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SPARQL - Querying the Web of Data
                      Seminar WS 2008/2009

    RDF and the Web of Data


                                  Olaf Hartig
               hartig@informatik.hu-berlin.de
52°57'N , 13°42'O




                                                                  ≈

                    ?



Olaf Hartig - Trustworthiness of Data on the Web                       2
RDF in General
 ●   Resource Description Framework (RDF)
 ●   A resource is basically everything
     ●   E.g. persons, places, Web documents, abstract concepts
 ●   Descriptions of resources
     ●   Attributes and features
     ●   Relations
 ●   The framework contains:
     ●   A data model, and
     ●   Languages and syntaxes



An Introduction to RDF and the Web of Data                        3
RDF Data Model
 ●   Atoms of knowledge are triples (subject, predicate, object)


 ●   Subject: resources
 ●   Predicate: properties
 ●   Object: resources or literals


 ●   Examples:
     ●   ( Mount Baker , last eruption , quot;1880quot; )
     ●   ( Mount Baker , location , Washington )


An Introduction to RDF and the Web of Data                         4
RDF Data Model
 ●   RDF is also a graph model
     ●   Triples as directed edges
     ●   Subjects and objects as vertices
     ●   Edges labeled by predicate

 ●   Example:
     ●   ( Mount Baker , last eruption , quot;1880quot; )
     ●   ( Mount Baker , location , Washington )

                                              location
                Mount Baker                                 Washington
                              last eruption
                                                         quot;1880quot;
An Introduction to RDF and the Web of Data                               5
Uniform Resource Identifier (URI)
 ●   Globally unique identifier for resources
 ●   Syntax:
     ●   URI schema (e.g. http, mailto, urn)
     ●   Colon character (“:”)
     ●   Scheme-specific part (often hierarchical)
 ●   Examples:
     http://dbpedia.org/resource/Mount_Baker
     http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf
     urn:isbn:0-486-27557-4




An Introduction to RDF and the Web of Data                      6
Uniform Resource Identifier (URI)
 ●   URIs extend the concept of URLs
     ●   URL of a Web document usually used as its URI
     ●   Attention: URIs identify not only Web documents
 ●   Example:
     ●   Me:
     http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf
     ●   RDF document about me:
     http://www.informatik.hu-berlin.de/~hartig/foaf.rdf
     ●   HTML document about me:
     http://www.informatik.hu-berlin.de/~hartig/index.html




An Introduction to RDF and the Web of Data                      7
Example (revisited)
   ●   (http://dbpedia.org/resource/Mount_Baker,
        http://dbpedia.org/property/lastEruption, quot;1880quot;)
   ●   (http://dbpedia.org/resource/Mount_Baker,
        http://dbpedia.org/property/location,
        http://dbpedia.org/resource/Washington)

                                      http://dbpedia.org/resource/Washington

                      http://dbpedia.org/property/location

         http://dbpedia.org/resource/Mount_Baker

        http://dbpedia.org/property/lastEruption

                                                            quot;1880quot;
An Introduction to RDF and the Web of Data                                     8
Compact URIs (CURIE)
 ●   Abbreviated Notation for URIs
 ●   Syntax:
     ●   Prefix name (references the prefix of the URI)
     ●   Colon character (“:”)
     ●   Reference part
 ●   URI by concatenating the prefix and the reference part
 ●   Examples:
     ●   dbpedia:Mount_Baker for
     http://dbpedia.org/resource/Mount_Baker
     ●   myfoaf:olaf for
     http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf

An Introduction to RDF and the Web of Data                      9
Example with CURIEs
 ●   Using
     ●   dbpedia for prefix http://dbpedia.org/resource/
     ●   p for prefix http://dbpedia.org/property/
 ●   we have
     ●   (dbpedia:Mount_Baker, p:lastEruption, quot;1880quot;)
     ●   (dbpedia:Mount_Baker, p:location, dbpedia:Washington)

                                             p:location
     dbpedia:Mount_Baker                                  dbpedia:Washington
                      p:lastEruption
                                                            quot;1880quot;


An Introduction to RDF and the Web of Data                                     10
Literals
 ●   Literals may occur in the object position of triples
 ●   Represented by strings
 ●   Literal strings interpreted by datatypes
     ●   Datatype identified by a URI
     ●   Common to use the XML Schema datatypes
     ●   No datatype: interpreted as xsd:string
 ●   Untyped literals may have language tags (e.g. @de)


                                             p:name
     dbpedia:Mount_Baker                              quot;Mount Bakerquot;@en
                      p:lastEruption
                                                      quot;1880quot;^^xsd:integer
An Introduction to RDF and the Web of Data                                  11
N3 – A Readable Syntax for RDF
 ●   Simple notation to list RDF triples:
     ●   Triples separated by a period (“.”) character
     ●   Example:
 <http://dbpedia.org/resource/Mount_Baker>
                <http://dbpedia.org/property/lastEruption>
                               quot;1880quot;^^xsd:integer .
 <http://dbpedia.org/resource/Mount_Baker>
                <http://dbpedia.org/property/location>
                               <http://dbpedia.org/resource/Washington> .




An Introduction to RDF and the Web of Data                                  12
N3 – A Readable Syntax for RDF
 ●   N3 allows the use of CURIEs:
     ●   @prefix directive binds a prefix to a namespace URI


 @prefix dbpedia : <http://dbpedia.org/resource/> .
 @prefix p : <http://dbpedia.org/property/> .
 @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .


 dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer .
 dbpedia:Mount_Baker p:location              dbpedia:Washington .


 dbpedia:Washington p:borderingstates dbpedia:Oregon .
 dbpedia:Washington p:borderingstates dbpedia:Idaho .

An Introduction to RDF and the Web of Data                          13
N3 – A Readable Syntax for RDF
 ●   N3 provides some syntactic sugar:
     ●   Property lists separated by a semicolon (“;”) character
     ●   Object lists separated by a comma (“,”) character
 @prefix dbpedia : <http://dbpedia.org/resource/> .
 @prefix p : <http://dbpedia.org/property/> .
 @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .


 dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer ;
                                             p:location   dbpedia:Washington .


 dbpedia:Washington p:borderingstates dbpedia:Oregon ,
                                                           dbpedia:Idaho .

