A Semantic Web Middleware for Virtual Data Integration on the Web

Abstract

Abstract. In this contribution a system is presented, which provides access to distributed data sources using Semantic Web technology. While it was primarily designed for data sharing and scientific collaboration, it is regarded as a base technology useful for many other Semantic Web applications. The proposed system allows to retrieve data using SPARQL queries, data sources can register and abandon freely, and all RDF Schema or OWL vocabularies can be used to describe their data, as long as they are accessible on the Web. Data heterogeneity is addressed by RDF-wrappers like D2R-Server placed on top of local information systems. A query does not directly refer to actual endpoints, instead it contains graph patterns adhering to a virtual data set. A mediator finally pulls and joins RDF data from different endpoints providing a transparent on-the-fly view to the end-user.The SPARQL protocol has been defined to enable systematic data access to remote endpoints. However, remote SPARQL queries require the explicit notion of endpoint URIs. The presented system allows users to execute queries without the need to specify target endpoints. Additionally, it is possible to execute join and union operations across different remote endpoints. The optimization of such distributed operations is a key factor concerning the performance of the overall system. Therefore, proven concepts from database research can be applied.

Executing SPARQL Queries over the Web of Linked Data

“…The DARQ engine [11], for instance, decomposes a SPARQL query in subqueries, forwards these subqueries to multiple, distributed query services, and, finally, integrates the results of the subqueries. Very similar to the DARQ approach the SemWIQ [12] system contains a mediator service that transparently distributes the execution of SPARQL queries. Both systems, however, do not actively discover relevant data sources that provide query services.…”

DAW: Duplicate-AWare Federated Query Processing over the Web of Data

“…DARQ combines service descriptions, query rewriting mechanisms and a cost-based optimisation approach to reduce the query processing time and the bandwidth usage. Langegger et al [13] describe a solution similar to DARQ that relies on a mediator to keep its service descriptions up-to-date. SPLENDID [8] uses VOID 7 descriptions for data source selection along with SPARQL ASK queries.…”

Mobile Querying of Online Semantic Web Data for Context-Aware Applications

“…For instance, in [9], properties found in a dataset are extracted and stored, which is equivalent to SIM1. In semwiq [10], lists of classes and properties are kept; this leads to information loss compared to SIM2 and SIM3, as it is no longer known which classes occur in the domain or range of the found properties. DARQ [11] relies on manually specified service descriptions, listing found properties together with value constraints on their subjects and objects.…”