Gottfried Vossen

The World Wide Web and Databases

The World Wide Web promises to transform human society by making virtually all types of information instantly available everywhere. Two prerequisites for this promise to be realized are a universal markup language and a universal query language. The power and flexibility of XML make it the leading candidate for a universal markup language. XML provides a way to label information from diverse data sources including structured and semi-structured documents, relational databases, and object repositories. Several XML-based query languages have been proposed, each oriented toward a specific category of information. Quilt is a new proposal that attempts to unify concepts from several of these query languages, resulting in a new language that exploits the full versatility of XML. The name Quilt suggests both the way in which features from several languages were assembled to make a new query language, and the way in which Quilt queries can combine information from diverse data sources into a query result with a new structure of its own.

Multidimensional normal forms for data warehouse design

A data warehouse is an integrated and time-varying collection of data derived from operational data and primarily used in strategic decision making by means of OLAP techniques. Although it is generally agreed that warehouse design is a non-trivial problem and that multidimensional data models as well as star or snowflake schemata are relevant in this context, there exist neither methods for deriving such a schema from an operational database nor measures for evaluating a warehouse schema. In this paper, a sequence of multidimensional normal forms is established that allow reasoning about the quality of conceptual data warehouse schemata in a rigorous manner. These normal forms address traditional database design objectives such as faithfulness, completeness, and freedom of redundancies as well as the notion of summarizability, which is specific to multidimensional database schemata.

A survey of database design transformations based on the Entity-Relationship model

Abstract At present, the Entity-Relationship (ER) model is the most important paradigm for conceptual database design. Since the model was introduced in the mid-seventies, a large body of literature has been published on transforming conceptual ER schemas or diagrams into logical data models. The purpose of this paper is to survey this literature. A first focus is on transformation approaches from the ER model to traditional data models, i.e. to the relational, the network, or the hierarchical model; this is then complemented by a discussion of more recent transformations to object-oriented models. A second focus is on considering the process of reverse engineering, i.e. transformations from a logical model back into the ER model; finally, an overview of direct transformations between various logical data models is presented.

Schema versioning in data warehouses: enabling cross-version querying via schema augmentation

As several mature implementations of data warehousing systems are fully operational, a crucial role in preserving their up-to-dateness is played by the ability to manage the changes that the data warehouse (DW) schema undergoes over time in response to evolving business requirements. In this paper we propose an approach to schema versioning in DWs, where the designer may decide to undertake some actions on old data aimed at increasing the flexibility in formulating cross-version queries, i.e., queries spanning multiple schema versions. First, we introduce a representation of DW schemata as graphs of simple functional dependencies, and discuss its properties. Then, after defining an algebra of schema graph modification operations aimed at creating new schema versions, we discuss how augmented schemata can be introduced to increase flexibility in cross-version querying. Next, we show how a history of versions for DW schemata is managed and discuss the relationship between the temporal horizon spanned by a query and the schema on which it can consistently be formulated.

A Multi-Level Model for Web Service Composition

Web services are rapidly becoming popular as a vehiclefor the design, integration, composition, and deploymentof distributed and heterogeneous software. However,while industry standards for the description, composition,and orchestration of Web services have been under discussion(and development) for quite some time already, theirconceptual underpinnings are still in their infancy. Indeed,conceptual models for service specification are rare so far,as are investigations based on them. This paper presents amulti-level service composition model that perceives servicespecification as going through several levels of abstraction,from transactional operations to the end user.Importantly, the model allows for specification of desirablecomposition properties at all levels. Different ways ofachieving these properties as well as implications of themodel are addressed.

Schema Versioning in Data Warehouses

As several mature implementations of data warehousing systems are fully operational, a crucial role in preserving their up-to-dateness is played by the ability to manage the changes that the data warehouse (DW) schema undergoes over time in response to evolving business requirements. In this paper we propose an approach to schema versioning in DWs, where the designer may decide to undertake some actions on old data aimed at increasing the flexibility in formulating cross-version queries, i.e., queries spanning multiple schema versions. After introducing an algebra of DW schema operations, we define a history of versions for data warehouse schemata and discuss the relationship between the temporal horizon spanned by a query and the schema on which it can consistently be formulated.

Transforming Relational Database Schemas into Object-Oriented Schemas according to ODMG-93

Many database applications are currently confronted with the problem of migrating from relational to object-oriented systems. A central task in such a process is schema conversion, which so far has to be done in a way that specifically depends on the target system. Recently, the ODMG-93 proposal has established a framework in which the core aspects of an object-oriented schema can be made precise, so that it becomes possible to do schema conversions in a system-independent way, by using the ODMG model as target. This paper presents a methodology for transforming relational schemas into object-oriented ones according to ODMG-93, thereby rendering it possible to do reverse engineering computer-assisted, and to automate database migrations. Essentially, the methodology is a three-step process who first goal is to complete a given relational schema, i.e., to make the semantic information it carries as explicit as possible using a variety of data dependencies. A completed schema is then transformed into an ODMG schema in a straightforward way, basically by generating classes from relation schemas. However, the result will in general not yet be optimal from an object-oriented perspective; so the initial object-oriented schema is finally improved to better exploit the options available in the object-oriented paradigm.

The WASA2 object-oriented workflow management system

Workflow management has gained increasing attention recently as an important technology to improve information system development in dynamic and distributed organizations. To develop a workflow application, selected business processes of an organization are modelled, optimized and specified as workflow schemas, using workflow languages [2]. Workflow schemas are used by workflow management systems to control the execution of workflow instances, i.e., representations of real-world business processes [3]. The first generation of workflow management systems (WFMS) were developed mainly to model and control the execution of business processes with fairly static structures, to be executed in homogeneous environments. Recently, the need for enhanced flexibility of workflow modeling and execution and the integration of applications in heterogeneous environments emerged in the workflow context [I]. The WASA project aims at supporting flexible and distributed workflows in heterogeneous environments [4]. This paper briefly overviews the conceptual design and implementation of the object-oriented workflow management system WASAZ, and sketches the proposed demo. References to work that relates to ours or that we started from are given in the cited WASA papers.

E-learning as a Web service (extended abstract)

E-learning has been a topic of increasing interest in recent years. It is often perceived as a group effort, where content authors, instructional designers, multimedia technicians, teachers, trainers, database administrators, and people from various other areas of expertise come together in order to serve a community of learners. In a typical e-learning scenario, many of the activities can be perceived and modeled as processes and consequently be executed as workflows; there are even prototypical system developments that are experimenting with this approach. On the other hand, there are increasingly many activities, which aim at providing services of any kind on the Web; these can occur as business-to-business or as business-to-consumer services and are generally subsumed under the term Web services. In this paper we suggest to combine the areas of e-learning and Web services, by providing electronic learning offerings as (individual or collections of) Web services as well. We elaborate on this by showing how content providers and content consumers (i.e., learners) can communicate appropriately through a Web service platform with its common description, publication, and retrieval functionalities. Finally, we indicate how a corresponding system can be realized.

Marketplaces for data: an initial survey

Data is becoming more and more of a commodity, so that it is not surprising that data has reached the status of tradable goods. An increasing number of data providers is recognizing this and is consequently setting up platforms for selling, buying, or trading data. We identify several categories and dimensions of data marketplaces and data vendors and provide a snapshot of the situation as of Summer 2012.