Linked Data

To reach the true potential of contents with structured data on the Semantic Web, structured contents are organized to create datasets that are linked to related entities of other datasets. The entities and relationships can be represented as directed multilabelled RDF graphs rather than a tree structure used in HTML and XML. A subset of the linked datasets explicitly provides a free license and called Linked Open Data. The union of these datasets forms the Linked Open Data Cloud, the decentralized core of the Semantic Web, where software agents can automatically find relationships between entities and make new discoveries. Linked Open Data is used by search engines, governments, social media, publishing agencies, media portals, researchers, and individuals.

Linked Data Principles

Those structured datasets derived from different resources that are published with typed links between them are called Linked Data (also known as Linking Data). Structured data should meet the following requirements to be called Linked Data:

  • A dereferenceable Uniform Resource Identifier (URI) such as a web address is assigned to each entity of the dataset rather than an application-specific identifier such as a database key or incremental numbers, making every data entity individually identifiable.
  • The data must be structured data in a standard Semantic Web format such as the Resource Description Framework).
  • The data format is non-proprietary (not vendor-specific).
  • The data entities are linked to other, related structured data entity URIs.
  • All data that fulfill the above requirements and are publicly available under an open license on the Web are called Linked Open Data (LOD).

Assume that you have a scanned image of a table on your local hard drive. If you want to publish that table as Linked Open Data, you create a structured data file based on the information of the table in a standard Semantic Web language, upload it to a publicly available web page under an open license, add a unique web address (typically with fragment identifier) to each row of the table, and link them to other, related data.
The benefits of Linked Data are recognized by more and more organizations, businesses, and individuals. Some industrial giants that already have LOD implementations are Amazon, BBC, Facebook, Flickr, Google, Thomson Reuters, New York Times, and Yahoo!, just to name a few.

Linked Data software are now widely deployed, as for example, Apache Marmotta, Callimachus, Neologism, LODStats, or available online such as Sindice Web Data Inspector and sameAs.