An Introduction to RDF and the Web of Data                                       14
N3 – A Readable Syntax for RDF
 ●   More syntactic sugar:
     ●   Shortcuts for number literals

 dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer ;
                                             geo:lat quot;48.777222quot;^^xsd:float ;
                                             geo:long quot;-121.813332quot;^^xsd:float .


 Equivalent:

 dbpedia:Mount_Baker p:lastEruption 1880 ;
                                             geo:lat 48.777222 ;
                                             geo:long -121.813332 .


An Introduction to RDF and the Web of Data                                         15
Classification
 ●   The predefined property rdf:type enables classifications
     ●   Object resource represents a category / class of things
     ●   Subject resource is an instance of that class
@prefix dbpedia: <http://dbpedia.org/resource/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns>.
@prefix umbel-sc: <http://umbel.org/umbel/sc/> .
@prefix yago:                           <http://dbpedia.org/class/yago/>.
@prefix skos:                           <http://www.w3.org/2004/02/skos/core#>.
dbpedia:Oregon rdf:type yago:StatesOfTheUnitedStates .
dbpedia:Mount_Baker rdf:type umbel-sc:Mountain .
umbel-sc:Mountain skos:definition quot;Each instance of ‣
  Mountain is a topographical feature of significantly ‣
  higher elevation ...quot;@en
An Introduction to RDF and the Web of Data                                    16
Classification
 ●   Class membership is not exclusive
     ●   I.e. instances may have multiple types
 dbpedia:Mount_Baker rdf:type umbel-sc:Mountain ,
                                             umbel-sc:Volcano .


 ●   Classes may be instances of other classes!
 dbpedia:Mount_Baker rdf:type umbel-sc:Mountain .
 umbel-sc:Mountain rdf:type umbel-ac:ExistingObjectType .


 ●   Syntactical distinction between classes and instances a
     priori impossible

An Introduction to RDF and the Web of Data                        17
RDF Schema in General
 ●   RDF Schema enables specification of schema knowledge
     ●   Definition of the vocabulary used in triples
     ●   Class hierarchies, property hierarchies
 ●   RDF Schema semantics enable elementary inferences




An Introduction to RDF and the Web of Data                  18
Predefined Classes
 ●   RDF Schema defines the following classes
     ●   rdfs:Resource – class of all resources
     ●   rdfs:Literal – class of all literals
     ●   rdfs:Class – class of all classes
                 it holds: ( rdfs:Class , rdf:type , rdfs:Class )
     ●   rdfs:Datatype – class of all datatypes
     ●   rdf:Property – class of all properties
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns>.
@prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> .
@prefix umbel-sc : <http://umbel.org/umbel/sc/> .


umbel-sc:Mountain rdf:type rdfs:Class .
An Introduction to RDF and the Web of Data                          19
Class Hierarchies
 ●   rdfs:subClassOf enables the definition of class hierarchies
 @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> .
 @prefix ex : <http://example.org/> .


 ex:Truck rdfs:subClassOf ex:MotorVehicle .
 ex:Van rdfs:subClassOf ex:MotorVehicle .
                                                              ex:MotorVehicle
 ex:MiniVan rdfs:subClassOf ex:Van .

                           rdfs:subClassOf                      rdfs:subClassOf

                                                              ex:Van
              ex:Truck
                                                          rdfs:subClassOf
                                             ex:MiniVan
An Introduction to RDF and the Web of Data                                      20
Class Hierarchies
 ●   Multiple inheritance allowed
                                                                ex:MotorVehicle
                        rdfs:subClassOf
              ex:Truck                           rdfs:subClassOf rdfs:subClassOf

                                             ex:Van        ex:PassengerVehicle
                 rdfs:subClassOf                             rdfs:subClassOf
                                              ex:MiniVan


 ●   rdfs:subClassOf is reflexive – e.g., it holds:
 ex:Truck rdfs:subClassOf ex:Truck .
An Introduction to RDF and the Web of Data                                        21
Class Hierarchies
 ●   rdfs:subClassOf is transitive
     ●   E.g., given
 ex:Van rdfs:subClassOf ex:MotorVehicle .
 ex:MiniVan rdfs:subClassOf ex:Van .
     ●   we can infer
 ex:MiniVan rdfs:subClassOf ex:MotorVehicle .

 ●   Entailment rule:
                                     ( A , rdfs:subClassOf , B )
                                     ( B , rdfs:subClassOf , C )
                                     ( A , rdfs:subClassOf , C )

An Introduction to RDF and the Web of Data                         22
Class Hierarchies
 ●   Another entailment rule:                     ( a , rdf:type , A )
                                             ( A , rdfs:subClassOf , B )
                                                 ( a , rdf:type , B )
 ●   E.g., from
 ex:Van rdfs:subClassOf ex:MotorVehicle .
 ex:MiniVan rdfs:subClassOf ex:Van .
 ex:MyRedVWT3 rdf:type ex:MiniVan .

 ●   we may infer
 ex:MyRedVWT3 rdf:type ex:Van .

 ●   and (exploiting transitivity)
 ex:MyRedVWT3 rdf:type ex:MotorVehicle .

An Introduction to RDF and the Web of Data                                 23
Property Hierarchies
 ●   Properties usually in predicate position
     ●   Entailment rule:
                                             (a,p,b)
                                   ( p , rdf:type , rdf:Property )

 ●   Properties are first class citizens (in contrast to OOP)
     ●   Not defined inside classes
     ●   Own hierarchy (specified by rdfs:subPropertyOf)

           ex:createdBy
                            rdfs:subPropertyOf               ex:authoredBy
An Introduction to RDF and the Web of Data                                   24
Property Hierarchies
 ●   Entailment rule:        ( a , p1 , b )
                  ( p1 , rdfs:subPropertyOf , p2 )
                                             ( a , p2 , b )
 ●   E.g., from
 ex:authoredBy rdfs:subPropertyOf ex:createdBy .
 dbpedia:The_Lord_of_the_Rings ex:authoredBy
                                                       dbpedia:J._R._R._Tolkien .

