1、Introduction to the Semantic Web,Questions,What is the Semantic Web? Why do we want it? How will we do it? Who will do it? When will it be done?,“XML is Lisps bastard nephew, with uglier syntax and no semantics. Yet XML is poised to enable the creation of a Web of data that dwarfs anything since the
2、 Library at Alexandria.”- Philip Wadler, Et tu XML? The fall of the relational empire, VLDB, Rome, September 2001.,“The web has made people smarter. We need to understand how to use it to make machines smarter, too.”- Michael I. Jordan, paraphrased from a talk at AAAI, July 2002 by Michael Jordan (U
3、C Berkeley),“The Semantic Web will globalize KR, just as the WWW globalize hypertext” - Tim Berners-Lee,“The multi-agent systems paradigm and the web both emerged around 1990. One has succeeded beyond imagination and the other has not yet made it out of the lab.”- Anonymous, 2001,IOHO,The web is lik
4、e a universal acid, eating through and consuming everything it touches. Web principles and technologies are equally good for wireless/pervasive computing The semantic web is our first serious attempt to provide semantics for XML sublanguages It will provide mechanisms for people and machines (agents
5、, programs, web services) to come together. In all kinds of networked environments: wired, wireless, ad hoc, wearable, etc.,Origins,Tim Berners-Lees original 1989 WWW proposal described a web of relationships among named objects unifying many info. management tasks. Capsule historyGuhas MCF (94) XML
6、+MCF=RDF (96)RDF+OO=RDFS (99)RDFS+KR=DAML+OIL (00)W3Cs SW activity (01)W3Cs OWL (03),http:/www.w3.org/History/1989/proposal.html,W3Cs Semantic Web Goals,Focus on machine consumption: “The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabl
7、ing computers and people to work in cooperation.“ - Berners-Lee, Hendler and Lassila, The Semantic Web, Scientific American, 2001,TBLs semantic web vision,Semantic web stack 2006,Why is this hard?,after Frank van Harmelen and Jim Hendler,What a web page looks like to a machine,after Frank van Harmel
8、en and Jim Hendler,OK, so HTML is not helpful,Maybe we can tell the machine what the different parts of the text represent?,title,time,speaker,location,abstract,biosketch,host,XML to the rescue?,XML fans propose creating a XML tag set to use for each application.For talks, we can choose , , etc.,aft
9、er Frank van Harmelen and Jim Hendler,XML machine accessible meaning,But, to your machine, the tags still look like this.The tag names carry no meaning.XML DTDs and Schemas have little or no semantics.,after Frank van Harmelen and Jim Hendler,XML Schema helps,XML Schemas provide a simple mechanism t
10、o define shared vocabularies.,XML Schema file,after Frank van Harmelen and Jim Hendler,XML Schema file 42,XML Schema file 1,But there are many schemas,after Frank van Harmelen and Jim Hendler,Theres no way to relate schema,Either manually or automatically. XML Schema is weak on semantics.,An Ontolog
11、y level is needed,We need a way to define ontologies in XML So we can relate them So machines can understand (to some degree) their meaning,XML Ontology 256,imports,imports,=,Ontologies add Structure Constraints mappings,Today and tomorrow,Simple ontologies like FOAF & DC in use today Weve crawled m
12、ore than 3M FOAF RDF files We hope to be able to make effective use ontologies like Cyc in the coming decade There are skeptics Its a great research topic The SW community has a roadmap and some experimental languages Industry is still holding back They are being conservative We need more experiment
13、ation and exploration,Semantic Web Languages,Semantic web languages today,Today there are, IOHO, two semantic web languages RDF Resource Description Framework http:/www.w3.org/RDF/ DAML+OIL Darpa Agent Markup Language http:/www.daml.org/ (deprecated) OWL Ontology Web Language http:/www.w3.org/2001/s
14、w/ Topic maps (http:/topicmaps.org/) are another species, not based on RDF Microformats, Common Logic, etc. offer other possibilities,Two Semantic Web Notions,The semantic web The idea of a web of machine understandable information Agnostic about the technology used to support it May involve more AI
15、 (e.g., NLP) Human end users in the center The Semantic Web The current vision of a semantic web as defined by the W3C community: a web of data Using W3C supported standards (i.e., RDF, OWL, SPARQL, XML, RIF, etc. By machines for machines with human oriented applications on top.,RDF is the first SW
16、language,XML Encoding,Graph,stmt(docInst, rdf_type, Document) stmt(personInst, rdf_type, Person) stmt(inroomInst, rdf_type, InRoom) stmt(personInst, holding, docInst) stmt(inroomInst, person, personInst),Triples,RDF Data Model,Good for Machine Processing,Good For Human Viewing,Good For Reasoning,RDF
17、 is a simple language for building graph based representations,The RDF Data Model,An RDF document is an unordered collection of statements, each with a subject, predicate and object (aka triples) A triple can be thought of as a labelled arc in a graph Statements describe properties of web resources
18、A resource is any object that can be pointed to by a URI: a document, a picture, a paragraph on the Web, E.g., http:/umbc.edu/finin/cv.html a book in the library, a real person (?) isbn:/5031-4444-3333 Properties themselves are also resources (URIs),URIs are a foundation,URI = Uniform Resource Ident
19、ifier “The generic set of all names/addresses that are short strings that refer to resources“ URLs (Uniform Resource Locators) are a subset of URIs, used for resources that can be accessed on the web URIs look like “normal” URLs, often with fragment identifiers to point to a document part: http:/ UR
