Yannis Stavrakas

Multidimensional Semistructured Data: Representing Context-Dependent Information on the Web

In this paper, we address a problem common in the frame of WWW, namely, representing information that assumes different facets under different contexts (sets of worlds). For expressing context-dependent (or multidimensional) data, we introduce Multidimensional Semistructured Data, where context is defined through (dimension, value) pairs. An extension of OEM called Multidimensional Object Exchange Model (MOEM) is introduced, for representing multidimensional data. We discuss the properties of MOEM and define a transformation that, for any given world, reduces MOEM to a conventional OEM holding under that world. As a case study, we show how MOEM can be used to represent changes over time in an OEM database.

Representing Changes in XML Documents Using Dimensions

In this paper, we present a method for representing the history of XML documents using Multidimensional XML (MXML). We demonstrate how a set of basic change operations on XML documents can be represented in MXML, and show that temporal XML snapshots can be obtained from MXML representations of XML histories. We also argue that our model is capable to represent changes not only in an XML document but to the corresponding XML Schema document as well.

Diachronic linked data: towards long-term preservation of structured interrelated information

The Linked Data Paradigm is a promising technology for publishing, sharing, and connecting data on the Web, which provides new perspectives for data integration and interoperability. However, the proliferation of distributed, interconnected linked data sources on the Web poses significant new challenges for consistently managing the vast number of potentially large datasets and their interdependencies. In this article we focus on the key problem of preserving evolving structured interlinked data. We argue that a number of issues, which hinder applications and users, are related to the temporal aspect that is intrinsic in Linked Data. We present three use cases to motivate our approach, we discuss problems that occur, and propose a direction for a solution.

Incorporating Dimensions in XML and DTD

In this paper we investigate various aspects of representing multidimensional information in the frame of the WWW. Multidimensional XML (MXML) is an extension of XML suitable for representing data that assume different facets, having different value or different structure, under different contexts. In Multidimensional XML, elements and attributes may depend on a number of dimensions, that define worlds under which variants of those elements or attributes hold. Moreover, we propose an extension of DTD that takes dimensions into account and is suitable for describing the logical structure of MXML documents. We also present a graph data model for MXML, and show how MXML can be reduced to conventional XML for a given world.

A Flexible Framework for Understanding the Dynamics of Evolving RDF Datasets

The dynamic nature of Web data gives rise to a multitude of problems related to the description and analysis of the evolution of RDF datasets, which are important to a large number of users and domains, such as, the curators of biological information where changes are constant and interrelated. In this paper, we propose a framework that enables identifying, analysing and understanding these dynamics. Our approach is flexible enough to capture the peculiarities and needs of different applications on dynamic data, while being formally robust due to the satisfaction of the completeness and unambiguity properties. In addition, our framework allows the persistent representation of the detected changes between versions, in a manner that enables easy and efficient navigation among versions, automated processing and analysis of changes, cross-snapshot queries spanning across different versions, as well as queries involving both changes and data. Our work is evaluated using real Linked Open Data, and exhibits good scalability properties.

Representing and querying histories of semistructured databases using multidimensional OEM

Multidimensional semistructured data (MSSD) are semistructured data that present different facets under different contexts. Context represents alternative worlds, and is expressed by assigning values to a set of user-defined variables called dimensions. The notion of context has been incorporated in the object exchange model (OEM), and the extended model is called Multidimensional OEM (MOEM), a graph model for MSSD. In this paper, we explain in detail how MOEM can represent the history of an OEM database. We discuss how MOEM properties are applied in the case of representing OEM histories, and show that temporal OEM snapshots can be obtained from MOEM. We present a system that implements the proposed ideas, and we use an example scenario to demonstrate how an underlying MOEM database accommodates changes in an OEM database. Furthermore, we show that MOEM is capable to model changes occurring not only in OEM databases, but in Multidimensional OEM databases as well. The use of multidimensional query language (MQL), a query language for MSSD, is proposed for querying the history of OEM databases and MOEM databases.

Implementing a Query Language for Context-Dependent Semistructured Data

In today’s global environment, the structure and presentation of information may depend on the underlying context of the user. To address this issue, in previous work we have proposed multidimensional semistructured data (MSSD), where an information entity can have alternative variants, or facets, each holding under some world, and MOEM, a data model suitable for representing MSSD. In this paper we briefly present MQL, a query language for MSSD that supports context-driven queries, and we attempt to motivate the direct use of context in data models and query languages by comparing MOEM and MQL with equivalent, context-unaware forms of representing and querying information. Specifically, we implemented an evaluation process for MQL during which MQL queries are translated to equivalent Lorel queries, and MOEM databases are transformed to corresponding OEM databases. The comparison between the two query languages and data models demonstrates the benefits of treating context as first-class citizen. We illustrate this query translation process using a cross-world MQL query, which has no direct counterpart in context-unaware query languages and data models.

A Web-Based System for Handling Multidimensional Information through MXML

In this paper we address an issue common in the frame of WWW, namely information entities that present different facets under different contexts (or worlds). Handling such multifacet or multidimensional entities requires a multidimensional paradigm for Web data, which consists of representation, manipulation and presentation issues. For representing multidimensional data we employ Multidimensional XML, a markup language that incorporates dimensions in XML. We discuss the presentation of multidimensional data through multidimensional XSL stylesheets. We describe the design of a system that implements the basic functionality of the multidimensional paradigm, and demonstrates how a user can interact with a multidimensional document and view different variants of the document under different worlds.

Representing time-dependent information in Multidimensional XML

Multidimensional XML (MXML) is an extension of XML that incorporates dimensions in order to represent in an elegant and concise way context-dependent data, that is, data which can exhibit different variations in value or structure (e.g. that can exist in different languages or various degrees of detail). We show that MXML dimensions can be used to express time and associate it with XML elements and attributes. We discuss the properties of the time domain and the features of MXML which are necessary in order to represent time, and give key points that motivate further investigation on the use of MXML for representing time-dependent data.

A query language for multi-version data web archives

The Data Web refers to the vast and rapidly increasing quantity of scientific, corporate, government and crowd-sourced data published in the form of Linked Open Data, which encourages the uniform representation of heterogeneous data items on the web and the creation of links between them. The growing availability of open linked datasets has brought forth significant new challenges regarding their proper preservation and the management of evolving information within them. In this paper, we focus on the evolution and preservation challenges related to publishing and preserving evolving linked data across time. We discuss the main problems regarding their proper modelling and querying and provide a conceptual model and a query language for modelling and retrieving evolving data along with changes affecting them. We present in details the syntax of the query language and demonstrate its functionality over a real-world use case of evolving linked dataset from the biological domain.