 ●   we may infer
 dbpedia:The_Lord_of_the_Rings ex:createdBy
                                                       dbpedia:J._R._R._Tolkien .


 ●   rdfs:subPropertyOf is reflexive and transitive too
An Introduction to RDF and the Web of Data                                      25
Property Restrictions
 ●   rdfs:domain and rdfs:range specify permitted subjects and
     objects, respectively

 @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> .
 @prefix xsd                 : <http://www.w3.org/2001/XMLSchema#> .
 @prefix p               : <http://dbpedia.org/property/> .
 @prefix ex : <http://example.org/> .
 @prefix foaf : <http://xmlns.com/foaf/0.1/> .


 ex:authoredBy rdfs:domain ex:Publication .
 ex:createdBy rdfs:range foaf:Person .
 p:lastEruption rdfs:range xsd:integer .


An Introduction to RDF and the Web of Data                             26
Property Restrictions
 ●   Entailment rules:

          ( p , rdfs:domain , A )            ( p , rdfs:range , A )
                 (a,p,x)                            (x,p,a)
              ( a , rdf:type , A )            ( a , rdf:type , A )

 ●   Beware: property restrictions are global and conjunctive
     ●   Let ( p , rdfs:domain , A ) and ( p , rdfs:domain , B );
         for each a with ( a , p , x ) holds
         ( a , rdfs:subClassOf , A ) and ( a , rdfs:subClassOf , B )
     ●   Same holds for rdfs:range
     ●   Hence, use the most general class

An Introduction to RDF and the Web of Data                             27
Property Restrictions
 ●   Extensional entailment rules:

        ( p , rdfs:domain , A )                   ( p2 , rdfs:domain , A )
     ( A , rdfs:subClassOf , B )             ( p1 , rdfs:subPropertyOf , p2 )
       ( p , rdfs:domain , B )                   ( p1 , rdfs:domain , A )


         ( p , rdfs:range , A )                    ( p2 , rdfs:range , A )
     ( A , rdfs:subClassOf , B )             ( p1 , rdfs:subPropertyOf , p2 )
         ( p , rdfs:range , B )                   ( p1 , rdfs:range , A )




An Introduction to RDF and the Web of Data                                  28
Further RDF Schema Properties
 ●   rdfs:label – alternative name of a resources
 ●   rdfs:comment – comment associated to a resource
 ●   rdfs:seeAlso – reference to a resource with more
                    information about the subject
 ●   rdfs:definedBy – reference to a resource with a definition
                     of the subject
 @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> .
 @prefix foaf : <http://xmlns.com/foaf/0.1/> .


 foaf:Person rdfs:label quot;Personquot;@en , quot;Personquot;@de ;
                           rdfs:comment quot;Class for persons.quot;@en ;
                           rdfs:definedBy <http://xmlns.com/foaf/0.1/> .

An Introduction to RDF and the Web of Data                             29
RDF Schema Summary
 ●   RDF Schema (RDFS) provides elementary means to define
     ●   vocabularies and
     ●   a machine-processable meaning of RDF data
 ●   RDF data that uses vocabulary described with RDFS can
     generically be processed by every RDFS-enabled software.
 ●   Web Ontology Language (OWL)
     ●   More comprehensive than RDFS
     ●   Property owl:sameAs – both URIs refer to the same thing
 ●   However, vocabulary-specific processing requires
     vocabulary-specific rules.



An Introduction to RDF and the Web of Data                         30
Common Vocabularies
 ●   FOAF (Friend of a Friend)
     ●   Persons and their main properties (e.g. name, email)
     ●   foaf:knows relation (enables specification of a network)
     ●   Namespace URI: http://xmlns.com/foaf/0.1/
 ●   DC (Dublic Core)
     ●   Enables description of created or published resources
     ●   Namespace URI: http://purl.org/dc/elements/1.1/
 ●   SKOS (Simple Knowledge Organisation Systems)
     ●   Thesauri, classification schemes, taxonomies, ...
     ●   Namespace URI: http://www.w3.org/2008/05/skos#

An Introduction to RDF and the Web of Data                          31
Common Vocabularies
 ●   SIOC (Semantically-Interlinked Online Communities)
     ●   Content and structure of online community sites
     ●   Weblogs, mailing lists, newsgroups,
     ●   Connections between channels and posts
     ●   Namespace URI: http://rdfs.org/sioc/ns#
 ●   DOAP (Description of a Project)
     ●   (Software) projects
     ●   Maintainer, programming language, source repository, ...
     ●   Namespace URI: http://usefulinc.com/ns/doap#



An Introduction to RDF and the Web of Data                          32
Web of Data
 ●   Triples may link different data spaces




                                                                                          My FOAF file
                      ...
          <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf>
              foaf:currentProject
                 <http://trdf.sourceforge.net/trdf> .
                      ...
      <http://trdf.sourceforge.net/trdf>




                                                                  My friend's FOAF file
        ...
         rdfs:seeAlso
 myfoaf:me
            <http://trdf.sourceforge.net/doap.rdf>
     foaf:knows
               ...
       <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> .
 <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf>
     rdfs:seeAlso
        <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf> .
             ...

An Introduction to RDF and the Web of Data                                                      33
Web of Data
 ●   Triples may link different data spaces




                                                                                               My FOAF file
                      ...
          <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf>
              foaf:currentProject
                 <http://trdf.sourceforge.net/trdf> .
                      ...
          <http://trdf.sourceforge.net/trdf>
              rdfs:seeAlso




                                                                           Project DOAP file
                 <http://trdf.sourceforge.net/doap.rdf>
                      ...

             ...
 <http://trdf.sourceforge.net/trdf> doap:name quot;tRDFquot; ;
                                             doap:created quot;2008-03-03quot; .
             ...

