What is Linked Data? Presented at the Linked Data for Libraries on Thursday, November 6, 2014 at Trinity College Dublin http://www.dri.ie/linked-data-libraries

1. What is Linked Data?
Linked Data for Libraries, 6th Nov 2014, Royal Irish Academy
Dr.
Christophe
Debruyne
Digital
Repository
of
Ireland
–
Royal
Irish
Academy
Insight
Centre
of
Data
Analy<cs
–
NUI
Galway
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

2. What is Linked Data?
• Linked Data started off as a initiative called the Linking
Open Data (LOD) project.
• Linked Data is a global initiative to publish and interlink
structured data on the Web using a combination of well
established technologies.
• Uniform Resource Identifiers – to name things;
• Resource Description Framework – to represent things;
• HTTP infrastructure – to obtain those representations.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

3. Web of Documents vs. Web of Data
• The Web of Documents were created by humans for
humans; the links between documents bore little
meaning for machines and documents provided little
structured information.
• Structured information can be found on the Web – such
as XML, CSV, etc. – but, …
• How do we link data rather than documents, and create
a global “database” of information?
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

4. Towards a Web of Documents
• We need appropriate methods (guidelines) and
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
standards.
• Tim Berners-Lee formulated four rules for creating
and publishing Linked Data on the Web.

5. Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
The four principles
Number
1
“Use
URIs
as
names
for
things.”

6. (1) Use URIs to name things
• Use Uniform Resource Identifiers (URIs) to name
everything you need to describe on the Web
• People, geographical locations, books, …
• Events, emotion, religion, …
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
• Examples
• http://dbpedia.org/resource/James_Joyce
• ftp://example.org/file.txt
• urn:ISSN:1535-3613
• But …

7. Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
The four principles
Number
1
“Use
URIs
as
names
for
things.”
Number
2
“Use
HTTP
URIs
so
that
people
can
look
up
those
names.”

8. (2) Use HTTP URIs to look up those names
• HTTP URIs allow one to reuse the existing HTTP
infrastructure to return something when one
performs an HTTP GET request.
• One can for instance put the HTTP URI in a
browser’s address bar and – hopefully – get a
result.
• http://dbpedia.org/resource/James_Joyce
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

9. Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
The four principles
Number
1
“Use
URIs
as
names
for
things.”
Number
2
“Use
HTTP
URIs
so
that
people
can
look
up
those
names.”
Number
3
“When
someone
looks
up
a
URI,
provide
useful
informa<on,
using
the
standards
(RDF,
SPARQL)”

10. (3) Provide useful Information for URI look-ups
• When entities are identified by URIs that use the http://
scheme, these entities can be looked up simply by
dereferencing the URI over the HTTP protocol.
• Simple, standardized mechanism for retrieving
resources via these URIs.
• Provide information suitable for the “consumer”
• Humans rather see HTML pages, PDFs, pictures, …
• Machines want machine-readable formats such as RDF
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

11. Resource Description Framework
• RDF is not (really) a language but a model (!!!)
• RDF is a W3C recommendation
• RDF is designed to be read by computers
• RDF is for describing resources on the Web
in terms of triples (subject – predicate – object)
• RDF uses URIs to identify and reference resources on
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
the Web
• RDF/XML is just one way of serializing RDF
• Others are Turtle, N3, etc.

12. Resource Description Framework
<?xml version="1.0" encoding="utf-8" ?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:foaf="http://xmlns.com/foaf/0.1/" >
<rdf:Description rdf:about="http://dbpedia.org/resource/James_Joyce">
<foaf:name>Joyce, James Augustine Aloysius</foaf:name>
<foaf:based_near rdf:resource="http://dbpedia.org/resource/Zurich" />
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
</rdf:Description>
</rdf:RDF>

13. foaf:name
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
foaf:based_near

14. Ontologies
• An ontology is an “explicit [formal] specification of a [shared]
conceptualization.” (Gruber, 1993)
• RDF is the data model.
• RDF, RDFS and OWL are ontology languages.
• RDF à Declare types and relations;
• RDFS à Declare type- & role hierarchies, domains and rages, etc.
• OWL à Properties of relations, Disjointness, etc.
• Popular ontologies for instance are Friend-of-a-Friend (FOAF),
Simple Knowledge Organization System (SKOS), Dublin Core terms
• Ontologies allows us to describe resources.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

15. Information resources and non-information resources
• Information resources are documents – referred to by a
URI – that describe non-information resources – named
with a URI – that represent things such as cars, people,
etc.
• The NIR http://dbpedia.org/resource/James_Joyce is
described by the following IRs:
• The web page http://dbpedia.org/page/James_Joyce
• The RDF doc http://dbpedia.org/data/James_Joyce
• Either is returned depending on what you need. How?
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