20、Is are unambiguous, unlike natural language terms the web provides a global namespace We assume references to the same URI are to the same thing,What does a URI mean?,Sometimes URIs denote a web resource http:/umbc.edu/finin/finin.jpg denotes a file We can use RDF to make assertions about the resour
21、ce, e.g., its an image and depicts a person with name Tim Finin, Sometimes concepts in the external world E.g., http:/umbc.edu/ denotes a particular University located in Baltimore This is done by social convention Cool URIs dont change http:/www.w3.org/Provider/Style/URI,The RDF Graph,An RDF docume
22、nt is an unordered collection of triples The subject of one triple can be the object of another So the result is a directed, labelled graph A triples object can also be a literal, e.g., a string.,Simple RDF Example,http:/umbc.edu/ finin/talks/idm02/,“Intelligent Information Systems on the Web and in
23、 the Aether”,http:/umbc.edu/,dc:Title,dc:Creator,bib:Aff,“Tim Finin”,“fininumbc.edu”,bib:name,bib:email,XML encoding for RDF,Intelligent Information Systems on the Web and in the AetherTim Fininfininumbc.edu,N triple representation,RDF can be encoded as a set of triples. .“Intelligent Information Sy
24、stems on the Web and in the Aether“ . _:j10949 “Tim Finin“ . _:j10949 “fininumbc.edu“ . _:j10949 . _:j10949 ._:j10949 .Note the gensym for the anonymous node _:j10949,Triple Notes,RDF triples have one of two forms:Triples are also easily mapped into logicbecoming: (,) With type(,) becoming () Exampl
25、e: subclass(man,person) sex(man,male) domain(sex,animal) man(adam) age(adam,100) Triples are easily stored and managed in DBMS Flat nature of a triple a good match for relational DBs,; Note: were not ; showing the actual ; URIs for clarity,N3 notation for RDF,N3 is a compact notation for RDF that is
26、 easier for people to read, write and edit. Aka Notation 3, developed by TBL himself. Translators exist between N3 and the XML encoding, such as the web form on http:/www.w3.org/DesignIssues/Notation3.html So, its just “syntactic sugar” But, XML is largely unreadable and even harder to write,N3 Exam
27、ple,prefix rdf: http:/www.w3.org/1999/02/22-rdf-syntax-ns# . prefix dc: http:/purl.org/dc/elements/1.1/ . prefix bib: http:/daml.umbc.edu/ontologies/bib/ .dc:title “Intelligent Information Systems on the Web and in the Aether“ ;dc:creator bib:Name “Tim Finin“;bib:Email “fininumbc.edu“bib:Aff: “http:
28、/umbc.edu/“ .,A usecase: FOAF,FOAF (Friend of a Friend) is a simple ontology to describe people and their social networks. See the foaf project page: http:/www.foaf-project.org/ We recently crawled the web and discovered over 1,000,000 valid RDF FOAF files. Most of these are from the http:/liveJ blo
29、gging system which encodes basic user info in foaf See http:/apple.cs.umbc.edu/semdis/wob/foaf/, Tim Finin 241037262c252e,FOAF Vocabulary,Basics Agent Person name nick title homepage mbox mbox_sha1sum img depiction (depicts) surname family_name givenname firstName,Personal Info weblog knows interest
30、 currentProject pastProject plan based_near workplaceHomepage workInfoHomepage schoolHomepage topic_interest publications geekcode myersBriggs dnaChecksum,Documents & Images Document Image PersonalProfileDocument topic (page) primaryTopic tipjar sha1 made (maker) thumbnail logo,Projects & Groups Pro
31、ject Organization Group member membershipClass fundedBy theme,Online Accts OnlineAccount OnlineChatAccount OnlineEcommerceAccount OnlineGamingAccount holdsAccount accountServiceHomepage accountName icqChatID msnChatID aimChatID jabberID yahooChatID,FOAF: why RDF? Extensibility!,FOAF vocabulary provi
32、des 50+ basic terms for making simple claims about people FOAF files can use other RDF terms too: RSS, MusicBrainz, Dublin Core, Wordnet, Creative Commons, blood types, starsigns, RDF guarantees freedom of independent extension OWL provides fancier data-merging facilities Result: Freedom to say what
33、 you like, using any RDF markup you want, and have RDF crawlers merge your FOAF documents with others and know when youre talking about the same entities.,After Dan Brickley, danbriw3.org,No free lunch!,Consequence: We must plan for lies, mischief, mistakes, stale data, slander Dataset is out of con
34、trol, distributed, dynamic Importance of knowing who-said-what Anyone can describe anyone We must record data provenance Modeling and reasoning about trust is critical Legal, privacy and etiquette issues emerge Welcome to the real world,After Dan Brickley, danbriw3.org,More RDF Vocabulary,RDF has te
35、rms for describing lists, bags, sequences, etc. RDF also can describe triples through reification Enabling statements about statements :john bdi:believes _:s. _:s rdf:type rdf:Statement. _:s rdf:subject . _:s rdf:predicate cat:salePrice . _:s rdf:object “19.95“ .,RDF is being used!,RDF has a solid s
36、pecification RDF is being used in a number of web standards CC/PP (Composite Capabilities/Preference Profiles) P3P (Platform for Privacy Preferences Project) RSS (RDF Site Summary) RDF Calendar ( iCalendar in RDF) And in other systems Netscapes Mozilla web browser, open directory (http:/dmoz.org/) A
37、dobe products via XMP (eXtensible Metadata Platform) Web communities: LiveJournal, Ecademy, and Cocolog In Microsofts VISTA: Connected Services Framework uses an RDF database and SPARQL,RDF Schema (RDFS),RDF Schema adds taxonomies for classes & properties subClass and subProperty and some metadata.
