Hans-Jörg Schek

The relational model with relation-valued attributes

Abstract Motivated by new applications of database systems we introduce relations which may have relation-valued attributes and propose a related algebra. Formal definitions for this extended relational model can be given by applying usual notions recursively. The main problem considered in this paper is the formal definition of an appropriate relational algebra for these non-first-normal-form relations. We allow the application of the basic relational operators to any relation-valued attribute within a relation. This leads to a (hierarchically) nested relational algebra.

Remarks on the algebra of non first normal form relations

Usually, the first normal form condition of the relational model of data is imposed. Presently, a broader class of data base applications like office information systems is considered where this restriction is not convenient. Therefore, an extension of the relational model is proposed consisting of Non First Normal Form (NF2) relations. The relational algebra is enriched mainly by so called nest and unnest operations which transform between NF2 relations and the usual ones. We state some properties of these operations and some rules which occur in combination with the operations of the usual relational algebra. Since we propose to use the NF2 model also for the internal data model these rules are important not only for theoretical reasons but also for a practical implementation.

Atomicity and isolation for transactional processes

Processes are increasingly being used to make complex application logic explicit. Programming using processes has significant advantages but it poses a difficult problem from the system point of view in that the interactions between processes cannot be controlled using conventional techniques. In terms of recovery, the steps of a process are different from operations within a transaction. Each one has its own termination semantics and there are dependencies among the different steps. Regarding concurrency control, the flow of control of a process is more complex than in a flat transaction. A process may, for example, partially roll back its execution or may follow one of several alternatives. In this article, we deal with the problem of atomicity and isolation in the context of processes. We propose a unified model for concurrency control and recovery for processes and show how this model can be implemented in practice, thereby providing a complete framework for developing middleware applications using processes.

Architecture and implementation of the Darmstadt database kernel system

The multi-layered architecture of the DArmStadt Data Base System (DASDBS) for advanced applications is introduced DASDBS is conceived as a family of application-specific database systems on top of a common database kernel system. The main design problem considered here is, What features are common enough to be integrated into the kernel and what features are rather application-specific? Kernel features must be simple enough to be efficiently implemented and to serve a broad class of clients, yet powerful enough to form a convenient basis for application-oriented layers. Our kernel provides mechanisms to efficiently store hierarchically structured complex objects, and offers operations which are set-oriented and can be processed in a single scan through the objects. To achieve high concurrency in a layered system, a multi-level transaction methodology is applied. First experiences with our current implementation and some lessons we have learned from it are reported.

Merging application-centric and data-centric approaches to support transaction-oriented multi-system workflows

Workflow management is primarily concerned with dependencies between the tasks of a workflow, to ensure correct control flow and data flow. Transaction management, on the other hand, is concerned with preserving data dependencies by preventing execution of conflicting operations from multiple, concurrently executing tasks or transactions. In this paper we argue that many applications will be served better if the properties of transaction and workflow models are supported by an integrated architecture. We also present preliminary ideas towards such an architecture.

A relational object model

The relational model and its extensions are often considered incompatible with object-orientation. However, on the one hand nested relations provide the complex object features demanded by object models. Particularly, powerful query languages exploit the complex data structure while keeping the advantages of the declarative, set-oriented paradigm. On the other hand, object models provide semantically rich constructs for advanced modeling, and abstractions of operations as well as data. In this paper, we show an evolutionary path from relational, essentially nested relational, to object-oriented data models and query languages. Basically, allowing nested relation schemes to be recursively defined yields the necessary w.r.t. structure. The query language, i.e., nested relational algebra, carries over to this “network” model. As a first step towards the object-oriented integration of cooperative systems, different views onto the objects have to be supported. We present a powerful view definition facility that basically allows object views as well as relational views to be defined in our object algebra.

Nested relations and complex objects in databases

The advanced information management prototype.- Verso: A database machine based on nested relations.- The two roles of nested relations in the DASDBS project.- A storage structure for Nested Relational Databases.- Four views of complex objects: A sophisticates introduction.- An introduction to the completeness of languages for complex objects and nested relations.- On the uniqueness of nested relations.- An introduction to the Nested Sequences of Tuples data model and algebra.- Recursively defined complex objects.- Query languages for Nested Relational Databases.- Nested relations and recursive queries.- Realization of nested relation interfaces for relational and network databases.- An approach to manage large inheritance networks within a DBS supporting nested relations.- On the normalization in Nested Relational Databases.- Complex objects modeling: An entity-relationship approach.- A data model for complex objects based on a semantic database model and nested relations.- ?-Acyclic database schemes and nested relations.

Scalable peer-to-peer process management - the OSIRIS approach

The functionality of applications is increasingly being made available by services. General concepts and standards like SOAP, WSDL, and UDDI support the discovery and invocation of single Web services. State-of-the-art process management is conceptually based on a centralized process manager. The resources of this coordinator limit the number of concurrent process executions, especially since the coordinator has to persistently store each state change for recovery purposes. In this paper, we overcome this limitation by executing processes in a peer-to-peer way exploiting all nodes of the system. By distributing the execution and navigation costs, we can achieve a higher degree of scalability allowing for a much larger throughput of processes compared to centralized solutions. This paper describes our prototype system OSIRIS, which implements such a true peer-to-peer process execution. We further present very promising results verifying the advantages over centralized process management in terms of scalability.

Multi-Level Transaction Management, Theoretical Art or Practical Need ?

A useful approach to the design and description of complex data management systems is the decomposition of a system into a hierarchically organized collection of levels. In such a system, transaction management is distributed among the levels. This paper presents the fundamental theory of multi-level concurrency control and recovery. A model for the computation of multi-level transactions is introduced by generalizing from the well known single-level theory. Three basic principles, called commutation, reduction, and abstraction are explained. Using them enables one to explain and prove seemingly ”tricky” implementation techniques as correct, by regarding them as multi-level algorithms. We show how the theory helps to understand and explain in a systematic framework techniques that are in use in todays DBMSs. We also discuss how and why multi-level algorithms may achieve better performance than single-level ones.

Digital library information-technology infrastructures

This paper charts a research agenda on systems-oriented issues in digital libraries. It focuses on the most central and generic system issues, including system architecture, user-level functionality, and the overall operational environment. With respect to user-level functionality, in particular, it abstracts the overall information lifecycle in digital libraries to five major stages and identifies key research problems that require solution in each stage. Finally, it recommends an explicit set of activities that would help achieve the research goals outlined and identifies several dimensions along which progress of the digital library field can be evaluated.