RDFa: putting RDF on the Web
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RDFa: putting RDF on the Web

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These slides explain (1) the motivation for using RDFa, for embedding structured data on web pages, (2) RDF as the foundation of RDFa, and (3) RDFa through examples.

These slides explain (1) the motivation for using RDFa, for embedding structured data on web pages, (2) RDF as the foundation of RDFa, and (3) RDFa through examples.

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RDFa: putting RDF on the Web RDFa: putting RDF on the Web Presentation Transcript

  • Digital Enterprise Research Institute www.deri.ie RDFa: putting RDF on the Web Benjamin.Heitmann@deri.org Chapter 1  Copyright 2007 Digital Enterprise Research Institute. All rights reserved. www.deri.org
  • Overview Digital Enterprise Research Institute www.deri.ie  Part 1: Motivation  Use case for embedding RDF on XHTML pages  Bonus: Explaining “what do I do?” at a Christmas party  Part 2: The foundation: RDF in a nutshell  data model  formal semantics  Part 3: RDFa with examples  the RDFa attributes  visible and invisible embedding  handle with care: CURIEs and implicit blank nodes  tools for consuming and publishing RDFa 2 of 28 Benjamin Heitmann
  • About me Digital Enterprise Research Institute www.deri.ie  Research interest: bringing the Semantic Web to the IT mainstream by applying software engineering methodologies  currently looking for a PhD Thesis topic  Master thesis topic: Transitioning web application frameworks towards the Semantic Web  worked with Eyal Oren (ActiveRDF) and Max Völkel (RDF2Go, RDFReactor)  Student research topic: simplifying RDF semantics 3 of 28 Benjamin Heitmann
  • Digital Enterprise Research Institute www.deri.ie Motivation Chapter 4  Copyright 2007 Digital Enterprise Research Institute. All rights reserved. www.deri.org
  • Publishing structured content Digital Enterprise Research Institute www.deri.ie  Current Web: written for humans (mostly)  Interesting information for machine agents exists  Example: music events in Galway (date, performer, venue)  Problem:  web page mark-up does not explicitly encode information  scraping of web pages is expensive:  webpage may change without warning  human intervention is necessary 5 of 28 Part 1: Motivation Benjamin Heitmann
  • A potential provider of event data 6 of 28 Part 1: Motivation Benjamin Heitmann
  • A potential consumer of event data 7 of 28 Part 1: Motivation Benjamin Heitmann
  • Possible solution Digital Enterprise Research Institute www.deri.ie  Embed machine readable data on the same web page  Benefits:  write and publish once  readableby humans and machine agents  easy maintenance for publisher  easy consuming of data after discovery  Two approaches:  Microformats: – fixed vocabulary, not extendable and customisable  RDFa – all the benefits of RDF: flexible and customisable – all the overhead of RDF: data model and formal semantics 8 of 28 Part 1: Motivation Benjamin Heitmann
  • A side note: Explaining “what do you do?” at a party Digital Enterprise Research Institute www.deri.ie  Quicker use case  suitable for explaining “what do you actually do?”  like, lets say, for the Christmas holidays  What can Google do today?  List Restaurants in Dublin  Can Google also give you this information?  Restaurants in Dublin, open on Thursday late, serving Pepperoni Pizza for under 15 Euro  RDFa (and the Semantic Web) could make this possible! 9 of 28 Benjamin Heitmann
  • Digital Enterprise Research Institute www.deri.ie The foundation: RDF in a nutshell Chapter 10  Copyright 2007 Digital Enterprise Research Institute. All rights reserved. www.deri.org
  • Show of hands Digital Enterprise Research Institute www.deri.ie  Who can answer these questions: 1.What is the RDF data model? 2.What is the formal semantics of RDF? 11 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • RDF data model: A graph Digital Enterprise Research Institute www.deri.ie  Graph with nodes and directed arcs  Graph consists of triples  Each triple has a subject, predicate and object  Contrast with other data models:  SQL: tables and relations  XML: tree of nodes with attributes 12 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • RDF data model: node types Digital Enterprise Research Institute www.deri.ie 1.Uniform Resource Blank Identifier (URI): URI Literal Node  basically like in the browser location field  globally unique Subject X X 2.Literal  like a string  can optionally have either a data type or a language Predicate X tag 3.Blank Node  place holder Object X X X  only locally unique 13 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • An example with all three types Digital Enterprise Research Institute www.deri.ie 14 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • RDF formal semantics Digital Enterprise Research Institute www.deri.ie  uses “model theory” to provide “a globally coherent notion of meaning”  provides basis for inference rules  specifies the semantics of RDF  Fundamental property: open world semantics and monotonic reasoning  Facts: I go to work from Monday to Friday.  Question: Will I go to work on the weekend?  Closed world semantics: no facts about the weekend -> answer: no.  Open world semantics: answer not possible  Monotonic reasoning: adding new data always possible 15 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • Model theory example Digital Enterprise Research Institute www.deri.ie  Three triples:  <ex:a> <ex:b> <ex:c> .  <ex:c> <ex:a> <ex:a> .  <ex:c> <ex:b> <ex:a> .  True with this interpretation: 16 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • What to remember about RDF Digital Enterprise Research Institute www.deri.ie  RDF is the foundation of the Semantic Web  defines the data model  provides base layer semantics  other standards extend these semantics (like RDF Schema and OWL)  domain ontologies provide domain specific semantics (like FOAF) on top of RDF Schema 17 of 28 Part 2: RDF Foundation Benjamin Heitmann
