Bernhard Haslhofer is a postdoc researcher at Cornell University studying linked data, user-contributed data, and data interoperability. He discusses Linked (Open) Data, which uses URIs and RDF to publish and link structured data on the web. The key principles are using URIs to identify things, providing useful information about those URIs when dereferenced, and including links to other URIs. Enabling technologies include URIs, RDF, RDFS/OWL for vocabularies, SPARQL for querying, and best practices for publishing vocabularies and data. Useful tools are also presented.

1. Linked (Open) Data
INFO 4302 - April 18, 2011
Bernhard Haslhofer - Cornell University

2. Who am I?
• Postdoc at Cornell Information Science
• Research areas
• linked data
• user-contributed data (annotations)
• (meta-)data interoperability
• Contact:
• bernhard.haslhofer@cornell.edu

3. Today we talk about...
http://www.youtube.com/watch?v=5Cb3ik6zP2I

4. Today we talk about...
• Movies, actors and other real-world entities
• How to make data about these entities
available on the Web (Linked Data)
• Enabling technologies, best-practices and
useful tools that help us in doing so
• Other Linked Data projects (BBC, LoC)

5. Web Architecture Recap

6. The World Wide Web (WWW)
• Internet != WWW != Google != Facebook
• Fundamental technologies
• URI - a simple and generic syntax for identifiers
• HTML - a markup language without formal schema
binding
• HTTP - a simple protocol to access and manipulate
resources and resource representations in a
distributed environment
• W3C Consortium (http://www.w3.org)

7. URIs
• Identification of resources via Uniform
Resource Identifiers (URIs)
•The generic syntax consists of a hierarchical sequence of components, scheme,
Generic Syntax:
authority, path, query, and fragment.
URI = scheme “:” hier-path [ “?” query ] [ “#” fragment ]
Scheme and hier-path are required, though the path may be empty.
Example URIs with components: URI
foo://example.com:8042/over/there?name=ferret#nose
_/ ________________/_________/ _________/ __/
URL
| | | | | URN
scheme authority path query fragment

8. URIs / Resources
• Information Resource
• web pages, images, product catalogs, etc
• all their essential characteristics can be conveyed in a
message
• e.g., http://www.flickr.com/user2/photos/image.jpg
• Non-Information Resource
• other things such as dogs, people, this classroom, concepts
• their essence is not information
• e.g., http://www.example.com/ontology/meter

9. HTTP
• A stateless request-response protocol in the
client-server computing model
• HTTP methods: GET, POST, PUT, DELETE, ...
• Agents may use a URI to access the
referenced resource = dereferencing the URI

10. HTTP Content Negotiation
• A URI is not (necessarily) a filename
• Conneg = making available multiple resource
representations via the same URI
Plain Text
text/plain
HTML (en)
URI text/html
HTML (jp)
http://example.com/The_Shining text/html
Resource

11. (X)HTML(5)
• A resource representation data format...
• ... for presentation markup
• rendered by user agents (typically browsers)
• focus on readability
• less formal, user-friendly syntax and semantics

12. Web Services
• Application-to-application communication
based on the Web architecture
• simple and open standards (HTTP, XML, JSON, ...)
• send data from Application A to Application B
through the Web
• usually define some API
Web
Application A Application B

13. Linked Data

14. Why Linked Data?

15. Why Linked Data?

16. Why Linked Data?

17. Why Linked Data?
• There is lots of information on the Web
• ...valuable information that can be (re-)used
• Problem
• information is usually expressed in the form of
HTML documents
• the underlying raw data are locked in closed data
silos (mostly DBMS)

18. (c) http://www.flickr.com/photos/docsearls/5500714140

19. Why Linked Data?
• The Web is successful because it provides
• Uniform encoding (HTML)
• Uniform addressing (URI)
• Uniform transportation (HTTP)
for the exchange of documents.
• Why not apply the same mechanism to the
underlying data?

