Abstract. The base of Semantic Web specifications is Resource Description Framework (RDF) as a standard for expressing metadata. RDF has a simple object model, allowing for easy design of knowledge bases. This implies that the size of knowledge bases can dramatically increase; therefore, it is necessary to take into account both scalability and space consumption when storing such bases. Some theoretical results related to blank node semantics can be exploited in order to design techniques that optimize, among others, space requirements in storing RDF descriptions. We present an algorithm, called REDD, that exploits these theoretical results and optimizes the space used by a RDF description.
MotivationThe realization of the Semantic Web (SW) vision [1] needs ontologies for generating or interpreting (semantic) metadata for resources. It is fundamental to have ontology creation and integration steps in order to share structural knowledge between ontology designers and users. Ontologies are to be expressed in RDF according to SW specifications, using languages such as RDFS 1 and OWL. 2 It is important to note that both RDFS and OWL ontologies can be expressed as RDF graphs, so that ontologies can be treated exactly as other RDF models. In RDF design, the least power principle was applied: data structures are to be kept as simple as possible. This imposes to have very simple basic components, that are URIs 3 , blank nodes and statements (or triples). These design decisions have the drawback that RDF descriptions tend to grow fast as the complexity of the knowledge they represent increases. This observation encourages SW research to investigate toward the most effective storage solutions for RDF knowledge bases, in order to minimize required space. Intuitively, the lesser the number of triples a software (say, a query engine) has to examine, the faster it will process them.