An Introduction to RDF and the Web of Data                                                           34
Web of Data
 ●
     Linked data principles1) (set of best practices for publish-
     ing and deploying data on the Web using RDF):
     1. Use URIs as names for things.
     2. Use HTTP URIs so that people can look up those names.
     3. When someone looks up a URI, provide useful RDF data.
     4. Include RDF statements that link to other URIs, so that
        they can discover related things.
 ●   These principles allow a true Web of data

 ●   RDF links have a machine-processable semantic
     (in contrast to links between Web documents)
                                      1)
                                           http://www.w3.org/DesignIssues/LinkedData.html
An Introduction to RDF and the Web of Data                                              35
Web of Data




                                                                                     My FOAF file
                           ...
               <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf>
                  foaf:currentProject
                      <http://trdf.sourceforge.net/trdf> .
                           ...

 ●   HTTP Request: GET http://trdf.sourceforge.net/trdf
 ●   Server response:
       303 See Other – http://trdf.sourceforge.net/doap.rdf
 ●   2nd Request: GET http://trdf.sourceforge.net/doap.rdf
                                                                  Re
                                                                     dir




                                                                                      Project
                      ...                                               e   cti
          <http://trdf.sourceforge.net/trdf> doap:name quot;tRDFquot; ;                 on
                                                  doap:created quot;2008-03-03quot; .
                      ...

An Introduction to RDF and the Web of Data                                                 36
Web of Data




                                             By courtesy of the Linking Open Data community project  As of September 2008
An Introduction to RDF and the Web of Data   License: CC-BY-SA                                                              37
                                             http://esw.w3.org/topic/SweoIG/TaskForces/CommunityProjects/LinkingOpenData
Web of Data
 ●   Selected open RDF datasets:
     Dataset                           Description                         Triples
 Dbpedia                   Structured information extracted from           ca. 117M
                           Wikipedia
 U.S.Census                2000 U.S. Census data                           ca. 1B
 GovTrack                  U.S. Congress data                              ca. 13M
 riese                     EuroStat data                                ca. 5M (3B)
 DBLP                      Bibliographic information on major computer ca. 28M
                           science journals and conference proceedings
 MusicBrainz               Data about artists, records, songs etc.         ca. 36M
 BBC                       Data about BBC Programmes                       ca. 2M
 CrunchBase                Directory of technology companies, people,      ca. 700K
                           and investors
An Introduction to RDF and the Web of Data                                            38
Open World Assumption
 ●   The absence of a triple is not relevant
 ●   The knowledge:
 @prefix dbpedia : <http://dbpedia.org/resource/> .
 @prefix p : <http://dbpedia.org/property/> .


 dbpedia:Washington p:borderingstates dbpedia:Oregon .

     does not mean Washington has one bordering state!
 ●   If you have no other triples doesn't mean they are not true.




An Introduction to RDF and the Web of Data                      39
Blank Nodes
                                                    myfoaf:olaf
 ●   Blank nodes represent unnamed,
     anonymous resources                               wot:hasKey
     ●   Not identified by a URI
                                                         rdf:type
 ●   Blank node identifiers                                   wot:PubKey
     ●   Identification of blank nodes                wot:hex_id
         in triple serializations
                                                 quot;E27D37D7quot;
     ●   Form: _:xyz
     ●   Significant only within a single RDF graph
 myfoaf:olaf wot:hasKey _:x .
 _:x rdf:type                     wot:PubKey ;
          wot:hex_id quot;E27D37D7quot; .
An Introduction to RDF and the Web of Data                                 40
Blank Nodes
                                                        myfoaf:olaf
 ●   Blank nodes break the global graph
     ●   Cannot be referenced                               wot:hasKey
     ●   Not reusable
                                                             rdf:type
     ●   Name your resources
         (linked data principle)                                 wot:PubKey
                                                          wot:hex_id
                                                    quot;E27D37D7quot;

 ●   Abbreviated syntax in N3:
 myfoaf:olaf wot:hasKey [ rdf:type                     wot:PubKey ;
                                             wot:hex_id quot;E27D37D7quot; ]


An Introduction to RDF and the Web of Data                                    41
Groups of Things
 ●   Containers are an open group
     ●   Contain resources or literals, possibly duplicates
     ●   rdf:Seq – ordered list
     ●   rdf:Bag – set (unordered)
     ●   rdf:Alt – for alternatives

 dbpedia:Mount_Etna ex:eruptions [ rdf:type rdf:Bag ;
                                             rdf:_1 quot;1669quot; ;
                                             rdf:_2 quot;1949quot; ;
                                             rdf:_3 quot;1971quot; ;
                                             rdf:_4 quot;2001quot; ] .



An Introduction to RDF and the Web of Data                       42
Groups of Things
 ●   Collections
     ●   Closed list of resources or literals, possibly duplicates
          ex:wheels                                 rdf:first
                                                                  ex:LeftFrontWheel
         ex:Car                              rdf:rest
                                                   rdf:first
                                                                  ex:RightFrontWheel
                                                rdf:rest
                                                      rdf:first
                                                                  ex:LeftRearWheel
                                                 rdf:rest
                                                      rdf:first
                                                                  ex:RightRearWheel
                                     rdf:rest
                                             rdf:nil
An Introduction to RDF and the Web of Data                                            43
Groups of Things
 ●   Collections in N3
 ex:Car ex:wheels _:a ;
 _:a rdf:first ex:LeftFrontWheel ; rdf:rest _:b .
 _:b rdf:first ex:RightFrontWheel ; rdf:rest _:c .
 _:c rdf:first ex:LeftRearWheel ; rdf:rest _:d .
 _:d rdf:first ex:RightRearWheel ; rdf:rest rdf:nil .

     ●   Shortcut
 ex:Car ex:wheels ( ex:LeftFrontWheel ex:RightFrontWheel
                                             ex:LeftRearWheel ex:RightRearWheel ) .


 ●   Generic access with SPARQL impossible

An Introduction to RDF and the Web of Data                                        44
Reification
 ●   Reification allows statements about statements

                                             represented by _:s


 ex:LinkedDataPrinciples rat:rating quot;greatquot; .


 _:s rdf:type rdf:Statement ;
          rdf:subject ex:LinkedDataPrinciples ;
          rdf:predicate rat:rating ;                      about
          rdf:object quot;greatquot; .


