Synchronous,asynchronous, and causally ordered communication

Summary.  This article studies characteristic properties of synchronous and asynchronous message communications in distributed systems. Based on the causality relation between events in computations with asynchronous communications, we characterize computations which are realizable with synchronous communications, which respect causal order, or where messages between two processes are always received in the order sent. It is shown that the corresponding computation classes form a strict hierarchy. Furthermore, an axiomatic definition of distributed computations with synchronous communications is given, and it is shown that several informal characterizations of such computations are equivalent when they are formalized appropriately. As an application, we use our results to show that the distributed termination detection algorithm by Dijkstra et al. is correct under a weaker synchrony assumption than originally stated. Received: November 1992/Accepted: July 1995     

Detecting causal relationships in distributed computations: In search of the holy grail

Summary The paper shows that characterizing the causal relationship between significant events is an important but non-trivial aspect for understanding the behavior of distributed programs. An introduction to the notion of causality and its relation to logical time is given; some fundamental results concerning the characterization of causality are presented. Recent work on the detection of causal relationships in distributed computations is surveyed. The issue of observing distributed computations in a causally consistent way and the basic problems of detecting global predicates are discussed. To illustrate the major difficulties, some typical monitoring and debugging approaches are assessed, and it is demonstrated how their feasibility is severely limited by the fundamental problem to master the complexity of causal relationships. Reinhard Schwarz received a diploma in computer science from the University of Kaiserslautern, Germany, in 1990. Since then, he is working as a research assistant at the computer science department. His research interests include debugging and monitoring of distributed systems, runtime support for object-oriented distributed programming, and distributed algorithms. Friedemann Mattern received the diploma in computer science from Bonn University, Germany, and the Ph.D. degree from the University of Kaiserslautern, Germany, in 1983 and 1989, respectively. Since 1991 he is a professor of computer science at the University of Saarland in Saarbrücken, Germany. His current research interests include programming of distributed systems, distributed applications, and distributed algorithms.The work presented in this paper was carried out as part of the PARAWAN project supported by the Bundesministerium für Forschung und Technologie (BMFT)     

Dynamic shader generation for GPU-based multi-volume ray casting

Real-time performance for rendering multiple intersecting volumetric objects requires the speed and flexibility of modern GPUs. This requirement has restricted programming of the necessary shaders to GPU experts only. A visualization system that dynamically generates GPU shaders for multi-volume ray casting from a user-definable abstract render graph overcomes this limitation.     

The sensor internet at work: Locating everyday items using mobile phones

We present a system for monitoring and locating everyday items using mobile phones. The system is based on phones which are enhanced with the capability to detect electronically tagged objects in their vicinity. It supports various functionalities: On the one hand, phones can store the context in which users leave registered items and thus help to locate them later on. On the other hand, object owners can search for their objects using the infrastructure of mobile phones carried by other users. We describe the design of our object location system and provide an algorithm which can be used to search for lost or misplaced items efficiently by selecting the most suitable sensors based on arbitrary domain knowledge. Furthermore, we demonstrate the practicability of such wide-area searching by means of user-held sensors in a series of simulations complemented by a real-world experiment.     

Process simulation and techno-economic assessment of SER steam gasification for hydrogen production

In the SER (sorption enhanced reforming) gasification process a nitrogen-free, high calorific product gas can be produced. In addition, due to low gasification temperatures of 600–750 °C and the use of limestone as bed material, in-situ CO₂ capture is possible, leading to a hydrogen-rich and carbon-lean product gas. In this paper, results from a bubbling fluidised bed gasification model are compared to results of process demonstration tests in a 200 kW_th pilot plant.Based upon that, a concept for the hydrogen production via biomass SER gasification is studied in terms of efficiency and feasibility. Capital and operational expenditures as well as hydrogen production costs are calculated in a techno-economic assessment study. Furthermore, market framework conditions are discussed under which an economic hydrogen production via SER gasification is possible.     

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Low cost chemical sensor device for supersensitive pentaerythritol tetranitrate (PETN) explosives detection based on titanium dioxide nanotubes

Within the last decade there has been a great increase in the need of trace and ultra-trace explosive detection. In this report, we demonstrate a new and versatile type of chemical explosive sensors based on metal oxide nanotubes easily made, even with the need of a low budget. We describe the step-by-step procedure to fabricate a sensing chip device, beginning with the synthesis of the starting materials to the point of supersensitive measurements of PETN explosive. As a result, the whole process actually is one of the most cost-effective methods to produce explosive sensing devices reported until now. The achieved chemical sensor device will be able to detect PETN explosive down to ∼112 ppt.