R Richard Vdovják

Index structures and algorithms for querying distributed RDF repositories

A technical infrastructure for storing, querying and managing RDFdata is a key element in the current semantic web development. Systems like Jena, Sesame or the ICS-FORTH RDF Suite are widelyused for building semantic web applications. Currently, none ofthese systems supports the integrated querying of distributed RDF repositories. We consider this a major shortcoming since the semanticweb is distributed by nature. In this paper we present an architecture for querying distributed RDF repositories by extending the existing Sesame system. We discuss the implications of our architectureand propose an index structure as well as algorithms forquery processing and optimization in such a distributed context.

Hera: development of semantic web information systems

As a consequence of the success of the Web, methodologies for information system development need to consider systems that use the Web paradigm. These Web Information Systems (WIS) use Web technologies to retrieve information from the Web and to deliver information in a Web presentation to the users. Hera is a model-driven methodology supporting WIS design, focusing on the processes of integration, data retrieval, and presentation generation. Integration and data retrieval gather from Web sources the data that composes the result of a user query. Presentation generation produces theWeb or hypermedia presentation format for the query result, such that the presentation and specifically its navigation suits the users browser. We show how in Hera all these processes lead to data transformations based on RDF(S) models. Proving the value of RDF(S) for WIS design, we pave the way for the development of Semantic Web Information Systems.

RAL: an algebra for querying RDF

To make the World Wide Web machine-understandable there is a strong demand both for languages describing metadata and for languages querying metadata. The Resource Description Framework (RDF), a language proposed by W3C, can be used for describing metadata about (Web) resources. RDF Schema (RDFS) extends RDF by providing means for creating application specific vocabularies (ontologies). While the two above languages are widely acknowledged as a standard means for describing Web metadata, a standardized language for querying RDF metadata is still an open issue. Research groups coming both from industry and academia are presently involved in proposing several RDF query languages. Due to the lack of an RDF algebra such query languages use APIs to describe their semantics and optimization issues are mostly neglected. This paper proposes RAL (an RDF algebra) as a reference mathematical study for RDF query languages and for performing RDF query optimization. We define the data model, we present the operators to manipulate the data, and we address the application of RAL for query optimization. RAL includes: extraction operators to retrieve the needed resources from the input RDF model, loop operators to support repetition, and construction operators to build the resulting RDF model.

Towards distributed processing of RDF path queries

A technical infrastructure for storing, querying and managing RDF data is a key element in the current semantic web development. Systems like Jena, Sesame or the ICS-FORTH RDF Suite are widely used for building semantic web applications. Currently, none of these systems support the integrated querying of distributed RDF repositories. We consider this a major shortcoming since the semantic web is distributed by nature. In this paper we present an architecture for querying distributed RDF repositories by extending the existing Sesame system. We discuss the implications of our architecture and propose an index structure as well as algorithms for query processing and optimisation in such a distributed context.

Embedding information retrieval in adaptive hypermedia: IR meets AHA!

This paper concentrates on the retrieval aspect in adaptive hypermedia (AH). Traditionally, AH research concentrates on applications that are ‘closed’, in the sense that they assume fixed content elements. Certain applications ask for an extension of the contents considered, with data obtained through information retrieval (IR). This paper addresses this issue of ‘opening up’ AH applications, and gives insight into research that applies techniques from IR and from the Semantic Web (SW) for the embedding of IR in AH. We look at this issue in the context of an abstract reference model (AHAM) and a concrete implementation framework (AHA!). The goal of this research is to define a framework for AH with extended IR functionality. We address the relevant issues for this framework, characterized by the application of concepts from the SW paradigm leading to an enriched notion of concept relevancy.

An RMM-Based Methodology for Hypermedia Presentation Design

Due to the rapid growth of the Web, there is an increasing need for methodologies that support the design of Web-based Information Systems (WIS). After investigating the application of existing hypermedia design methodologies in the context of automated hypermedia presentation design we propose a specification framework for this context. The framework considers the possibility of dynamically gathering information from a collection of structured, but also possibly heterogeneous sources (relational or object-oriented databases, XML repositories etc.). The methodology associated with the framework shows two levels of abstraction: the logical level, and the presentation level. At the logical level the application diagram captures the design of slices, thus specifying the content related grouping of data elements and their relationships. At the presentation level, the presentation diagram bridges the logical level and the actual implementation by specifying how the design of slices is translated into hypermedia mechanisms, e.g. hyperlinks.

Modeling User Input and Hypermedia Dynamics in Hera

Methodologies for the engineering of Web applications typically provide models that drive the generation of the hypermedia navigation structure in the application. Most of these methodologies and their models consider link following as the only materialization of the navigation structure. In this paper we see how extended user input can dynamically influence the navigation structure. By means of Hera it is shown how one can define this extended user input and capture the functional aspects related to the hypermedia dynamics in the RDF(S)-based design models. For this purpose we discuss the definition of input controls, the representation of state information, and the embedding of both in the application model. We also present the XML/RDF-based architecture implementing this.

Methodologies for Web information system design

The Web information space is rapidly growing in the size and the diversity of both its data and its audience. A consequence is that Web information systems (WIS) in many applications replace existing traditional (not Web based) information systems. Since the nature of WIS differs from the nature of traditional information systems there is a strong demand for design methodologies specifically oriented towards WIS design. The complexity of WIS implies the need for an effective design process and a rigorous and systematic design approach. We argue that besides the quality of the navigation that is typical for Web (hypermedia) data also the adaptation of the presented content is a desired feature of a modern WIS. In this paper we briefly describe the navigation and adaptation design in selected WIS design methodologies, RMM, OOHDM, UWE, and particularly Hera.

Providing the Semantic Layer for WIS Design

Designing Web-based information systems requires the use of a thorough design methodology. Particularly, when the content of the system is gathered from different information sources available via the Web, the specification of how data is to be retrieved requires appropriate design tools. Concretely, a solution to the problem of how to facilitate the design of integrating heterogeneous information sources is needed, in order to be able to provide a uniform access to data gathered from different sources. In this paper we propose the use of the Hera methodology extended with the Semantic Layer, which concentrates on the integration aspect. The presented integration framework provides a coherent and meaningful (with respect to a given conceptual model) view of the integrated heterogeneous information sources.

EROS: explorer for RDFS-based ontologies

1. THE CHALLENGE IN VISUALIZING RDFS-BASED ONTOLOGIES RDFS is an acknowledged backbone of the Semantic Web architecture. Browsing and querying RDFS-based ontologies is becoming a characteristic (user) activity in Semantic Web applications. We argue that due to peculiarities of the RDFS language a new interface is needed to convey RDFS-based ontologies to the end-user in a comprehensible form. The two main approaches currently used, the tree-based approach and the graph-based approach, do not fulfill the above requirement completely. The tree paradigm does not help the user in grasping other concept relationships than that used to construct the tree structure (most of the time being the rdfs:subClassOf relationship). The graph approach, on the other hand, displays all concept relationships but as a result introduces the full complexity of a directed labeled graph in which it is very difficult to spot the hierarchical structure of the ontology “hidden” behind the special kind of edges.