 _:s dc:creator myfoaf:olaf .


An Introduction to RDF and the Web of Data                        45
RDF/XML – An XML syntax for RDF
 @prefix dbpedia : <http://dbpedia.org/resource/> .                      N3
 @prefix p : <http://dbpedia.org/property/> .
 @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

 dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer .
 dbpedia:Mount_Baker p:location              dbpedia:Washington .

 <rdf:RDF xmlns:rdf=quot;http://www.w3.org/1999/02/22-rdf-syntax-ns#quot;
                 xmlns:p=quot;http://dbpedia.org/property/quot;>
   <rdf:Description rdf:about=quot;http://dbpedia.org/resource/Mount_Bakerquot;>
      <p:lastEruption ‣
             rdf:datatype=quot;http://www.w3.org/2001/XMLSchema#integerquot; ‣
             >1880</p:lastEruption>
      <p:location rdf:resource=quot;http://dbpedia.org/resource/Washingtonquot;/>
   </rdf:Description>
 </rdf:RDF>                                                         RDF/XML
An Introduction to RDF and the Web of Data                                46
Further Reading
 ●   W3C RDF Specifications – http://www.w3.org/RDF/
     ●   RDF Primer
     ●   RDF: Concepts and Abstract Syntax
     ●   RDF Vocabulary Description Language 1.0: RDF Schema
     ●   RDF Semantics
     ●   RDF/XML Syntax Specification (Revised)
     ●   RDF Test Cases
 ●   Information about Linked Data – http://linkeddata.org/




An Introduction to RDF and the Web of Data                     47

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An Introduction to RDF and the Web of Data