  • Digital Enterprise Research Institute www.deri.ie RDFa with examples Chapter 18  Copyright 2007 Digital Enterprise Research Institute. All rights reserved. www.deri.org
  • What is RDFa? Digital Enterprise Research Institute www.deri.ie  syntax for embedding an RDF graph in an XHTML document  uses XHTML attributes for expressing RDF properties  properties about  same page  or external URI  properties can reuse visible page content or be invisible  XHTML documents with RDFa are backwards compatible  GRDDL transformation to extract RDF exists 19 of 28 Part 3: RDFa Benjamin Heitmann
  • RDFa Example: Graph Digital Enterprise Research Institute www.deri.ie 20 of 28 Part 3: RDFa Benjamin Heitmann
  • RDFa Example: Source Digital Enterprise Research Institute www.deri.ie <div xmlns:foaf=quot;http://xmlns.com/foaf/0.1/quot; about=quot;#mequot; rel=quot;foaf:knowsquot;> <ul> <li typeof=quot;foaf:Personquot;> <a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/bobquot;>Bob</a> </li> <li typeof=quot;foaf:Personquot;> <a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/evequot;>Eve</a> </li> <li typeof=quot;foaf:Personquot;> <a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/manuquot;>Manu</a> </li> </ul> </div> 21 of 28 Part 3: RDFa Benjamin Heitmann
  • What just happened? Digital Enterprise Research Institute www.deri.ie  RDFa works a little bit like RDF/XML  xmlns:foaf declares the FOAF namespace  about=”#me” defines the subject of a triple  rel=”foaf:knows” defines the predicate  typeof=”foaf:Person” defines a resource type  typeof without explicit URLs leads to blank nodes  property=”foaf:name” uses the literal “Bob”  rel=”foaf:homepage” uses the href  That’s 12 triples for just 8 XML nodes 22 of 28 Part 3: RDFa Benjamin Heitmann
  • CURIEs versus URIs Digital Enterprise Research Institute www.deri.ie  Notall of the 10 RDFa attributes can use URIs  CURIE (Compact URI )  example: foaf:Person  curie := [ [ prefix ] ':' ] reference  prefix is either a defined name space or the default namespace  URI (like http://dbpedia.org/resource/London)  SafeCURIE  CURIE in square brackets, example: [wiki:Biome]  prevents ambiguities between URIs and CURIEs  Idea: Subject and Object can be external, use URIs. Predicate should be internal, so use CURIE. 23 of 28 Part 3: RDFa Benjamin Heitmann
  • Attributes for subjects Digital Enterprise Research Institute www.deri.ie @about : URI or SafeCURIE define a subject of a triple @src : URI a not clickable resource object, like a picture or multimedia object 24 of 28 Part 3: RDFa Benjamin Heitmann
  • Attributes for predicates Digital Enterprise Research Institute www.deri.ie @rel : CURIEs express relationship between resources @rev : CURIEs express reverse relationship between resources @property : CURIEs express relationship between a subject and a literal 25 of 28 Part 3: RDFa Benjamin Heitmann
  • Attributes for objects Digital Enterprise Research Institute www.deri.ie @resource : URI or SafeCURIE an invisible resource object, that is not clickable @href : URI a resource object that is clickable like a link @content : string invisible literal object for a triple @datatype : XML data for a literal @typeof : specify class of a subject 26 of 28 Part 3: RDFa Benjamin Heitmann
  • Another RDFa example Digital Enterprise Research Institute www.deri.ie <html xmlns=quot;http://www.w3.org/1999/xhtmlquot; xmlns:cal=quot;http://www.w3.org/2002/12/cal/ical#quot; xmlns:xsd=quot;http://www.w3.org/2001/XMLSchemaquot; > <head><title>Jo's Friends and Family Blog</title></head> <body> <p typeof=quot;cal:Veventquot;> I'm holding <span property=quot;cal:summaryquot;> one last summer Barbecue </span>, on <span property=quot;cal:dtstartquot; content=quot;2007-09-16T16:00:00-05:00quot; datatype=quot;xsd:dateTimequot;> September 16th at 4pm </span>. </p> </body> </html> 27 of 28 Part 3: RDFa Benjamin Heitmann
  • Publishing and consuming RDFa Digital Enterprise Research Institute www.deri.ie  Publishing RDFa  use any CMS that has custom templates  Drupal  Wordpress (use pods plugin for wp 2.7)  Consuming RDFa  use GRDDL to convert any web page with RDFa to RDF/XML  Operator Plug-in for Firefox  Take a look at http://rdfa.info/wiki/Tools for more 28 of 28 Part 3: RDFa Benjamin Heitmann
  • Summary Digital Enterprise Research Institute www.deri.ie  RDF provides the data model and formal semantics for the Semantic Web  RDFa embeds the data model in XHTML pages  very flexible, can express any RDF graph  lotsof nested tags  can reuse existing visible content  or can be completely invisible  beware of the difference between URIs and CURIEs  watch out for implicit blank nodes  generic tools for publishing and consuming exist 29 of 28 Benjamin Heitmann