Yannis Tzitzikas

Dynamic Taxonomies and Faceted Search

Current access paradigms for the Web, i.e., direct access via search engines or database queries and navigational access via static taxonomies, have recently been criticized because they are too rigid or simplistic to effectively cope with a large number of practical search applications. A third paradigm, dynamic taxonomies and faceted search, focuses on user-centered conceptual exploration, which is far more frequent in search tasks than retrieval using exact specification, and has rapidly become pervasive in modern Web data retrieval, especially in critical applications such as product selection for e-commerce. It is a heavily interdisciplinary area, where data modeling, human factors, logic, inference, and efficient implementations must be dealt with holistically. Sacco, Tzitzikas, and their contributors provide a coherent roadmap to dynamic taxonomies and faceted search. The individual chapters, written by experts in each relevant field and carefully integrated by the editors, detail aspects like modeling, schema design, system implementation, search performance, and user interaction. The basic concepts of each area are introduced, and advanced topics and recent research are highlighted. An additional chapter is completely devoted to current and emerging application areas, including e-commerce, multimedia, multidimensional file systems, and geographical information systems. The presentation targets advanced undergraduates, graduate students and researchers from different areas from computer science to library and information science as well as advanced practitioners. Given that research results are currently scattered among very different publications, this volume will allow researchers to get a coherent and comprehensive picture of the state of the art.

On Graph Features of Semantic Web Schemas

In this paper, we measure and analyze the graph features of semantic Web (SW) schemas with focus on power-law degree distributions. Our main finding is that the majority of SW schemas with a significant number of properties (respectively, classes) approximate a power law for total-degree (respectively, the number of subsumed classes) distribution. Moreover, our analysis revealed some emerging conceptual modeling practices of SW schema developers: (1) each schema has a few focal classes that have been analyzed in detail (that is, they have numerous properties and subclasses), which are further connected with focal classes defined in other schemas, (2) class subsumption hierarchies are mostly unbalanced (that is, some branches are deep and heavy, while others are shallow and light), (3) most properties have as domain/range classes that are located high at the class subsumption hierarchies, and (4) the number of recursive/multiple properties is significant. The knowledge of these features is essential for guiding synthetic SW schema generation, which is an important step toward benchmarking SW repositories and query language implementations.

On the foundations of computing deltas between RDF models

The ability to compute the differences that exist between two RDF models is an important step to cope with the evolving nature of the Semantic Web (SW). In particular, RDF Deltas can be employed to reduce the amount of data that need to be exchanged and managed over the network and hence build advanced SW synchronization and versioning services. By considering Deltas as sets of change operations, in this paper we study various RDF comparison functions in conjunction with the semantics of the underlying change operations and formally analyze their possible combinations in terms of correctness, minimality, semantic identity and redundancy properties.

Mediators over ontology-based information sources

We propose a model for providing integrated and unified access to multiple information sources of the kind of Web catalogs. We assume that each source comprises two parts: (a) an ontology i.e. a set of terms structured by a subsumption relation, and (b) a database that stores objects of interest under the terms of the ontology. We assume that these objects belong to an underlying domain that is common to all sources (e.g. a set of Web pages of interest), and that different sources may use different ontologies with terms that correspond to different natural languages or to different levels of granularity. Information integration is obtained through a mediator comprising two parts: (a) an ontology, and (b) a set of articulations to the sources. Here, by articulation to a source we mean a set of relationships between terms of the mediator and terms of that source. Information requests (queries) are addressed to the mediator whose task is to analyze each query into sub-queries, translate them into queries to the appropriate sources, then merge the results to answer the original query. We study the querying and answering process in such a model and present algorithms for handling the main tasks of the mediator, namely, query translation between the mediator and the sources, source selection and result merging to produce the final answer.

