Linked data

In computing, linked data (often capitalized as Linked Data) is a method of publishing structured data so that it can be interlinked and become more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but rather than using them to serve web pages for human readers, it extends them to share information in a way that can be read automatically by computers. This enables data from different sources to be connected and queried.

Tim Berners-Lee, director of the World Wide Web Consortium (W3C), coined the term in a design note about the Semantic Web project.

Principles

Tim Berners-Lee outlined four principles of linked data in his Design Issues: Linked Data note, paraphrased along the following lines:

1. Use URIs to name (identify) things.
2. Use HTTP URIs so that these things can be looked up (interpreted, "dereferenced").
3. Provide useful information about what a name identifies when it's looked up, using open standards such as RDF, SPARQL, etc.
4. Refer to other things using their HTTP URI-based names when publishing data on the Web.

Tim Berners-Lee gave a presentation on linked data at the TED 2009 conference. In it, he restated the linked data principles as three "extremely simple" rules:

1. All kinds of conceptual things, they have names now that start with HTTP.
2. If I take one of these HTTP names and I look it up [..] I will get back some data in a standard format which is kind of useful data that somebody might like to know about that thing, about that event.
3. When I get back that information it's not just got somebody's height and weight and when they were born, it's got relationships. And when it has relationships, whenever it expresses a relationship then the other thing that it's related to is given one of those names that starts with HTTP.

Components

• URIs (specifically, of the dereferenceable variety)
• HTTP
• Structured data using controlled vocabulary terms and dataset definitions expressed in Resource Description Framework serialization formats such as RDFa, RDF/XML, N3, Turtle, or JSON-LD
• Linked Data Platform

Linked open data

Linked open data is linked data that is open content. Tim Berners-Lee gives the clearest definition of linked open data in differentiation with linked data. He defines linked data by identifying its four components, and then adds a fifth rule - open content - to define linked open data. Large linked open data sets include DBpedia and Freebase.

History

The term "linked open data" has been in use since at least February, 2007, when the "Linking Open Data" mailing list was created. The mailing list was initially hosted by the SIMILE project at the Massachusetts Institute of Technology.

Linking Open Data community project

The above diagram shows which Linking Open Data datasets are connected, as of August 2014. This was produced by the Linked Open Data Cloud project, which was started in 2007. Some sets may include copyrighted data which is freely available.

The goal of the W3C Semantic Web Education and Outreach group's Linking Open Data community project is to extend the Web with a data commons by publishing various open datasets as RDF on the Web and by setting RDF links between data items from different data sources. In October 2007, datasets consisted of over two billion RDF triples, which were interlinked by over two million RDF links. By September 2011 this had grown to 31 billion RDF triples, interlinked by around 504 million RDF links. A detailed statistical breakdown was published in 2014.

European Union projects

There are a number of European= Union projects involving linked data. These include the linked open data around the clock (LATC) project, the PlanetData project, the DaPaaS (Data-and-Platform-as-a-Service) project, and the Linked Open Data 2 (LOD2) project. Data linking is one of the main goals of the EU Open Data Portal, which makes available thousands of datasets for anyone to reuse and link.

Datasets

• DBpedia – a dataset containing extracted data from Wikipedia; it contains about 3.4 million concepts described by 1 billion triples, including abstracts in 11 different languages
• GeoNames provides RDF descriptions of more than 7,500,000 geographical features worldwide.
• UMBEL – a lightweight reference structure of 20,000 subject concept classes and their relationships derived from OpenCyc, which can act as binding classes to external data; also has links to 1.5 million named entities from DBpedia and YAGO
• FOAF – a dataset describing persons, their properties and relationships

Dataset instance and class relationships

Clickable diagrams that show the individual datasets and their relationships within the DBpedia-spawned LOD cloud (as shown by the figures to the right) are available.

See also

• Network model – an older type of database management system
• Citation analysis – for citations between scholarly articles
• Authority control – about controlled headings in library catalogs
• Hyperdata
• Linked data page
• Schema.org
• Resource Description Framework
• Uniform Resource Identifier
• Hypertext Transfer Protocol
• Web Ontology Language

Further reading

• Ahmet Soylu, Felix Mödritscher, and Patrick De Causmaecker. 2012. “Ubiquitous Web Navigation through Harvesting Embedded Semantic Data: A Mobile Scenario.” Integrated Computer-Aided Engineering 19 (1): 93–109.
• Linked Data: Evolving the Web into a Global Data Space (2011) by Tom Heath and Christian Bizer, Synthesis Lectures on the Semantic Web: Theory and Technology, Morgan & Claypool
• How to Publish Linked Data on the Web, by Chris Bizer, Richard Cyganiak and Tom Heath, Linked Data Tutorial at Freie Universität Berlin, Germany, 27 July 2007.
• The Web Turns 20: Linked Data Gives People Power, part 1 of 4, by Mark Fischetti, Scientific American 2010 October 23
• Linked Data Is Merely More Data – Prateek Jain, Pascal Hitzler, Peter Z. Yeh, Kunal Verma, and Amit P. Sheth. In: Dan Brickley, Vinay K. Chaudhri, Harry Halpin, and Deborah McGuinness: Linked Data Meets Artificial Intelligence. Technical Report SS-10-07, AAAI Press, Menlo Park, California, 2010, pp. 82–86.
• Moving beyond sameAs with PLATO: Partonomy detection for Linked Data – Prateek Jain, Pascal Hitzler, Kunal Verma, Peter Z. Yeh, Amit Sheth. In: Proceedings of the 23rd ACM Hypertext and Social Media conference (HT 2012), Milwaukee, WI, USA, June 25–28, 2012.
• Freitas, André, Edward Curry, João Gabriel Oliveira, and Sean O’Riain. 2012. “Querying Heterogeneous Datasets on the Linked Data Web: Challenges, Approaches, and Trends.” IEEE Internet Computing 16 (1): 24–33.
• Linked Data on the Web – Chris Bizer, Tom Heath, Kingsley Uyi Idehen, Tim Berners-Lee. In Proceedings WWW2008, Beijing, China
• Interlinking Open Data on the Web – Chris Bizer, Tom Heath, Danny Ayers, Yves Raimond. In Proceedings Poster Track, ESWC2007, Innsbruck, Austria
• Ontology Alignment for Linked Open Data – Prateek Jain, Pascal Hitzler, Amit Sheth, Kunal Verma, Peter Z. Yeh. In proceedings of the 9th International Semantic Web Conference, ISWC 2010, Shanghai, China
• Linked open drug data for pharmaceutical research and development - J Cheminform. 2011; 3: 19. Samwald, Jentzsch, Bouton, Kallesøe, Willighagen, Hajagos, Marshall, Prud'hommeaux, Hassenzadeh, Pichler, and Stephens (May 2011)
• Interview with Sören Auer, head of the LOD2 project about the continuation of LOD2 in 2011, June 2011
• Linked Open Data: The Essentials - Florian Bauer and Martin Kaltenböck (January 2012)
• The Flap of a Butterfly Wing - semanticweb.com Richard Wallis (February 2012)

External links

• LinkedData at the W3C Wiki
• LinkedData.org
• OpenLink Software white papers
• Data from Northwind SQL schema as linked data, use case demo
• Linked data for the discipline of numismatics, use case demo
• Interactive LOD demo
• American Art Collaborative, consortium of art museums in the United States committed to establishing a critical mass of linked open data on the subject of American art