  • 1. SPARQL - Querying the Web of Data Seminar WS 2008/2009 RDF and the Web of Data Olaf Hartig hartig@informatik.hu-berlin.de
  • 2. 52°57'N , 13°42'O ≈ ? Olaf Hartig - Trustworthiness of Data on the Web 2
  • 3. RDF in General ● Resource Description Framework (RDF) ● A resource is basically everything ● E.g. persons, places, Web documents, abstract concepts ● Descriptions of resources ● Attributes and features ● Relations ● The framework contains: ● A data model, and ● Languages and syntaxes An Introduction to RDF and the Web of Data 3
  • 4. RDF Data Model ● Atoms of knowledge are triples (subject, predicate, object) ● Subject: resources ● Predicate: properties ● Object: resources or literals ● Examples: ● ( Mount Baker , last eruption , quot;1880quot; ) ● ( Mount Baker , location , Washington ) An Introduction to RDF and the Web of Data 4
  • 5. RDF Data Model ● RDF is also a graph model ● Triples as directed edges ● Subjects and objects as vertices ● Edges labeled by predicate ● Example: ● ( Mount Baker , last eruption , quot;1880quot; ) ● ( Mount Baker , location , Washington ) location Mount Baker Washington last eruption quot;1880quot; An Introduction to RDF and the Web of Data 5
  • 6. Uniform Resource Identifier (URI) ● Globally unique identifier for resources ● Syntax: ● URI schema (e.g. http, mailto, urn) ● Colon character (“:”) ● Scheme-specific part (often hierarchical) ● Examples: http://dbpedia.org/resource/Mount_Baker http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf urn:isbn:0-486-27557-4 An Introduction to RDF and the Web of Data 6
  • 7. Uniform Resource Identifier (URI) ● URIs extend the concept of URLs ● URL of a Web document usually used as its URI ● Attention: URIs identify not only Web documents ● Example: ● Me: http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf ● RDF document about me: http://www.informatik.hu-berlin.de/~hartig/foaf.rdf ● HTML document about me: http://www.informatik.hu-berlin.de/~hartig/index.html An Introduction to RDF and the Web of Data 7
  • 8. Example (revisited) ● (http://dbpedia.org/resource/Mount_Baker, http://dbpedia.org/property/lastEruption, quot;1880quot;) ● (http://dbpedia.org/resource/Mount_Baker, http://dbpedia.org/property/location, http://dbpedia.org/resource/Washington) http://dbpedia.org/resource/Washington http://dbpedia.org/property/location http://dbpedia.org/resource/Mount_Baker http://dbpedia.org/property/lastEruption quot;1880quot; An Introduction to RDF and the Web of Data 8
  • 9. Compact URIs (CURIE) ● Abbreviated Notation for URIs ● Syntax: ● Prefix name (references the prefix of the URI) ● Colon character (“:”) ● Reference part ● URI by concatenating the prefix and the reference part ● Examples: ● dbpedia:Mount_Baker for http://dbpedia.org/resource/Mount_Baker ● myfoaf:olaf for http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf An Introduction to RDF and the Web of Data 9
  • 10. Example with CURIEs ● Using ● dbpedia for prefix http://dbpedia.org/resource/ ● p for prefix http://dbpedia.org/property/ ● we have ● (dbpedia:Mount_Baker, p:lastEruption, quot;1880quot;) ● (dbpedia:Mount_Baker, p:location, dbpedia:Washington) p:location dbpedia:Mount_Baker dbpedia:Washington p:lastEruption quot;1880quot; An Introduction to RDF and the Web of Data 10
  • 11. Literals ● Literals may occur in the object position of triples ● Represented by strings ● Literal strings interpreted by datatypes ● Datatype identified by a URI ● Common to use the XML Schema datatypes ● No datatype: interpreted as xsd:string ● Untyped literals may have language tags (e.g. @de) p:name dbpedia:Mount_Baker quot;Mount Bakerquot;@en p:lastEruption quot;1880quot;^^xsd:integer An Introduction to RDF and the Web of Data 11
  • 12. N3 – A Readable Syntax for RDF ● Simple notation to list RDF triples: ● Triples separated by a period (“.”) character ● Example: <http://dbpedia.org/resource/Mount_Baker> <http://dbpedia.org/property/lastEruption> quot;1880quot;^^xsd:integer . <http://dbpedia.org/resource/Mount_Baker> <http://dbpedia.org/property/location> <http://dbpedia.org/resource/Washington> . An Introduction to RDF and the Web of Data 12
  • 13. N3 – A Readable Syntax for RDF ● N3 allows the use of CURIEs: ● @prefix directive binds a prefix to a namespace URI @prefix dbpedia : <http://dbpedia.org/resource/> . @prefix p : <http://dbpedia.org/property/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer . dbpedia:Mount_Baker p:location dbpedia:Washington . dbpedia:Washington p:borderingstates dbpedia:Oregon . dbpedia:Washington p:borderingstates dbpedia:Idaho . An Introduction to RDF and the Web of Data 13
  • 14. N3 – A Readable Syntax for RDF ● N3 provides some syntactic sugar: ● Property lists separated by a semicolon (“;”) character ● Object lists separated by a comma (“,”) character @prefix dbpedia : <http://dbpedia.org/resource/> . @prefix p : <http://dbpedia.org/property/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer ; p:location dbpedia:Washington . dbpedia:Washington p:borderingstates dbpedia:Oregon , dbpedia:Idaho . An Introduction to RDF and the Web of Data 14
  • 15. N3 – A Readable Syntax for RDF ● More syntactic sugar: ● Shortcuts for number literals dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer ; geo:lat quot;48.777222quot;^^xsd:float ; geo:long quot;-121.813332quot;^^xsd:float . Equivalent: dbpedia:Mount_Baker p:lastEruption 1880 ; geo:lat 48.777222 ; geo:long -121.813332 . An Introduction to RDF and the Web of Data 15
  • 16. Classification ● The predefined property rdf:type enables classifications ● Object resource represents a category / class of things ● Subject resource is an instance of that class @prefix dbpedia: <http://dbpedia.org/resource/> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns>. @prefix umbel-sc: <http://umbel.org/umbel/sc/> . @prefix yago: <http://dbpedia.org/class/yago/>. @prefix skos: <http://www.w3.org/2004/02/skos/core#>. dbpedia:Oregon rdf:type yago:StatesOfTheUnitedStates . dbpedia:Mount_Baker rdf:type umbel-sc:Mountain . umbel-sc:Mountain skos:definition quot;Each instance of ‣ Mountain is a topographical feature of significantly ‣ higher elevation ...quot;@en An Introduction to RDF and the Web of Data 16
  • 17. Classification ● Class membership is not exclusive ● I.e. instances may have multiple types dbpedia:Mount_Baker rdf:type umbel-sc:Mountain , umbel-sc:Volcano . ● Classes may be instances of other classes! dbpedia:Mount_Baker rdf:type umbel-sc:Mountain . umbel-sc:Mountain rdf:type umbel-ac:ExistingObjectType . ● Syntactical distinction between classes and instances a priori impossible An Introduction to RDF and the Web of Data 17
