Irini Fundulaki

Ontology-Based Integration of XML Web Resources

This paper deals with some modeling aspects that have to be addressed in the context of the integration of heterogeneous and autonomous XML resources. We propose an integration system, but the emphasis of this paper is neither on its algorithmic aspects nor on its technical details. Instead, we focus on the significance of offering appropriate high-level primitives and mechanisms for representing the semantics of XML data. We posit that support for such primitives and mechanisms is a pre-requisite for realizingthe goals of the semantic Web.

Specifying access control policies for XML documents with XPath

Access control for XML documents is a non-trivial topic, as can be witnessed from the number of approaches presented in the literature. Trying to compare these, we discovered the need for a simple, clearand unambiguous language to state the declarative semantics of an access control policy. All current approaches state the semantics in natural language, which has none of the above properties. This makes it hard to assess whether the proposed algorithms are correct (i.e., really implement the described semantics). It is also hard to assess the proposed policy on its merits, and to compare it to others (for file systems for instance). This paper shows how XPath can be used to specify the semantics of an access control policy for XML documents. Using XPath has great advantages: it is standard technology, widely used and it has clear and easy syntax and semantics. We use the developed framework to give a formal specification of the five most prominent approaches of access controlfor XML documents from the literature.

On Detecting High-Level Changes in RDF/S KBs

An increasing number of scientific communities rely on Semantic Web ontologies to share and interpret data within and across research domains. These common knowledge representation resources are usually developed and maintained manually and essentially co-evolve along with experimental evidence produced by scientists worldwide. Detecting automatically the differences between (two) versions of the same ontology in order to store or visualize their deltas is a challenging task for e-science. In this paper, we focus on languages allowing the formulation of concise and intuitive deltas, which are expressive enough to describe unambiguously any possible change and that can be effectively and efficiently detected. We propose a specific language that provably exhibits those characteristics and provide a change detection algorithm which is sound and complete with respect to the proposed language. Finally, we provide a promising experimental evaluation of our framework using real ontologies from the cultural and bioinformatics domains.

Querying XML Sources Using an Ontology-Based Mediator

In this paper we propose a mediator architecture for the querying and integration of Web-accessible XML data sources. Our contributions are (i) the definition of a simple but expressive mapping language, following the local as view approach and describing XML resources as local views of some global schema, and (ii) efficient algorithms for rewriting user queries according to existing source descriptions. The approach has been validated by the STYX prototype.

Heuristics-based query optimisation for SPARQL

Query optimization in RDF Stores is a challenging problem as SPARQL queries typically contain many more joins than equivalent relational plans, and hence lead to a large join order search space. In such cases, cost-based query optimization often is not possible. One practical reason for this is that statistics typically are missing in web scale setting such as the Linked Open Datasets (LOD). The more profound reason is that due to the absence of schematic structure in RDF, join-hit ratio estimation requires complicated forms of correlated join statistics; and currently there are no methods to identify the relevant correlations beforehand. For this reason, the use of good heuristics is essential in SPARQL query optimization, even in the case that are partially used with cost-based statistics (i.e., hybrid query optimization). In this paper we describe a set of useful heuristics for SPARQL query optimizers. We present these in the context of a new Heuristic SPARQL Planner (HSP) that is capable of exploiting the syntactic and the structural variations of the triple patterns in a SPARQL query in order to choose an execution plan without the need of any cost model. For this, we define the variable graph and we show a reduction of the SPARQL query optimization problem to the maximum weight independent set problem. We implemented our planner on top of the MonetDB open source column-store and evaluated its effectiveness against the state-of-the-art RDF-3X engine as well as comparing the plan quality with a relational (SQL) equivalent of the benchmarks.

Coloring RDF Triples to Capture Provenance

Recently, the W3C Linking Open Data effort has boosted the publication and inter-linkage of large amounts of RDF datasets on the Semantic Web. Various ontologies and knowledge bases with millions of RDF triples from Wikipedia and other sources, mostly in e-science, have been created and are publicly available. Recording provenance information of RDF triples aggregated from different heterogeneous sources is crucial in order to effectively support trust mechanisms, digital rights and privacy policies. Managing provenance becomes even more important when we consider not only explicitly stated but also implicit triples (through RDFS inference rules) in conjunction with declarative languages for querying and updating RDF graphs. In this paper we rely on colored RDF triples represented as quadruples to capture and manipulate explicit provenance information.

Semantic Web and Databases

Service Oriented Computing: Opportunities and Challenges.- Data Semantics Revisited.- Dynamic Agent Composition from Semantic Web Services.- Ontology-Extended Component-Based Workflows : A Framework for Constructing Complex Workflows from Semantically Heterogeneous Software Components.- Data Procurement for Enabling Scientific Workflows: On Exploring Inter-ant Parasitism.- XSDL: Making XML Semantics Explicit.- Refining Semantic Mappings from Relational Tables to Ontologies.- Triadic Relations: An Algebra for the Semantic Web.- Semantically Unlocking Database Content Through Ontology-Based Mediation.- Representation and Reasoning About Changing Semantics in Heterogeneous Data Sources.- Context Mediation in the Semantic Web: Handling OWL Ontology and Data Disparity Through Context Interchange.- HCOME: A Tool-Supported Methodology for Engineering Living Ontologies.- Query Answering by Rewriting in GLAV Data Integration Systems Under Constraints.- Utilizing Resource Importance for Ranking Semantic Web Query Results.- Querying Faceted Databases.- Constructing and Querying Peer-to-Peer Warehouses of XML Resources.

On Provenance of Queries on Semantic Web Data

Capturing trustworthiness, reputation, and reliability of Semantic Web data manipulated by SPARQL requires researchers to represent adequate provenance information, usually modeled as source data annotations and propagated to query results along with a query evaluation. Alternatively, abstract provenance models can capture the relationship between query results and source data by taking into account the employed query operators. The authors argue the benefits of the latter for settings in which query results are materialized in several repositories and analyzed by multiple users. They also investigate how relational provenance models can be leveraged for SPARQL queries, and advocate for new provenance models.

Controlling access to RDF graphs

One of the current barriers towards realizing the huge potential of Future Internet is the protection of sensitive information, i.e., the ability to selectively expose (or hide) information to (from) users depending on their access privileges. Given that RDF has established itself as the de facto standard for data representation over the Web, our work focuses on controlling access to RDF data. We present a high-level access control specification language that allows fine-grained specification of access control permissions (at triple level) and formally define its semantics. We adopt an annotation-based enforcement model, where a user can explicitly associate data items with annotations specifying whether the item is accessible or not. In addition, we discuss the implementation of our framework, propose a set of dimensions that should be considered when defining a benchmark to evaluate the different access control enforcement models and present the results of our experiments conducted on different Semantic Web platforms.

Integrating ontologies and thesauri for RDF schema creation and metadata querying

Abstract.In this paper we present a new approach for building metadata schemas by integrating existing ontologies and structured vocabularies (thesauri). This integration is based on the specification of inclusion relationships between thesaurus terms and ontology concepts and results in application-specific metadata schemas incorporating the structural views of ontologies and the deep classification schemes provided by thesauri. We will also show how the result of this integration can be used for RDF schema creation and metadata querying. In our context, (metadata) queries exploit the inclusion semantics of term relationships, which introduces some recursion. We will present a fairly simple database-oriented solution for querying such metadata which avoids a (recursive) tree traversal and is based on a linear encoding of thesaurus hierarchies.