Database Reference
In-Depth Information
14.1 Semantic Web
The
semantic web
is a proposal oriented to represent web content in a
machine-processable way. The basic layer for data representation on the
semantic web recommended by the World Wide Web Consortium (W3C)
is the resource description framework (RDF). In a semantic web scenario,
domain ontologies are used to define a common terminology for the concepts
involved in a particular domain. These ontologies are expressed in RDF or in
languages defined on top of RDF like the Web Ontology Language (OWL)
1
and are especially useful for describing unstructured, semistructured, and
text data. Many applications attach metadata and semantic annotations to
the information they produce (e.g., in medical applications, medical images,
and laboratory tests). We expect that, in the near future, large repositories
of semantically annotated data will be available, opening new opportunities
for enhancing current decision-support systems.
14.1.1 Introduction to RDF and RDFS
The
resource description framework
(RDF)
2
is a formal language for
describing structured information. One of the main goals of RDF is to
enable the composition of distributed data to allow data exchange over the
web. To uniquely identify resources, RDF uses
internationalized resource
identifiers
(IRIs). IRIs generalize the concept of
universal resource
locators
(URLs) since they do not necessarily refer to resources located
on the web. Further, IRIs generalize the concept of the
uniform resource
identifiers
(URIs): while URIs are limited to a subset of the ASCII character
set, IRIs may contain Unicode characters.
RDF can be used to express assertions over resources. These assertions
are expressed in the form of
subject-predicate-object
triples, where
subject
are
resources or
blank nodes
,
predicate
are resources, and
object
are resources
or
literals
(i.e., data values). Blank nodes are used to represent resources
without an IRI, typically with a structural function, for example, to group
a set of statements. A set of RDF triples or
RDF data set
can be seen as a
directed graph where subjects and objects are nodes and predicates are arcs.
RDF provides a way to express statements about resources using named
properties and values. However, sometimes it is also needed to define kinds
or classes of resources and the specific properties describing those resources.
A set of reserved words, called
RDF Schema
(RDFS),
3
is used to define
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