The Semantic Web aims at easy integration and usage of content by building on a semi-structured data model where data semantics are explicitly specified through ontologies. However, ontologies and thereby ontology-based applications themselves suffer from heterogeneity. Therefore a new level of data independence is required to allow the customization of information, e.g. towards the needs of other agents, which can be achieved by exploiting database view principles. This paper addresses this issue and presents a new view mechanism for the data models underlying the Semantic Web, RDF and RDFS.

this paper we report upon a graph-based approach to such an integration. Our use of graphs has two key advantages : firstly, graphs are formally defined, well-understood structures; secondly, it is widely accepted that graph-based formalisms considerably enhance the usability of complex systems [19]. Graphs have been used in conjunction with a number of conventional data models, for example the hierarchical and network models [35], the entity-relationship model [9] and a recent extension thereof for complex objects [27], and various semantic data models [16, 20, 31]. Graphs or hypergraphs [6] have also been used more recently in [12, 17, 23, 25, 33, 36] as a data modelling tool in their own right. We give a comparison between this recent work and our own approach in Section 4 of the paper. Directed graphs have also been the foundation of Hypertext databases [11, 33]. Such databases are graphs consisting of nodes which refer to units of stored information (typically text) and of named links. Each link connects two nodes, the "source" and the "destination". Links are traversed either forwards (from source to destination) or backwards (from destination to source). The process of traversing named links and examining the text associated with nodes is called

1 Introduction A Formal Temporal Object-Oriented Data Model Elisa Bertino Elena Ferrari Giovanna Guerrini snapshot tuple timestamping attribute timestamping bertino@hermes.mc.dsi.unimi.it, ferrarie@dsi.unimi.it guerrini@disi.unige.it Temporal databases are an active and fast growing research area. Although many extensions to the relational data model have been proposed in order to incorporate time, there is no comparable amount of work in the context of object-oriented data models. This paper presents Chimera, a temporal extension of the Chimera object-oriented data model. The main contribution of this work is to define a formal temporal object-oriented data model and to address on a formal basis several issues deriving from the introduction of time in an object-oriented context. Dipartimento di Scienze dell'Informazione - Universit`a di Milano Via Comelico, 39/41 - I20133 Milano, Italy Dipartimento di Informatica e Scienze dell'Informazione - Universit`a di Genova Vi...

. This paper presents a framework for data integration that is based on using "Squirrel integration mediators" that use materialization to support integrated views over multiple databases. These mediators generalize techniques from active databases to provide incremental propagation of updates to the materialized views. A framework based on "View Decomposition Plans" for optimizing the support of materialized integrated views is introduced. The paper describes the Squirrel mediator generator currently under development, which can generate the mediators based on high-level specifications. The integration of information by Squirrel mediators is expressed primarily through an extended version of a standard query language, that can refer to data from multiple information sources. In addition to materializing an integrated view of data, these mediators can monitor conditions that span multiple sources. The Squirrel framework also provides efficient support for the problem of "object matchin...

The definition of a view mechanism... In this paper, we formally introduce a view model for object-oriented databases. Our view model is comparable to existing view models for what concerns the supported features; however, our model is the only one for which a formal definition is given. This formal definition of object-oriented view mechanisms is useful both for understanding what views are and as a basis for further investigations on view properties. The paper introduces the model, discussing all the supported features both from a theoretical and practical point of view. A comparison of our model with other models is also presented.

We examine the problem of merging persistent object bases (POBs) and the attendant problem of determining when two objects are equivalent. In particular, two or more source POBs are combined in a single target POB in which physically different objects in the sources may be merged into a single object in the target. Although persistent object base research has not focused on these problems in the past, any attempt to model (portions of) the real world in an object base butts up against them. In fact, the object-centered nature of POBs is consistent with and almost motivates our two-phase solution to the problem, in which an `object identification' phase precedes a `constraint resolution' phase. Families of keys are used to identify objects, after which various mechanisms are used to merge the remaining data. For example, preference specifications are used to resolve constraints violated in the target -- by preferring facts from one of the sources over the others. 1 Introduction One of ...

Abstract. Two major problems in schema integration are to identify correspondences between different conceptual schemas and to verify that the proposed correspondences are consistent with the semantics of the schemas. This problem can only be effectively addressed if the conceptual schema is expressed in a semantically rich modelling formalism. We introduce such a modelling formalism, the distinguishing feature of which is the use of case grammar. We show that it is easier to identify correspondences between schemas expressed in this formalism than in schemas formulated in traditional modelling languages. The main reason for this is that case grammar standardizes the terminology in conceptual schemas by providing a set of meaningful and established labels for conceptual relations. 1

The Semantic Web aims at easy integration and usage of content by building on a semi-structured data model where data semantics are explicitly specified through ontologies. The use of ontologies in real-world applications such as community portals has shown that a new level of data independence is required for ontology-based applications. For example, the customization of information towards the needs of specific user communities is often need. This paper extends previous work [22, 21] on this issue and presents a view language for the fundamental data models of the Semantic Web, viz. RDF and RDFS, and how it can be implemented. The basic novelty of the view language is the semantically appropriate classification of views into inheritance taxonomies based on query semantics. Additionally, the underlying distinction between unary predicates (classes) and binary predicates (properties) taken in RDF/S is maintained in the view language. So-called external ontologies allow the integration of multiple source databases, offer control over the publishing of data and enable the generation of views spanning across databases.

Fragments of a method to formally specify object-oriented models of a universe of discourse are presented. The task of finding such models is divided into three subtasks, object classification, event specification, and the specification of the life cycle of an object. Each of these subtasks is further subdivided, and for each of the subtasks heuristics are given that can aid the analyst in deciding how to represent a particular aspect of the real world. The main sources of inspiration are Jackson System Development, algebraic specification of data- and object types, and algebraic specification of processes. Keywords: Formal specification, Object-oriented modeling methods. 1. Introduction 1.1. The problem Conceptual modeling is the process of finding an explicit model of a universe of discourse (UoD). Other terms used for "conceptual modeling" are "information analysis" and "business area analysis". The result of this is a conceptual model (CM) of the current or of desired ve...

We present a general approach for modelling temporal aspects of objects in a logic programming framework. Change is formulated in the context of a database which stores explicitly a record of all changes that have occurred to objects and thus (implicitly) all states of objects in the database. A snapshot of the database at any given time is an object-oriented database, in the sense that it supports an object-- based data model. An object is viewed as a collection of simple atomic formulas, with support for an explicit notion of object identity, classes and inheritance. The event calculus is a treatment of time and change in first-order classical logic augmented with negation as failure. The paper develops a variant of the event calculus for representing changes to objects, including change in internal state of objects, creation and deletion of objects, and mutation of objects over time. Implementation strategies for practical application to temporal object-oriented databases...