Semantic Markup Languages: A Gentle Introduction Yolanda Gil - PowerPoint PPT Presentation

Semantic Markup Languages: A Gentle Introduction Yolanda Gil USC/Information Sciences Institute gil@isi.edu Yolanda Gil 1

Outline ν I: The Big Picture ™ The Semantic Web http://www.scientificamerican.com/2001/0501issue/0501berners-lee.html http://www.w3.org/DesignIssues/Semantic.html ν II: A Gentle Introduction ™ XSD, RDFS, (DAML), OWL http://trellis.semanticweb.org/expect/web/semanticweb/comparison.html ν 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 Yolanda Gil 2

I: THE BIG PICTURE Yolanda Gil 3

The Semantic Web W3C’s Tim Berners-Lee: “Weaving the Web”: “I have a dream for the Web… and it has two parts.” ν The first Web enables communication between people ™ The Web shows how computers and networks enable the information space while getting out of the way ν The new Web will bring computers into the action ™ Step 1 -- Describe: putting data on the Web in machine- understandable form -- a Semantic 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 Yolanda Gil 4

Semantics and Meaning according to TBL ν “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.” ν “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.” ν 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.” Yolanda Gil 5

And There You Have I Yolanda Gil 6

II: THE GENTLE INTRODUCTION Yolanda Gil 7

The Layer Cake [TBL,XML2000] Yolanda Gil 8

The Layer Cake [TBL,XML2000] Yolanda Gil 9

URIs: Uniform Resource Identifiers (aka URLs) http://trellis.semanticweb.org/semanticweb/slides/ ftp://www.allinone.org/all.gz ν The Web is an information space. URIs are the points in that space. ν Short strings that identify resources in the web: documents, images, downloadable files, services, electronic mailboxes, and other resources. ν 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. Yolanda Gil 10

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Unicode ν A character encoding system, like ASCII, designed to help developers who want to create software applications that work in any language in the world ν Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language Yolanda Gil 12

The Layer Cake [TBL,XML2000] Yolanda Gil 13

Why XML (eXtensible Markup Language) Problems wit h HTML HTML design - HTML is intended for presentation of informat ion as Web pages. - HTML cont ains a fixed set of markup tags . This design is not appropriat e for dat a: - Tags don’ t convey meaning of t he dat a inside t he t ags. - Tags are not ext ensible. Yolanda Gil 14

The Design of XML ν Tags can be used to represent the meaning of data/information ™ separates syntax (structural representation) from semantics => only syntax is considered in XML ν There is no fixed set of markup tags - new tags can be defined ν Underlying data model is a tree structure ν “XML is the new ASCII” -- Tim Bray http://www.w3.org/TR/2000/REC-xml-20001006 Yolanda Gil 15

Simple XML Example <Bookst ore> <Book ID=“ 101” > <Aut hor>John Doe</ Aut hor> <Tit le>Int roduct ion t o XML</ Tit le> <Dat e>12 June 2001</ Dat e> <ISBN>121232323</ ISBN> <Publisher>XYZ</ Publisher> </ Book> <Book ID=“ 102” > <Aut hor>Foo Bar</ Aut hor> <Tit le>Int roduct ion t o XSL</ Tit le> <Dat e>12 June 2001</ Dat e> <ISBN>12323573</ ISBN> <Publisher>ABC</ Publisher> </ Book> </ Bookst ore> XML by it self is j ust hierarchically st ruct ured t ext Yolanda Gil 16

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XSD: XML Schema Definition – Writ t en in t he same synt ax as XML document s (unlike XML DTDs!) – Element s and at t ribut es – Enhanced set of primit ve dat at ypes. • Wide range of primit ive dat a t ypes, support ing t hose found in dat abases (st ring, boolean, decimal, int eger, dat e, et c.) • Can creat e your own dat at ypes (complexType) - Can derive new t ype definit ions on t he basis of old ones (refinement ) – Can have const raint s on at t ribut es • Examples: maxlengt h, precision, enumerat ion, maxInclusive (upper bound), minInclusive (lower bound), et c. Yolanda Gil 18

XSD (XML Schema) Example Yolanda Gil 19

< ?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> �I XSL [XML Stylesheet Language] < �U Yolanda Gil 20

XML: Tools/Software Yolanda Gil 21

Summary of the XML+ NS +XSD Layer The Power of Simplicity ν “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 ν Keeps the principles of SGML in place but its spec is thin enough to wave ϑ ν To say you are “Using XML” is sort of like saying you are using ASCII ν Using XSD (XML Schema) makes a lot more sense Yolanda Gil 22

The Layer Cake [TBL,XML2000] Yolanda Gil 23

Where XML & XML Schemas Fail • No semant ics! • Will XML scale in t he met adat a world? 1. The order in which element s appear in an XML document is oft en meaningful. This seems highly unnat ural in t he met adat a world. Furt hermore, maint aining t he correct order of millions of dat a it ems is impract ical. 2. XML allows const ruct ions t hat mix up some t ext along wit h child element s, which are hard t o handle. Ex. Yolanda Gil 24

RDF (Resource Description Framework) • RDF provides a way of describing resources via met adat a (dat a about dat a) It rest rict s t he descript ion of resources t o triplets (subject,predicate,object) • It provides int eroperabilit y bet ween applicat ions t hat exchange machine underst andable informat ion on t he Web. • The original broad goal of RDF was t o define a mechanism for describing resources t hat makes no assumpt ions about a part icular applicat ion domain, nor defines (a priori) t he semant ics of any applicat ion domain. • Uses XML as t he int erchange synt ax. Provides a lightweight ont ology syst em. • The formal specificat ion of RDF is available at : ht t p:/ / www.w3.org/ TR/ REC-rdf-synt ax/ Yolanda Gil 25

RDF Syntax Subj ect , Predicat e and Obj ect Triplet s (Tuples) • Subj ect : The resource being described. • Predicat e: A propert y of t he resource • Obj ect : The value of t he propert y A combinat ion of t hem is said t o be a St at ement (or a rule) Yolanda Gil 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> Yolanda Gil 27

RDF Schema • A schema defines t he t erms t hat will be used in t he RDF st at ement s and gives specific meanings t o t hem. http://www.w3.org/TR/rdf-schema/ Example: <rdf:RDF xml:lang="en" xmlns:rdf="ht t p:/ / www.w3.org/ 1999/ 02/ 22-rdf-synt ax-ns#" xmlns:rdfs="ht t p:/ / www.w3.org/ 2000/ 01/ rdf-schema#"> <rdf:Descript ion ID="Mot orVehicle"> <rdf:t ype resource="ht t p:/ / www.w3.org/ 2000/ 01/ rdf-schema#Class"/ > <rdfs:subClassOf rdf:resource="ht t p:/ / www.w3.org/ 2000/ 01/ rdf-schema#Resource"/ > </ rdf:Descript ion> <rdf:Descript ion ID="PassengerVehicle"> <rdf:t ype resource="ht t p:/ / www.w3.org/ 2000/ 01/ rdf-schema#Class"/ > <rdfs:subClassOf rdf:resource="#Mot orVehicle"/ > </ rdf:Descript ion> <rdf:Descript ion ID="Truck"> <rdf:t ype resource="ht t p:/ / www.w3.org/ 2000/ 01/ rdf-schema#Class"/ > <rdfs:subClassOf rdf:resource="#Mot orVehicle"/ > </ rdf:Descript ion> Yolanda Gil 28