  • 18. RDF Schema in General ● RDF Schema enables specification of schema knowledge ● Definition of the vocabulary used in triples ● Class hierarchies, property hierarchies ● RDF Schema semantics enable elementary inferences An Introduction to RDF and the Web of Data 18
  • 19. Predefined Classes ● RDF Schema defines the following classes ● rdfs:Resource – class of all resources ● rdfs:Literal – class of all literals ● rdfs:Class – class of all classes it holds: ( rdfs:Class , rdf:type , rdfs:Class ) ● rdfs:Datatype – class of all datatypes ● rdf:Property – class of all properties @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns>. @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> . @prefix umbel-sc : <http://umbel.org/umbel/sc/> . umbel-sc:Mountain rdf:type rdfs:Class . An Introduction to RDF and the Web of Data 19
  • 20. Class Hierarchies ● rdfs:subClassOf enables the definition of class hierarchies @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> . @prefix ex : <http://example.org/> . ex:Truck rdfs:subClassOf ex:MotorVehicle . ex:Van rdfs:subClassOf ex:MotorVehicle . ex:MotorVehicle ex:MiniVan rdfs:subClassOf ex:Van . rdfs:subClassOf rdfs:subClassOf ex:Van ex:Truck rdfs:subClassOf ex:MiniVan An Introduction to RDF and the Web of Data 20
  • 21. Class Hierarchies ● Multiple inheritance allowed ex:MotorVehicle rdfs:subClassOf ex:Truck rdfs:subClassOf rdfs:subClassOf ex:Van ex:PassengerVehicle rdfs:subClassOf rdfs:subClassOf ex:MiniVan ● rdfs:subClassOf is reflexive – e.g., it holds: ex:Truck rdfs:subClassOf ex:Truck . An Introduction to RDF and the Web of Data 21
  • 22. Class Hierarchies ● rdfs:subClassOf is transitive ● E.g., given ex:Van rdfs:subClassOf ex:MotorVehicle . ex:MiniVan rdfs:subClassOf ex:Van . ● we can infer ex:MiniVan rdfs:subClassOf ex:MotorVehicle . ● Entailment rule: ( A , rdfs:subClassOf , B ) ( B , rdfs:subClassOf , C ) ( A , rdfs:subClassOf , C ) An Introduction to RDF and the Web of Data 22
  • 23. Class Hierarchies ● Another entailment rule: ( a , rdf:type , A ) ( A , rdfs:subClassOf , B ) ( a , rdf:type , B ) ● E.g., from ex:Van rdfs:subClassOf ex:MotorVehicle . ex:MiniVan rdfs:subClassOf ex:Van . ex:MyRedVWT3 rdf:type ex:MiniVan . ● we may infer ex:MyRedVWT3 rdf:type ex:Van . ● and (exploiting transitivity) ex:MyRedVWT3 rdf:type ex:MotorVehicle . An Introduction to RDF and the Web of Data 23
  • 24. Property Hierarchies ● Properties usually in predicate position ● Entailment rule: (a,p,b) ( p , rdf:type , rdf:Property ) ● Properties are first class citizens (in contrast to OOP) ● Not defined inside classes ● Own hierarchy (specified by rdfs:subPropertyOf) ex:createdBy rdfs:subPropertyOf ex:authoredBy An Introduction to RDF and the Web of Data 24
  • 25. Property Hierarchies ● Entailment rule: ( a , p1 , b ) ( p1 , rdfs:subPropertyOf , p2 ) ( a , p2 , b ) ● E.g., from ex:authoredBy rdfs:subPropertyOf ex:createdBy . dbpedia:The_Lord_of_the_Rings ex:authoredBy dbpedia:J._R._R._Tolkien . ● we may infer dbpedia:The_Lord_of_the_Rings ex:createdBy dbpedia:J._R._R._Tolkien . ● rdfs:subPropertyOf is reflexive and transitive too An Introduction to RDF and the Web of Data 25
  • 26. Property Restrictions ● rdfs:domain and rdfs:range specify permitted subjects and objects, respectively @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> . @prefix xsd : <http://www.w3.org/2001/XMLSchema#> . @prefix p : <http://dbpedia.org/property/> . @prefix ex : <http://example.org/> . @prefix foaf : <http://xmlns.com/foaf/0.1/> . ex:authoredBy rdfs:domain ex:Publication . ex:createdBy rdfs:range foaf:Person . p:lastEruption rdfs:range xsd:integer . An Introduction to RDF and the Web of Data 26
  • 27. Property Restrictions ● Entailment rules: ( p , rdfs:domain , A ) ( p , rdfs:range , A ) (a,p,x) (x,p,a) ( a , rdf:type , A ) ( a , rdf:type , A ) ● Beware: property restrictions are global and conjunctive ● Let ( p , rdfs:domain , A ) and ( p , rdfs:domain , B ); for each a with ( a , p , x ) holds ( a , rdfs:subClassOf , A ) and ( a , rdfs:subClassOf , B ) ● Same holds for rdfs:range ● Hence, use the most general class An Introduction to RDF and the Web of Data 27
  • 28. Property Restrictions ● Extensional entailment rules: ( p , rdfs:domain , A ) ( p2 , rdfs:domain , A ) ( A , rdfs:subClassOf , B ) ( p1 , rdfs:subPropertyOf , p2 ) ( p , rdfs:domain , B ) ( p1 , rdfs:domain , A ) ( p , rdfs:range , A ) ( p2 , rdfs:range , A ) ( A , rdfs:subClassOf , B ) ( p1 , rdfs:subPropertyOf , p2 ) ( p , rdfs:range , B ) ( p1 , rdfs:range , A ) An Introduction to RDF and the Web of Data 28
  • 29. Further RDF Schema Properties ● rdfs:label – alternative name of a resources ● rdfs:comment – comment associated to a resource ● rdfs:seeAlso – reference to a resource with more information about the subject ● rdfs:definedBy – reference to a resource with a definition of the subject @prefix rdfs : <http://www.w3.org/2000/01/rdf-schema#> . @prefix foaf : <http://xmlns.com/foaf/0.1/> . foaf:Person rdfs:label quot;Personquot;@en , quot;Personquot;@de ; rdfs:comment quot;Class for persons.quot;@en ; rdfs:definedBy <http://xmlns.com/foaf/0.1/> . An Introduction to RDF and the Web of Data 29
  • 30. RDF Schema Summary ● RDF Schema (RDFS) provides elementary means to define ● vocabularies and ● a machine-processable meaning of RDF data ● RDF data that uses vocabulary described with RDFS can generically be processed by every RDFS-enabled software. ● Web Ontology Language (OWL) ● More comprehensive than RDFS ● Property owl:sameAs – both URIs refer to the same thing ● However, vocabulary-specific processing requires vocabulary-specific rules. An Introduction to RDF and the Web of Data 30
  • 31. Common Vocabularies ● FOAF (Friend of a Friend) ● Persons and their main properties (e.g. name, email) ● foaf:knows relation (enables specification of a network) ● Namespace URI: http://xmlns.com/foaf/0.1/ ● DC (Dublic Core) ● Enables description of created or published resources ● Namespace URI: http://purl.org/dc/elements/1.1/ ● SKOS (Simple Knowledge Organisation Systems) ● Thesauri, classification schemes, taxonomies, ... ● Namespace URI: http://www.w3.org/2008/05/skos# An Introduction to RDF and the Web of Data 31
  • 32. Common Vocabularies ● SIOC (Semantically-Interlinked Online Communities) ● Content and structure of online community sites ● Weblogs, mailing lists, newsgroups, ● Connections between channels and posts ● Namespace URI: http://rdfs.org/sioc/ns# ● DOAP (Description of a Project) ● (Software) projects ● Maintainer, programming language, source repository, ... ● Namespace URI: http://usefulinc.com/ns/doap# An Introduction to RDF and the Web of Data 32