22. What is Linked Data?
• A method to build a Web of Data
• Architectural style, set of standards
Web

23. What is Linked Data?
• A set of four principles
• use URIs as names for things
• use HTTP URIs so that people can look up those
names
• when someone looks up a URI, provide useful
information, using the standards (RDF, SPARQL)
• include links to other URIs, so that they can
discover more things

24. Enabling Technologies

25. Uniform Resource Identifiers (URI)
• Name and identify things (resources)
• Dereferencable HTTP URIs
http://dbpedia.org/resource/
The_Shining_(film)
http://data.linkedmdb.org/
resource/film/2014
http://rdf.freebase.com/ns/m/
04fjzv

26. Resource Description Framework (RDF)
• A model for representing data on the Web
• Several statements (triples) form a graph
http://dbpedia.org/ontology/ http://xmlns.com/foaf/0.1/
Film Person
rdf:type rdf:type
http://dbpedia.org/resource/ http://dbpedia.org/resource/
dbpprop:starring
The_Shining_(film) Jack_Nicholson
foaf:name
rdfs:label rdfs:label
dbpedia-owl:birthDate
!" (#$) The Shining (film) 1937-04-22 Jack Nicholson

27. RDF serialization (RDF/XML, N3, Turtle, etc.)
• Data formats for RDF resource representations
7.2.2.3 RDF Serialization Formats: RDF/XML, N3, Turtle, N-Triple, etc
• Used to transfer RDF data between apps
Data formats for RDF resource representations
Used to transfer RDF data from application-to-application
N3/Turtle example:
@prefix rdf:<http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix dbpedia-owl:<http://dbpedia.org/ontology/> .
<http://dbpedia.org/resource/The_Shining_%28film%29>
rdf:type dbpedia-owl:Work , dbpedia-owl:Film .
@prefix dbpprop:<http://dbpedia.org/property/> .
@prefix ns9:<http://dbpedia.org/datatype/> .
<http://dbpedia.org/resource/The_Shining_%28film%29>
dbpprop:runtime"146.0"^^ns9:minute ;
© Prof. Dr. Wolfgang Klas und Dr. Bernhard Haslhofer, WS 2010/11 - Multimediale Systeme 2
7 Linked (Open) Data 7-15

28. RDF Vocabulary Description Language (RDFS)
• A language for describing the syntax and
semantics of vocabularies in a machine-
understandable way
http://dbpedia.org/ontology/
Work
rdfs:subClassOf
http://dbpedia.org/ontology/
Film

29. OWL - Web Ontology Language
• A more expressive (formal) language for defining the
syntax and semantics of vocabularies
• Solves RDFS shortcomings but introduces quite some
complexity
http://www.w3.org/2002/07/ http://dbpedia.org/ontology/
owl#ObjectProperty Work
rdf:type rdfs:domain
http://dbpedia.org/ontology/ http://dbpedia.org/ontology/
rdfs:range
starring Person
rdfs:label
starring

30. Simple Knowledge Organization System (SKOS)
• A language for describing controlled vocabularies
(taxonomies, thesauri, classification schemes)
http://dbpedia.org/resource/
Category:1980s_horror_films
skos:subject rdf:type
http://dbpedia.org/resource/ skos:broader
http://www.w3.org/2004/02/
The_Shining_(film) skos/core#Concept
rdf:type
http://dbpedia.org/resource/
Category:1980s_films

31. Links between Resources
• OWL defines properties for linking resources
http://dbpedia.org/resource/ http://dbpedia.org/resource/
dbpprop:starring
The_Shining_(film) Jack_Nicholson
owl:sameAs
owl:sameAs
owl:sameAs
http://data.linkedmdb.org/
resource/film/2014
http://data.nytimes.com/
N5761411277431266513
http://rdf.freebase.com/ns/m/
04fjzv

32. SPARQL
• A query language and protocol for accessing
7.2.2.7 SPARQL - RDF Query Language
RDF data on the Web
A query language and protocol for accessing RDF data on the Web
SELECT DISTINCT ?x
WHERE {?x skos:subject <http:dbpedia.org/resource/Cate-
gory:1980s_horror_films>}
LIMIT 10

33. Vocabulary / Data
Publishing Best Practices

34. Publishing Vocabularies
• Hash-based URIs
• e.g., http://example.com/example1#ClassA
• Suited to group the description of a moderate number of
related terms into one RDF document
• Agent can retrieve terms with a single request
• Slash-based URIs
• e.g., http://example.com/example1/ClassB
• Suited to split terms in large vocabularies into one
document per term
• No need to download a massive document

35. Provide either:
human-readable content from vocabulary URI

36. or:
machine-readable content from vocabulary URI
... depending on what is requested.

