Abstract
As Web-based content becomes an increasingly important knowledge management resource, Webbased technologies are developing to help harness that resource in a more effective way. The current state of these Web-based technology – the ‘first generation’ or ‘syntactic’ Web – gives rise to well known, serious problems when trying to accomplish in a non-trivial way essential management tasks like indexing, searching, extracting, maintaining and generating information. These tasks would, in fact, require some sort of ‘deep understanding’ of the information dealt with: in a ‘syntactic’ Web context, on the contrary, computers are only used as tools for posting and rendering information by brute force. Faced with this situation, Tim Berners-Lee first proposed a sort of ‘Semantic Web’ (SW) where the access to information is based mainly on the processing of the semantic properties of this information: “… the Semantic Web is an extension of the current Web in which information is given well-defined meaning (emphasis added), better enabling computers and people to work in co-operation” (Berners-Lee et al., 2001: 35). The Semantic Web’s challenge consists then in being able to manage information on the Web by ‘understanding’ its proper semantic content (its meaning), and not simply by matching some keywords.
Key Terms in this Chapter
SPARQL: A simple query language for accessing RDF structures. As the majority of the query languages developed within a Web context, SPARQL is based on a strict ‘pattern-matching’ approach, which means that no inference facilities are directly associated with SPARQL. As the majority of the Web query languages, SPARQL makes use of a SQL-like format, employing then operators in the style of SELECT and WHERE.
OWL: The Web Ontology Language (OWL) is a semantic markup language for publishing and sharing ontologies on the World Wide Web. OWL is developed as a vocabulary extension of RDF and is derived from the DAML+OIL Web Ontology Language. An OWL ontology is an RDF graph, which is in turn a set of RDF triples. OWL includes three specific sub-languages, characterized by an increasing level of complexity and expressiveness, OWL Lite, OWL DL – DL stands for Description Logics, a particular, logic-oriented, knowledge representation language introduced to supply a formal foundation for frame-based systems – and OWL Full. The new OWL 2 proposals extends the ‘standard’ OWL Ontology Language with a small set of features like an increased expressiveness of properties, qualified cardinality constructors, extended datatype support, and a sort of meta-modeling device called “punning”.
Semantic Web Rules: Still a ‘hot’ topic in a Semantic Web context. The present proposals (like RuleML, TRIPLE or SWRL) are based on an expansion of the classical ‘logic programming’ paradigm where the inferential properties of Prolog/Datalog are extended to deal with RDF/OWL knowledge bases.
Linked Data Movement: The general aim of the “Linked Data” movement is that of popularizing the use of the Semantic Web techniques through the availability of large amounts of interlinked data in RDF format, where every RDF triple must be conceived as a hyperlink that can be followed by specific Linked Data browsers and crawlers. In particular, the Linking Open Data project aims at identifying datasets that are available on the Web under open licenses (such as Wikipedia, Musicbrainz, Geonames, Wordnet, and DBLP), at re-publishing these datasets in RDF format and at associating them with each other on the Web. Estimations about the size of the Web of Linked Data that stems directly from this effort amount to over two billion RDF triples, interlinked by around three million RDF links.
Resource Description Framework (RDF): An example of ‘metadata’ language (metadata = data about data) used to describe generic ‘things’ (‘resources’, according to the RDF jargon) on the Web. An RDF document is a list of statements under the form of triples having the classical format: <object, property, value>, where the elements of the triples can be URIs (Universal Resource Identifiers), literals (mainly, free text) and variables. RDF statements are normally written into XML format (the so-called ‘RDF/XML syntax’).
Extensible Markup Language (XML): Has been created to overcome some difficulties proper to HTML (Hypertext Markup Language) that – developed as a means for instructing the Web browsers how to display a given Web page – is a ‘presentation-oriented’ markup tool. XML is called ‘extensible’ because, at the difference of HTML, is not characterized by a fixed format, but it lets the user design its own customized markup languages (using, e.g., a specific DTD, Document Type Description) for limitless different types of documents; XML is then a ‘content-oriented’ markup tool.
Semantic Web Architecture: A layered architecture proposed by Berners-Lee for the Semantic Web applications. In this architecture, ontologies occupy a central place: they are built on the top of the RDF (Resource Description Framework) and RDFS (RDF Schema) layers, which are in turn built on the top of the XML layer.
Semantic Web Services: A Web service is a Web site that does not simply supply static information, but that allows also to execute automatically some ‘actions’ (services), like the sale of a product. To do this, Web services make use of XML-based standards like WSDL, a description protocol, and SOAP, a messaging protocol, characterized by a low level of semantic expressiveness. For example, WSDL can describe the interface of the different services, and how these services are deployed via SOAP, but it is very limited in its ability to express what the overall competences of this service are. Semantic Web Services are Web Services that can specify not only their interfaces, but also describe in full, under the form of OWL-based ontologies, their capabilities, and the prerequisites and consequences of their use. For example, OWL-S is a specification that enables Web users and software agents to automatically discover, invoke, select, compose and monitor Web-based services.
RDF Schema (RDFS): Provides a mechanism for constructing specialized RDF vocabularies through the description of domain-specific properties. This is obtained mainly by describing the properties in terms of the classes of resource to which they apply: for example, we could define the creator property saying that it has the resource document as ‘domain’ ( document is the value or ‘object’ of this property) and the resource person as ‘range’ (this property must always be associated with a resource person , its ‘subject’). Other basic modeling primitives of RDFS allow setting up hierarchies (taxonomies), both hierarchies of concepts thanks to the use of class and subclass-of statements, and hierarchies of properties thanks to the use of property and subproperty-of statements. Instances of a specific class (concept) can be declared making use of the type statement.
XML Schema: A more complete methodology, instead of using DTDs, for specifying the semantics of a set of XML markup elements. XML Schema supplies a complete grammar for describing the structure of the elements allowing, e.g., to define the cardinality of the offspring elements, default values, etc.