  • 33. Web of Data ● Triples may link different data spaces My FOAF file ... <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> foaf:currentProject <http://trdf.sourceforge.net/trdf> . ... <http://trdf.sourceforge.net/trdf> My friend's FOAF file ... rdfs:seeAlso myfoaf:me <http://trdf.sourceforge.net/doap.rdf> foaf:knows ... <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> . <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> rdfs:seeAlso <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf> . ... An Introduction to RDF and the Web of Data 33
  • 34. Web of Data ● Triples may link different data spaces My FOAF file ... <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> foaf:currentProject <http://trdf.sourceforge.net/trdf> . ... <http://trdf.sourceforge.net/trdf> rdfs:seeAlso Project DOAP file <http://trdf.sourceforge.net/doap.rdf> ... ... <http://trdf.sourceforge.net/trdf> doap:name quot;tRDFquot; ; doap:created quot;2008-03-03quot; . ... An Introduction to RDF and the Web of Data 34
  • 35. Web of Data ● Linked data principles1) (set of best practices for publish- ing and deploying data on the Web using RDF): 1. Use URIs as names for things. 2. Use HTTP URIs so that people can look up those names. 3. When someone looks up a URI, provide useful RDF data. 4. Include RDF statements that link to other URIs, so that they can discover related things. ● These principles allow a true Web of data ● RDF links have a machine-processable semantic (in contrast to links between Web documents) 1) http://www.w3.org/DesignIssues/LinkedData.html An Introduction to RDF and the Web of Data 35
  • 36. Web of Data My FOAF file ... <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> foaf:currentProject <http://trdf.sourceforge.net/trdf> . ... ● HTTP Request: GET http://trdf.sourceforge.net/trdf ● Server response: 303 See Other – http://trdf.sourceforge.net/doap.rdf ● 2nd Request: GET http://trdf.sourceforge.net/doap.rdf Re dir Project ... e cti <http://trdf.sourceforge.net/trdf> doap:name quot;tRDFquot; ; on doap:created quot;2008-03-03quot; . ... An Introduction to RDF and the Web of Data 36
  • 37. Web of Data By courtesy of the Linking Open Data community project As of September 2008 An Introduction to RDF and the Web of Data License: CC-BY-SA 37 http://esw.w3.org/topic/SweoIG/TaskForces/CommunityProjects/LinkingOpenData
  • 38. Web of Data ● Selected open RDF datasets: Dataset Description Triples Dbpedia Structured information extracted from ca. 117M Wikipedia U.S.Census 2000 U.S. Census data ca. 1B GovTrack U.S. Congress data ca. 13M riese EuroStat data ca. 5M (3B) DBLP Bibliographic information on major computer ca. 28M science journals and conference proceedings MusicBrainz Data about artists, records, songs etc. ca. 36M BBC Data about BBC Programmes ca. 2M CrunchBase Directory of technology companies, people, ca. 700K and investors An Introduction to RDF and the Web of Data 38
  • 39. Open World Assumption ● The absence of a triple is not relevant ● The knowledge: @prefix dbpedia : <http://dbpedia.org/resource/> . @prefix p : <http://dbpedia.org/property/> . dbpedia:Washington p:borderingstates dbpedia:Oregon . does not mean Washington has one bordering state! ● If you have no other triples doesn't mean they are not true. An Introduction to RDF and the Web of Data 39
  • 40. Blank Nodes myfoaf:olaf ● Blank nodes represent unnamed, anonymous resources wot:hasKey ● Not identified by a URI rdf:type ● Blank node identifiers wot:PubKey ● Identification of blank nodes wot:hex_id in triple serializations quot;E27D37D7quot; ● Form: _:xyz ● Significant only within a single RDF graph myfoaf:olaf wot:hasKey _:x . _:x rdf:type wot:PubKey ; wot:hex_id quot;E27D37D7quot; . An Introduction to RDF and the Web of Data 40
  • 41. Blank Nodes myfoaf:olaf ● Blank nodes break the global graph ● Cannot be referenced wot:hasKey ● Not reusable rdf:type ● Name your resources (linked data principle) wot:PubKey wot:hex_id quot;E27D37D7quot; ● Abbreviated syntax in N3: myfoaf:olaf wot:hasKey [ rdf:type wot:PubKey ; wot:hex_id quot;E27D37D7quot; ] An Introduction to RDF and the Web of Data 41
  • 42. Groups of Things ● Containers are an open group ● Contain resources or literals, possibly duplicates ● rdf:Seq – ordered list ● rdf:Bag – set (unordered) ● rdf:Alt – for alternatives dbpedia:Mount_Etna ex:eruptions [ rdf:type rdf:Bag ; rdf:_1 quot;1669quot; ; rdf:_2 quot;1949quot; ; rdf:_3 quot;1971quot; ; rdf:_4 quot;2001quot; ] . An Introduction to RDF and the Web of Data 42
  • 43. Groups of Things ● Collections ● Closed list of resources or literals, possibly duplicates ex:wheels rdf:first ex:LeftFrontWheel ex:Car rdf:rest rdf:first ex:RightFrontWheel rdf:rest rdf:first ex:LeftRearWheel rdf:rest rdf:first ex:RightRearWheel rdf:rest rdf:nil An Introduction to RDF and the Web of Data 43
  • 44. Groups of Things ● Collections in N3 ex:Car ex:wheels _:a ; _:a rdf:first ex:LeftFrontWheel ; rdf:rest _:b . _:b rdf:first ex:RightFrontWheel ; rdf:rest _:c . _:c rdf:first ex:LeftRearWheel ; rdf:rest _:d . _:d rdf:first ex:RightRearWheel ; rdf:rest rdf:nil . ● Shortcut ex:Car ex:wheels ( ex:LeftFrontWheel ex:RightFrontWheel ex:LeftRearWheel ex:RightRearWheel ) . ● Generic access with SPARQL impossible An Introduction to RDF and the Web of Data 44
  • 45. Reification ● Reification allows statements about statements represented by _:s ex:LinkedDataPrinciples rat:rating quot;greatquot; . _:s rdf:type rdf:Statement ; rdf:subject ex:LinkedDataPrinciples ; rdf:predicate rat:rating ; about rdf:object quot;greatquot; . _:s dc:creator myfoaf:olaf . An Introduction to RDF and the Web of Data 45
  • 46. RDF/XML – An XML syntax for RDF @prefix dbpedia : <http://dbpedia.org/resource/> . N3 @prefix p : <http://dbpedia.org/property/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . dbpedia:Mount_Baker p:lastEruption quot;1880quot;^^xsd:integer . dbpedia:Mount_Baker p:location dbpedia:Washington . <rdf:RDF xmlns:rdf=quot;http://www.w3.org/1999/02/22-rdf-syntax-ns#quot; xmlns:p=quot;http://dbpedia.org/property/quot;> <rdf:Description rdf:about=quot;http://dbpedia.org/resource/Mount_Bakerquot;> <p:lastEruption ‣ rdf:datatype=quot;http://www.w3.org/2001/XMLSchema#integerquot; ‣ >1880</p:lastEruption> <p:location rdf:resource=quot;http://dbpedia.org/resource/Washingtonquot;/> </rdf:Description> </rdf:RDF> RDF/XML An Introduction to RDF and the Web of Data 46
  • 47. Further Reading ● W3C RDF Specifications – http://www.w3.org/RDF/ ● RDF Primer ● RDF: Concepts and Abstract Syntax ● RDF Vocabulary Description Language 1.0: RDF Schema ● RDF Semantics ● RDF/XML Syntax Specification (Revised) ● RDF Test Cases ● Information about Linked Data – http://linkeddata.org/ An Introduction to RDF and the Web of Data 47