39. Publishing Data
• Distinguish between non-information and
information resource
• Sample non-information resource
• http://dbpedia.org/resource/The_Shining_(film)
• Sample information resource
• http://dbpedia.org/page/The_Shining_(film) - HTML
• http://dbpedia.org/data/The_Shining_(film) - RDF

40. Publishing Data
GET http://dbpedia.org/resource/The_Shining_(film)
Accept: application/rdf+xml
303 See Other
Location: http://dbpedia.org/data/The_Shining_(film)
GET http://dbpedia.org/data/The_Shining_(film)
Accept: application/rdf+xml
200 OK
...
<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF ...

41. The Linking Open Data
Community Project

42. Linking? Open? Data Project?
• Open Data: a philosophy, practice, or policy that data are
freely available to everyone without restrictions from
copyright, patents, a.s.o.
• Linked Data: method / best practices for exposing, sharing,
and connecting data using URIs and RDF
• Linking Open Data: a W3C community project with the
goal to extend the Web with a data commons by publishing
various open data sets as RDF on the Web and by setting
links between data items from different sources

56. Useful Tools

57. RDF APIs
• Java
• Jena Semantic Web Framework (http://openjena.org/)
• Sesame RDF API (http://www.openrdf.org/)
• PHP
• ARC (http://arc.semsol.org/)
• Ruby
• RDF.rb: Linked Data for Ruby (http://rdf.rubyforge.org/)
• Python
• RDFLib (http://www.rdflib.net/)
• C
• Redland RDF Libraries (http://librdf.org/)

58. RDF Stores
• OpenLink Virtuoso (http://virtuoso.openlinksw.com/
dataspace/dav/wiki/Main/)
• 4Store (http://4store.org/)
• AllegroGraph (http://www.franz.com/agraph/
allegrograph/)
• Oracle 11g (http://www.oracle.com/technetwork/
database/options/semantic-tech/ index.html)
• ...and many more: http://www.w3.org/2001/sw/wiki/Tools

59. RDF / Linked Data Wrappers
• D2RQ - SPARQL / Linked Data for relational
databases (http://www4.wiwiss.fu-berlin.de/
bizer/d2rq/)
• OAI2LOD Server - expose any OAI-PMH
source as Linked Data
• TripFS - filesystem as Linked Data
• TripCel - XLS spreadsheets as Linked Dat
• ...

60. Linked Data debugging
Startup your console / terminal
- native on Linux / Mac OS X
- Windows: http://www.cygwin.com/
Dereference resources with cURL (http://curl.haxx.se/)
curl -I -H "Accept: application/rdf+xml" http://
dbpedia.org/resource/The_Shining_%28film%29
curl -H "Accept: application/rdf+xml" http://
dbpedia.org/data/The_Shining_%28film%29

61. Linked Data debugging
Install the Raptor RDF Syntax Library (http://
librdf.org/raptor/)
- Mac: brew install raptor
Use the rapper utility to dereference URIs
rapper http://dbpedia.org/resource/The_Shining_%28film
%29
rapper -o rdfxml http://dbpedia.org/resource/
The_Shining_%28film%29

62. Readings

63. Required Reading
• T. Heath, C. Bizer. Linked Data: Evolving the Web into a
Global Data Space, Chapters 1-5
http://linkeddatabook.com/editions/1.0/

64. Recommended Readings
• Linked Data Web Site: http://linkeddata.org
• Linked Data / Semantic Web Introduction: http://
www.linkeddatatools.com/semantic-web-basics
• Tim Berners-Lee. Linked Data Design Issues: http://
www.w3.org/DesignIssues/LinkedData.html
• Best Practice Recipes for Publishing RDF Vocabularies:
http://www.w3.org/TR/swbp-vocab-pub/
• How to Publish Linked Data on the Web: http://
www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/