16. Content negotiation – part of the HTTP infrastructure
Resource identifiers:
• HTTP URIs not only as a name, but also for a Web look-up.
• Non-information resources can have multiple representations:
HTML, RDF/XML, ...
HTTP URI dereferencing:
• To dereference → “To obtain the address of a data item held
in another location from a pointer”
• URI pointing to a IR returns the representation.
• URI pointing to a NIR returns a redirect to an IR describing
that NIR.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

17. Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
Content negotiation
Image
from
hSp://www.w3.org/TR/swbp-­‐vocab-­‐pub/

18. What should be returned?
• RDF should be at least be represented as RDF/XML.
• All RDF triples with the NIR’s URI as the subject in the
triples. Triples where the NIR is a object are optional,
but nice to have.
• Descriptions about related resources and metadata
(e.g. publisher, creation date, etc.) should be attached
to the information resource.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

19. Content negotiation in Linked Logainm
Linked Logainm is a collaborative project undertaken
by the DRI, Insight @ NUI Galway, Fiontar at DCU, the
National Library of Ireland and the Placenames Branch
of the Department of Arts, Heritage and the Gaeltacht.
The Linked Logainm project has created a new open
dataset, which allows Irish place names to be linked
across the world by cutting edge technologies
developed in Ireland.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

20. Content negotiation in Linked Logainm
Westport is identified by the URI http://data.logainm.ie/place/132920
$ curl -H "Accept:text/rdf+n3" http://
data.logainm.ie/place/13292
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix spatial: <http://geovocab.org/spatial#> .
<http://data.logainm.ie/place/132920> rdf:type
spatial:Feature .
@prefix ns2: <http://data.logainm.ie/category/> .
<http://data.logainm.ie/place/132920> rdf:type ns2:B .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix ns4: <http://linkedgeodata.org/triplify/> .
<http://data.logainm.ie/place/132920> owl:sameAs
ns4:node52244000 ,
<http://id.worldcat.org/fast/1210744> ,
<http://sws.geonames.org/2960970/> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
<http://data.logainm.ie/place/132920> foaf:name "Westport"@en
…
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

21. Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
The four principles
Number
1
“Use
URIs
as
names
for
things.”
Number
2
“Use
HTTP
URIs
so
that
people
can
look
up
those
names.”
Number
3
“When
someone
looks
up
a
URI,
provide
useful
informa<on,
using
the
standards
(RDF,
SPARQL)”
Number
4
“Include
links
to
other
URIs,
so
that
they
can
discover
more
things.”

22. (4) Include links to other URIs
• Not only within the same dataset
<http://dbpedia.org/resource/James_Joyce>
dbpedia-owl:birthPlace
<http://dbpedia.org/resource/Dublin"> .
• But also across datasets
<http://dbpedia.org/page/Dublin>
owl:sameAs
<http://sws.geonames.org/7778677/> .
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy

23. Example of linking across datasets
dbpedia-­‐owl:popula<onTotal
gn:popula<on
gn:name
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy
hSp://
dbpedia.org/
resource/Dublin
Dublin
rdfs:label
527612
owl:sameAs
hSp://
sws.geonames.org
/7778677/
hSp://
sws.geonames.org
/7778677/
527612
Dublin
City

24. Linked Data for Librarians – November 6, 2014 Linking
Open
Data
cloud
diagram
2014,
by
Max
Schmachten b–e Rrogy,a
lC Irhisrhi sA<caadnem
Byi zer,
Anja
Jentzsch
and
Richard
Cyganiak.
hSp://lod-­‐cloud.net/

25. References
• Berners-Lee, T. (2006). Linked Data - Design Issues.
Retrieved October 20 2014, http://www.w3.org/
DesignIssues/LinkedData.html
• Gruber, T. R. (1993) Towards principles for the design of
ontologies used for knowledge sharing. In Guarino, N. and
Poli, R., eds. Formal Ontology in Conceptual Analysis and
Knowledge representation. Kluwer Academic Publishers,
Deventer, The Netherlands.
• Hitzler, P., Krotzsch, M., and Rudolph, S. (2010) Foundations
of Semantic Web Technologies. CRC Press, Boca Raton,
FL, the US.
• van Hooland, S. and Verborgh, R. (2014) Linked Data for
Libraries, Archives and Museums. How to clean, link and
publish your metadata. Facet Publishing, London, The UK.
Linked Data for Librarians – November 6, 2014 – Royal Irish Academy