A Semantic Web Middleware for Virtual Data Integration on the Web

A Semantic Web Middleware for Virtual Data Integration on the Web
A Semantic Web Middleware for Virtual Data Integration on the Web
Bibliographical Metadata
Subject:	Querying Distributed RDF Data Sources
Year:	2008
Authors:	Andreas Langegger, Wolfram Wöß, Martin Blochl
Content Metadata
Implementation:	SemWIQ

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.

Conclusion

In this contribution a mediator-based system for virtual data integration based on SemanticWeb technology has been presented. The system is primarily developed for sharing scientific data, but because of its generic architecture, it is supposed to be used for many other Semantic Web applications. In this paper query federation based on SPARQL and Jena/ARQ has been demonstrated in detail and several concepts for query optimization which is currently on the agenda have been discussed. Additional contributions can be expected after the implementation of additional features mentioned before.

Future work

For a mediator, minimizing response time is usually more important than maximizing throughput. Because ARQ is pipelined, response time is very good. However, shipping data is costly, so another goal is the minimization of the amount of data transferred. When using the REST-based SPARQL protocol a second requirement is to minimize the number of required requests. Query 2 and 3 show bad performance mainly because of bad join ordering and not pushing down filters to local sub-plans.

Approach

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Implementations

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Research Problem

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Evaluation

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