38、domain and range constraints on properties Several widely used KB tools can import and export in RDFS,Stanford Protg KB editor Java, open sourced extensible, lots of plug-ins provides reasoning & server capabilities,RDFS Vocabulary,Terms for classes rdfs:Class rdfs:subClassOf Terms for properties rd
39、fs:domain rdfs:range rdfs:subPropertyOf Special classes rdfs:Resource rdfs:Literal rdfs:Datatype,Terms for collections rdfs:member rdfs:Container rdfs:ContainerMembershipProperty Special properties rdfs:comment rdfs:seeAlso rdfs:isDefinedBy rdfs:label,RDFS introduces the following terms and gives ea
40、ch a meaning w.r.t. the rdf data model,RDF and RDF Schema,John Smith ,RDFS supports simple inferences,An RDF ontology plus some RDF statements may imply additional RDF statements. This is not true of XML. Note that this is part of the data model and not of the accessing or processing code.,prefix rd
41、fs: . prefix : . parent rdfs:domain person;rdfs:range person. mother rdfs:subProperty parent;rdfs:domain woman;rdfs:range person. eve mother cain.,parent a property. person a class. woman subClass person. mother a property. eve a person;a woman;parent cain. cain a person.,New and Improved! 100% Bett
42、er than XML!,N3 example,prefix rdf: . prefix rdfs: . prefix : .rdfs:comment “This is an N3 example”. :Person a rdfs:Class. :Woman a rdfs:Class; rdfs:subClassOf :Person. :eve a :Woman; :age “100”. :sister a rdf:Property; rdfs:domain :Person; rdfs:range :Woman. :eve :sister a :Woman; :age 98. :eve :be
43、lieve :eve :age “100”. is :spouse of is :sister of :eve :age 99. :eve.:sister.:spouse :age 99.,This defines the “empty prefix” as refering to “this document”,Heres how you declare a namespace., Is an alias for the URI of this document.,“person is a class”. The “a” syntax is sugar for rdf:type proper
44、ty.,“Woman is a class and a subclass of person”. Note the ; syntax.,“eve is a woman whose age is 100.”,“sister is a property from person to woman”,“eve has a sister who is a 98 year old woman”. The brackets introduce an anonymous resource.,“eve believes that her age is 100”. The braces introduce a r
45、eified triple.,“the spouse of the sister of eve is 99”.,“the spouse of the sister of eve is 99”.,Is RDF(S) better than XML?,Q: For a specific application, should I use XML or RDF? A: It depends XMLs model is a tree, i.e., a strong hierarchy applications may rely on hierarchy position relatively simp
46、le syntax and structure not easy to combine trees RDFs model is a loose collections of relations applications may do “database”-like search not easy to recover hierarchy easy to combine relations in one big collection great for the integration of heterogeneous information,From where will the markup
47、come?,A few authors will add it manually. More will use annotation tools. SMORE: Semantic Markup, Ontology and RDF Editor Intelligent processors (e.g., NLP) can understand documents and add markup (hard) Machine learning powered information extraction tools show promise Lots of web content comes fro
48、m databases & we can generate SW markup along with the HTML See http:/ebiquity.umbc.edu/,From where will the markup come?,In many tools, part of the metadata information is present, but thrown away at output e.g., a business chart can be generated by a tool it “knows” the structure, the classificati
49、on, etc. of the chart but, usually, this information is lost storing it in metadata is easy! So “semantic web aware” tools can produce lots of metadata E.g., Adobes use of its XMP platform,Problems with RDFS,RDFS too weak to describe resources in sufficient detail, e.g.: No localised range and domai
50、n constraints Cant say that the range of hasChild is person when applied to persons and elephant when applied to elephants No existence/cardinality constraints Cant say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents No transitive, inverse or symmetrical properties Cant say that isPartOf is a transitive property, that hasPart is the inverse of isPartOf or that touches is symmetrical We need RDF terms providing these and other features.,
