Georg Lausen

Modeling and analysis of the behavior of information systems

Many widely used specification techniques for information systems are based on a hierarchy of information flow diagrams. A method is introduced which preserves the benefits of these techniques and adds the precision of the Petri net formalism. Information-flow diagram hierarchies are formalized by notions of net theory. The bottom-level nets of a hierarchy are treated as Petri nets. The behavior model of the information system is the Petri net derived by repeatedly replacing each part of a net by its associated refinement. As a prerequisite for such replacements, the data abstractions relation information flows of different level are specified by a semantic hierarchy data model. The nets in the hierarchy are appended by dynamic counterparts of the abstractions so that a consistent replacement becomes possible. The interface behavior of the nets in the hierarchy is analyzed, using the concept of behavior constraints as a formal measure of correct interface behavior. The behavior model can be derived in an iterative bottom-up way by first analyzing a net for fulfillment of its associated behavior constraint and afterward integrating it into the next-higher-level net. >

Formal aspects of optimistic concurrency control in a multiple version database system

Abstract Formal aspects of optimistic concurrency control in a multiple version database system are discussed. It is distinguished between element-optimistic and set-optimistic methods. In an element-optimistic method the unit of scheduling for the concurrency control is one action/transaction, while in a set-optimistic method a set of transactions is scheduled. The underlying decision problems of serializability are defined and shown to be NP-complete in a model, which is typical for most modern transaction oriented database management systems. Therefore it is most probable, that both optimistic concurrency control types cannot be implemented efficiently in the general case.

Integrated concurrency control in shared B-trees

The traditional approach to concurrency control in sharedB-trees is based on locking. Recently new methods have been proposed called optimistic methods. In contrast to locking these methods achieve correct operations on theB-tree by a restart mechanism. In this paper we present a new approach to concurrency control, which integrates locking and the optimistic method. Practical applications are pointed out in which this approach can be expected to be superior to either locking or the optimistic method.ZusammenfassungDer traditionelle Ansatz für eine Concurrency Control innerhalbB-Bäumen basiert auf Sperren. In der letzten Zeit wurden sogenannte optimistische Verfahren vorgeschlagen. Im Gegensatz zu Sperren erreichen diese Verfahren korrekte Operationen auf denB-Bäumchen durch einen Restart-Mechanismus. In dieser Arbeit wird ein neuer Ansatz für eine Concurrency Control vorgeschlagen, der Sperren und die optimistischen Verfahren integriert. Es werden praktische Anwendungen aufgezeigt, in denen erwartet werden kann, daß dieser Ansatz dem starren Ansatz entweder Sperren oder optimistisches Verfahren überlegen ist.

Serializability Problems of Interleaved Database Transactions

The interleaved execution of database transactions produces correctness problems. It is called correct — or serializable —, if it is equivalent to a serial execution of the same transactions. An execution of a set of transactions is described by the sequence of the read/write actions — called schedule —, a reads-from rolation ϕ and an overwrite relation ω.

Maximal concurrency by locking

The purpose of a database concurrency control mechanism is to control a transaction system in such a way that only serializable executions of transactions are possible; that is, safety is enforced. Locking is an appropriate means to achieve safety. In this paper the following problem is considered:Can the set of all serializable schedules (in the sense of conflict-preserving serializability) of a transaction system be defined by locking, i.e., can maximal concurrency be achieved by locking?An efficient algorithm exists for the problem in the case of two-transaction systems, but in general the problem is NP-complete. In the case in which maximal concurrency indeed is achievable by locking a locking policy realizing maximal concurrency can be constructed efficiently. Finally, an easy-to-test sufficient condition under which maximal concurrency is achieved by locking is discussed.

Optimistic concurrency control and recovery in a multi personal computer system

In this paper we present an attractive concurrency control and recovery method to manage the transactions of a multi user Personal Computer system on a shared data base in a star constellation.

Formale Beschreibung von Anforderungen: Eine netzorientierte Vorgehensweise zur konzeptuellen Modellierung von Informationssystemen

Das formale Beschreiben von Anforderungen fur Softwaresysteme, die unter Verwendung von Datenbanksystemen realisiert werden, bezeichnet man als konzeptuelle Modellierung. Es werden die Grundlagen einer Methode fur die konzeptuelle Modellierung eingefuhrt. Als Modellierungsaspekte werden die Verhaltens-, Datenstruktur- und Transaktionsmodeliierung betrachtet. Die Besonderheit der Methode liegt in der konsequenten Verwendung von Konzepten der Netztheorie. Ein rechnergestutzter Arbeitsplatz zur konzeptuellen Modellierung und Softwareentwicklung wird skizziert.

Datenbank-Pascal an extendable database management system for personal computer networks

Datenbank-Pascal is a database management system (dbms) for personal computers which supports a three level architecture. On different levels of extension it is running on different scaled personal computers (8/16/32 bit). The dbms programming interface is integrated into the Pascal programming language. A multi-user dbms environment can be achieved by connecting several personal computers to a disk-system which contains the shared database. First, the architecture of the dbms is described. Then, the data models on the conceptual and external level and the concepts for the Pascal language integration are explained. Finally, the paper contains a high-level description of the transaction management in the disk-based personal computer network. This work was partially supported by the Deutsche Forschungsgemeinschaft, Project-No. Stu 98/2.

Designing the Conceptual Database Schema - A Framework for an Interactive Approach

A framework for interactive conceptual database design methods is proposed, which captures top-down and bottom-up approaches, as well. Several constraints are defined, which are to be fulfilled by the final conceptual schema. Some of them can be completely or partially tested by an automatic design tool. Further we show, how inconsistencies as homonyms, redundancies and synonyms can be automatically tested. Finally we sketch by examples, how bottom-up and top-down approaches can be used within our framework.