A literature survey on Linked Data for a spring 2009 class at the Tetherless World Constellation.

1. Joshua Shinavier
The state of the art in
Linked Data
Advanced Semantic Web, Spring 2009
Literature Survey

2. Outline
• Linked Data
• Linking Open Data
• describing linked datasets
• growing the data web
• keeping Linked Data connected
• indexing and searching
• applications
• navigation
• state of the data web
2

3. Linked Data overview
• resource -- an item of interest
• URI -- global identifier for a resource
• representation -- data corresponding to the state
of a resource
• information resource -- a “document” containing
information
• non-information resource -- anything else
• associated description -- representation describing
a Semantic Web resource
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4. The Linking Open Data initiative
• “bootstrap” the data web with large, interconnected data sets
to reach a critical mass of semantics
• strict adherence to W3C standards
• identification and transportation (URI, HTTP) of resource
descriptions
• interpretation (RDF, RDFS, OWL) of resource descriptions
• LOD grows as data providers:
• publish structured data on the Web
• set RDF links between entities in different data sources
• transition of the web from a distributed document repository
into a universal, ubiquitous database [Erling 09]
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5. The LOD cloud
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6. LOD data sets
6

7. Link sets in LOD
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8. Describing linked datasets
• voiD (Vocabulary of Interlinked Datasets)
[Alexander, Cyganiak, Hausenblas, Zhao 09]
• describes data sets the link sets between them
• DING (Dataset RankING) [Toupikov, Umbrich,
Delbru, Hausenblas, Tummarello 09]
• ranking of linked datasets using formal
descriptions
• modeling of the Linked Data domain [Halpin,
Presutti 09]
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9. Keeping Linked Data connected
• network-shaped Entity Name System to enable
systematic reuse of URIs [Bouquet, Stoermer,
Cordioli, Tummarello 08]
• similar to DNS for interlinking hypertext
• n2Mate framework [Peterson, Cregan, Atkinson,
Brisbin 08]
• use social networking principles to facilitate
vocabulary and instance reuse
• graph-based disambiguation of Semantic Web
entities with idMesh [Cudré-Mauroux, Haghani,
Jost, Aberer, de Meer 09]
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10. Managing co-reference
• many conflated resources in DBpedia [Jaffri,
Glaser, Millard 08]
• representative of LOD as a whole
• Co-Reference Resolution Service [Glaser, Jaffri,
Millard 09]
• when co-reference is context-specific,
owl:sameAs is inappropriate
• stores co-reference information as a first-class
entity
• ontology-level alignment should precede data-level
alignment [Nikolov, Uren, Motta 09]
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11. Growing the data web
• how to get data out there?
• challenges of the read-write Semantic Web
• user awareness of social context of data (e.g.
licensing, privacy)
• view update problem
• is the wiki model applicable?
• incentives for posting data on the SW
• validating existing Linked Data with Vapour
[Berrueta, Fernandez, Frade 08]
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12. Examples of LOD data sets
• DBpedia [Auer, Bizer, Kobilarov, Lehmann,
Cyganiak, Ives 07]
• extracts structured information from Wikipedia
• linking hub for the LOD cloud
• RDF Book Mashup [Bizer, Cyganiak, Gauss 07]
• product metadata from Amazon.com
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13. Music and movies as Linked Data
• Linked Movie Database [Hassanzadeh, Consens 09]
• combines data from IMDb, Freebase, OMDB,
DBPedia, RottenTomatoes.com, Stanford Movie
Database
• interlinked music datasets [Raimond, Sutton,
Sandler 08]
• combines data from Jamendo on DBTune, BBC
John Peel sessions, SBSimilarity, Musicbrainz,
DBpedia, Geonames
• links artists, albums, tracks, personal music
collections
• generated links based similarity of resources,
similarity of neighbors
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14. Other sources of data
• the hypertext Web itself [Li, Zhao 08]
• extraction of semantic links from hypertext links and
hierarchical relationships among Web documents
• RDF representation of HTML DOM from using SparqPlug
[Coetzee, Heath, Motta 08]
• multimedia metadata
• interlinking multimedia fragments [Hausenblas, Troncy,
Bürger, Raimond 09]
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15. Other sources of data (cont.)
• XML Business Reporting Language (XBRL) [Garcia, Gil
09]
• mapping data to RDF and schemas to OWL
facilitates interoperability
• large thesauri [Neubert 09]
• as interlinking hubs for professional communities
• enterprise data, e.g. technical documentation [Servant
08]