Modeling and querying provenance by extending CIDOC CRM

This paper elaborates on the problem of modeling provenance for both physical and digital objects. In particular it discusses provenance according to OAIS (ISO 14721:2003) and how it relates with the conceptualization of CIDOC CRM ontology (ISO 21127:2006). Subsequently it introduces an extension of the CIDOC CRM ontology, able to capture the modeling and the query requirements regarding the provenance of digital objects. Over this extension the paper provides a number of indicative examples of modeling provenance in various domains. Subsequently, it introduces a number of indicative provenance query templates, and finally it describes an implementation using Semantic Web technologies.

On Computing Deltas of RDF/S Knowledge Bases

The ability to compute the differences that exist between two RDF/S Knowledge Bases (KB) is an important step to cope with the evolving nature of the Semantic Web (SW). In particular, RDF/S deltas can be employed to reduce the amount of data that need to be exchanged and managed over the network in order to build SW synchronization and versioning services. By considering deltas as sets of change operations, in this article we introduce various RDF/S differential functions which take into account inferred knowledge from an RDF/S knowledge base. We first study their correctness in transforming a source to a target RDF/S knowledge base in conjunction with the semantics of the employed change operations (i.e., with or without side-effects on inferred knowledge). Then we formally analyze desired properties of RDF/S deltas such as size minimality, semantic identity, redundancy elimination, reversibility, and composability, as well as identify those RDF/S differential functions that satisfy them. Subsequently, we experimentally evaluate the computing time and size of the produced deltas over real and synthetic RDF/S knowledge bases.

Ostensive Automatic Schema Mapping for Taxonomy-Based Peer-to-Peer Systems

This paper considers Peer-to-Peer systems in which peers employ taxonomies for describing the contents of their objects and for formulating semantic-based queries to the other peers of the system. As each peer can use its own taxonomy, peers are equipped with inter-taxonomy mappings in order to carry out the required translation tasks. As these systems are ad-hoc, the peers should be able to create or revise these mappings on demand and at run-time. For this reason, we introduce an ostensive data-driven method for automatic mapping and specialize it for the case of taxonomies.

Integrating Heterogeneous and Distributed Information about Marine Species through a Top Level Ontology

One of the main characteristics of biodiversity data is its cross-disciplinary feature and the extremely broad range of data types, structures, and semantic concepts which encompasses. Moreover, biodiversity data, especially in the marine domain, is widely distributed, with few well-established repositories or standard protocols for their archiving, access, and retrieval. Our research aims at providing models and methods that allow integrating such information either for publishing it, browsing it, or querying it. For providing a valid and reliable knowledge ground for enabling semantic interoperability of marine data, in this paper we motivate a top level ontology, called MarineTLO that we have designed for this purpose, and discuss its use for creating MarineTLO-based warehouses in the context of a research infrastructure.

Mediators over taxonomy-based information sources

Abstract.We propose a mediator model for providing integrated and unified access to multiple taxonomy-based sources. Each source comprises a taxonomy and a database that indexes objects under the terms of the taxonomy. A mediator comprises a taxonomy and a set of relations between the mediator’s and the sources’ terms, called articulations. By combining different modes of query evaluation at the sources and the mediator and different types of query translation, a flexible, efficient scheme of mediator operation is obtained that can accommodate various application needs and levels of answer quality. We adopt a simple conceptual modeling approach (taxonomies and intertaxonomy mappings) and we illustrate its advantages in terms of ease of use, uniformity, scalability, and efficiency. These characteristics make this proposal appropriate for a large-scale network of sources and mediators.

Exploratory web searching with dynamic taxonomies and results clustering

This paper proposes exploiting both explicit and mined metadata for enriching Web searching with exploration services. On-line results clustering is useful for providing users with overviews of the results and thus allowing them to restrict their focus to the desired parts. On the other hand, the various metadata that are available to a WSE (Web Search Engine), e.g. domain/language/date/filetype, are commonly exploited only through the advanced (form-based) search facilities that some WSEs offer (and users rarely use). We propose an approach that combines both kinds of metadata by adopting the interaction paradigm of dynamic taxonomies and faceted exploration. This combination results to an effective, flexible and efficient exploration experience.