. The Layer Cake [TBL,XML2000]


  41. To Learn More About These Languages... http://trellis.semanticweb.org/: more detailed tutorial slides http://trellis.semanticweb.org/expect/web/semanticweb/flairs02.pdf http://trellis.semanticweb.org/expect/web/semanticweb/comparison.html

  42. The View from W3Chttp://www.w3.org/TR/ • XML Schema Part 0: Primer 02 May 2001, David C. Fallside • XML Schema Part 1: Structures 02 May 2001, Henry S. Thompson, David Beech, Murray Maloney, N. Mendelsohn • XML Schema Part 2: Datatypes 02 May 2001, Paul V. Biron, Ashok Malhotra • Extensible Markup Language (XML) 1.0 (Second Edition) 6 October 2000, Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler • Namespaces in XML 14 January 1999, Tim Bray, Dave Hollander, Andrew • Resource Description Framework (RDF) Model and Syntax Specification 22 February 1999, Ora Lassila, Ralph R. Swick • Resource Description Framework (RDF) Schemas 3 March 2000, Dan Brickley, R.V. Guha • RDF Model Theory 25 September 2001, Patrick Hayes • XML Schema: Formal Description 25 September 2001, Allen Brown, Matthew Fuchs, Jonathan Robie, Philip Wadler RECOMMENDATIONS CAND REC WORKING DRAFTS

  43. W3C Review Stages • A Working Draft represents work in progress and a commitment by W3C to pursue work in this area. A Working Draft does not imply consensus by a group or W3C. A Candidate Recommendation is work that has received significant review from its immediate technical community. It is an explicit call to those outside of the related Working Groups or the W3C itself for implementation and technical feedback. • A Proposed Recommendation is work that (1) represents consensus within the group that produced it and (2) has been proposed by the Director to the Advisory Committee for review. • A Recommendation is work that represents consensus within W3C and has the Director's stamp of approval. W3C considers that the ideas or technology specified by a Recommendation are appropriate for widespread deployment and promote W3C's mission.


  45. W3C’s Semantic Web Principles • Everything identifiable is in the Semantic Web (URIs!) • Partial information • Anyone can say anything about anything • Web of trust • All statements on the Web occur in some context • Evolution • Allow combining independent work done by different communities • Minimalist design • Make the simple things simple, and the complex things possible • Standardize no more than is necessary

  46. Hypertext: Then and Now • SOTA circa 1990: Dynatext’s electronic book • A book had to be compiled (like a program) in order to be displayed efficiently • A central link database, to make sure there were no broken links • Text that was fixed and consistent (a whole book) • WWW: • Links can be added and used at any time • Distributed (must live with broken links!) • Decentralized

  47. Knowledge Representation: Now and Tomorrow “To webize KR in general is, in many ways, the same as to webize hypertext. Replace identifiers with URIs. Remove any requirement for global consistency. Put any significant effort into getting critical mass. Sit back.” -- TBL

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