• MARC21 bibliographic records [Styles, Ayers, Shabir
08]
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16. Mapping tools
• D2R Server for customizable mappings from
relational databases to ontologies [Bizer, Cyganiak
06]
• browser-based tools for defining RDB-to-RDF
mappings [Zhou, Xu, Chen, Idehen 08]
• Triplify [Auer, Dietzold, Lehmann, Hellmann,
Aumueller 09]
• from generic data silos to Linked Data using
OpenLink Data Spaces [Idehen, Erling 08]
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17. Aggregated resources
• Open Archives Initiative Protocol for Metadata
Harvesting (OAI-PMH)
• can be made Web-accessible with OAI2LOD
Server [Haslhofer, Schandl 08]
• Open Archives Initiative - Object Reuse and
Exchange (OAI-ORE) [Van de Sompel, Lagoze,
Nelson, Warner, Sanderson, Johnston 09]
• adheres to Web principles
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18. User-driven Linked Data
• existing Linked Data datasets are more
appropriate for machine than human
consumption
• template-generated interlinks are of limited quality
• data from existing silos quickly becomes out of
date
• need human involvement to grow the data web
organically
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19. User-driven Linked Data (cont.)
• direct modification using SPARQL/Update
• e.g. in Tabulator [Berners-Lee, Hollenbach, Lu, Presbrey,
Prud’hommeaux, Schraefel 08]
• User Contributed Interlinking [Halb, Raimond, Hausenblas]
• semantic wikis
• Loomp [Roesch, Heese 09]
• semantic annotation of content using a text editor
interface
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20. User-driven Linked Data (cont.)
• public data from existing social networks
• wrappers for Web 2.0 services [Passant 08]
• unifying personal identity across various
networks [Rowe 09]
• Semantically Interlinked Online Communities
(SIOC)
• integrating social media sites (forums, blogs,
wikis, etc. with the data web [Bojars, Passant,
Cyganiak, Breslin 08]
• Meaning of a Tag (MOAT) ontology gives meaning
to tags on Web 2.0 [Passant, Laublet 08]
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21. Usability and licensing
• usability (for humans) of Linked Data [Halb,
Raimond, Hausenblas 08]
• current LOD datasets are primarily for machine
consumption
• low semantic strength of current LOD link sets
• provenance information for Linked Data [Hartig
09]
• Open Data Commons license [Miller, Styles, Heath
08]
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22. Indexing and searching
• W3C’s TAP semantic search [Guha, McCool 01]
• Swoogle [Ding, Finin, Joshi, Pan, Cost, Peng, Reddivari,
Doshi, Sachs 04]
• adapts PageRank concept to ontologies
• SWSE [Hogan, Harth, Umbrich, Decker 07]
• MultiCrawler [Harth, Umbrich, Decker 06]
• RDF Gateway search
• Watson document-based search
• Falcons [Cheng, Ge, Wu, Qu 08]
• textual search using class hierarchies for query restriction
• Sindice Semantic Web index [Tummarello, Delbru, Oren 07]
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23. Link discovery
• Silk link discovery framework [Volz, Bizer, Gaedke,
Kobilarov 09]
• find relationships between entities within
different data sources
• generation of owl:sameAs links
• value of Web of Data depends on the amount and
quality of links between data sources
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24. Navigation
• like early Web, it’s easy to get “Lost in Hyperspace”
• Tabulator generic Linked Data browser [Berners-
Lee, Chen, Chilton, Connolly, Dhanaraj,
Hollenbach, Lerer, Sheets 06]
• encourage deployment of Linked Data
• test, refine and promote Linked Data standards
• faceted views over large-scale linked data with
Virtuoso Cluster Edition [Erling 09]
• Explorator RDF browser [Araujo, Schwabe 09]
• exploratory search using direct manipulation
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25. Navigation (cont.)
• DBPedia Mobile map view and faceted Linked
Data browser [Becker, Bizer 08]
• explore the geospatial Semantic Web
• uses current GPS position as a starting point
• potential for Linked Data publishing
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26. Navigation (cont.)
• Fenfire generic Linked Data browser [Hastrup,
Cyganiak, Bojars 08]
• uses graph views rather than tables or outlines
• shows graph data as directly as possible
• related to Fentwine [Fallenstein, Lukka 04]
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27. Navigation (cont.)
• Humboldt [Kobilarov,
Dickinson 08]
• exploratory browsing
• faceted views
• “resource at a time”
• uses a “pivot” operation
to refocus the view
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28. Navigation (cont.)
• zLinks plugin [Bergman, Giasson 08]
• WordPress plugin with supporting server
• relates hypertext links with contextually
relevant Linked Data
• WOWY (WordNet, OpenCyc, Wikipedia, YAGO)
• distinguish between types of resources
• disambiguate alternate senses
28

