Theory and Practice of Transactional Method Caching

Nowadays, tiered architectures are widely accepted for constructing large scale information systems. In this context application servers often form the bottleneck for a system's efficiency. An application server exposes an object oriented interface consisting of set of methods which are accessed by potentially remote clients. The idea of method caching is to store results of read-only method invocations with respect to the application server's interface on the client side. If the client invokes the same method with the same arguments again, the corresponding result can be taken from the cache without contacting the server. It has been shown that this approach can considerably improve a real world system's efficiency. This paper extends the concept of method caching by addressing the case where clients wrap related method invocations in ACID transactions. Demarcating sequences of method calls in this way is supported by many important application server standards. In this context the paper presents an architecture, a theory and an efficient protocol for maintaining full transactional consistency and in particular serializability when using a method cache on the client side. In order to create a protocol for scheduling cached method results, the paper extends a classical transaction formalism. Based on this extension, a recovery protocol and an optimistic serializability protocol are derived. The latter one differs from traditional transactional cache protocols in many essential ways. An efficiency experiment validates the approach: Using the cache a system's performance and scalability are considerably improved

Temporized and localized rule sets

Constraint management plays an important role in design applications where constraints reect design restrictions and design decisions. ECA rules are a widely used mechanism to enforce constraints. The paper argues that such rules must be augmented for design environments by a spatial and a temporal dimension of validity, resulting in so-called area-event-condition-action (AECA) rules. The spatial dimension allows to restrict constraints locally in the design space, and to control interaction between designers. The temporal dimension permits designers to retract their designs to earlier stages. The paper introduces the concept of AECA rules, motivates them by examples from building design, discusses rule management, and then introduces two important issues, conflict detection during collaboration, and backtracking during design revision

Bereichsdynamische Konsistenzüberwachung

Eine Datenbankunterstützung des integrierten architektonischen Entwurfes stellt mannigfaltige Anforderungen, die von heutigen Datenbanksystemen nicht erfüllt werden können. Insbesondere die Überwachung von Constraints stellt hier eine große Herausforderung dar. Dieser Bericht hat zum Ziel, zunächst die Anforderungen an einen Mechanismus zur Überwachung der Constraints auszuarbeiten und anschließend ein Konzept vorzustellen, das in der Lage ist diese Anforderungen zu erfüllen

Object-oriented protocol hierarchies for distributed workflow systems

Distributed software systems such as groupware and workflow systems will play a key role in the near future. While numerous models which promise highly sophisticated functionality are proposed in the literature their implementation is still a difficult and very expensive task. Therefore existing systems fall far behind their promises. Entities of the workflow level are often autonomous. Consequently, they are related to each other in more than a fixed client/server configuration: they often perform their activities in collaboration. Workflow models also contain a lot of information about the system\u27s dynamics. If one uses objects as an implementation mode

Multi-object cooperation in distributed object bases

It is an emerging trend to build large information systems in a component-based fashion where the components follow the concept of object. Applications are constructed by organizing pre-built objects such that they cooperate with each other to perform some task. However, considerable programming effort is required to express multi-object constraints in terms of the traditional message-passing mechanism. This observation lead many authors to suggest communication abstractions in object models. One promising approach is to separate multi-object constraints from the objects and collect them into a separate construct. We call this construct an alliance. Unlike other approaches we allow alliances to involve large sets of long-lived objects which may dynamically vary during the - also potentially long - life-time of the alliance. Alliances are not only visible at the specification level but are also computational entities which enforce multi-object constraints at run-time. They do so in an unreliable world, i.e., we do not assume that objects will always meet their obligations in a cooperation. Since objects may often be distributed across a network, we demonstrate that alliances are an ideal place to deal with aspects of distribution in an application-specific manner. We illustrate our thesis by one of the key questions of distributed object management: where shall objects be located and when shall they migrate to which node? We show that alliances allow for customized distribution policies which are neither "hardwired" into the objects nor necessitate a centralized distribution control

Modulare Aufbereitung von multimedialen Lerninhalten für eine heterogene Lernumgebung

Der Einsatz multimedialer Lerninhalte ist durch die vielfältig vorhandenen und sich ständig verbessernden technischen Möglichkeiten ein aktuell weit verbreitetes Thema. Ihre Erstellung gestaltet sich allerdings äußerst aufwendig. Daher beschäftigen wir uns mit der kooperativen Entwicklung von multimedialen Lehr-/Lerninhalten, die von verscheidenen Autoren in unterschiedlichen Kontexten für verschiedene Zielgruppen eingesetzt werden können. Unser Ansatz basiert auf dem Modularisierungskonzept zur Unterstützung der Wiederverwendbarkeit von Lernmaterialien. Um solche "universellen" Lernmodule zu entwickeln, die sowohl die Bedürfnisse verschiedener Zielgruppen als auch verschiedener Autoren erfüllen, werden mehrere Modularisierungsebenen eingeführt und die Erzeugung verschiedener Modulsichten ermöglicht