29. Navigation (cont.)
• mapping of Linked Data to a file system model
[Schandl 09]
• enables use of this data within desktop
applications
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30. Other applications
• how to use the data that is out there?
• emerging applications which exploit Linked
Data [Hausenblas 09]
• integrating data sources related to drug and
clinical trials [Jentzsch, Andersson, Hassanzadeh,
Stephens, Bizer 09]
• mashups
• MashQL [Jarrar, Dikaiakos 09]
• Internet is a database, mashup is a query
over that database
• benefit of specialized, independent Linked Data
services acting together [Bojars, Passant, Giasson,
Breslin 07]
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31. The gray area
• U-P2P framework for peer-to-peer linked data [Davoust,
Esfandiari 09]
• data replication provides a measure of popularity
• Linked Data with Named Graphs
• e.g. interlinks with embedded provenance information
[Zhao, Klyne, Shotton 08]
• Ripple scripting language [Shinavier 07]
• embeds Turing-complete programs in the Web of Data
31

32. State of the data web
• where are we with the Linked Data graph?
• size
• number and type of links
• usefulness to end users
• network characteristics
• single-point-of-access (e.g. DBpedia, GeoNames)
vs. distributed datasets (e.g. FOAF-o-sphere,
SIOC-land)
• syntactic and semantic analysis of the LOD
dataset [Hausenblas, Halb, Raimond, Heath 08]
32

33. Statistics of the data web
• today’s Linked Data is very different than the first-
generation data web [Halpin 09]
• LOD data accounts for the vast majority of data
• power-law distributions are emerging
• data web is not growing organically
• Web standards are generally adhered to
• is Linked Data useful to ordinary users?
• sampling of Linked Data using Live.com query
logs and FALCON-S semantic search engine
33

34. Query popularity follows a power law
• ...
34

35. URI frequency... not so much
• ...
35

36. Data publishing lacks a “long tail”
• ...
36

37. A few dominant ontologies are emerging
# of URIs by vocabulary
37

38. (DBpedia bias)
# of URIs by domain name
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39. Graph analysis for the data web
• common network analysis techniques can be used
to investigate interoperability and structural
patterns of the LOD cloud [Rodriguez 09]
• results based on March 2009 statistics of the LOD
data set graph:
• LOD graph is not strongly connected
• diameter of 8 is large given relatively small size
of the cloud
• data sets have nearly identical incoming and
outgoing link patterns (⇒ majority of reciprocal
owl:sameAs links)
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40. Ranking and clustering of LOD data sets
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41. • Original slide show:
• http://tw.rpi.edu/proj/portal.wiki/images/f/f0/
LinkedData.pdf
• References:
• http://tw.rpi.edu/proj/portal.wiki/images/e/e0/
LinkedDataSurvey.pdf
• BibTeX:
• http://tw.rpi.edu/proj/portal.wiki/images/3/37/
LinkedDataSurvey.bbl
41