Transactions on replicated data in Mobile Ad Hoc Nets - and some further issues of large scalable networks

In the Coco/Da project1 data centric middleware functions for Mobile Ad Hoc Nets have been investigated and developed. Transactional support2 and data replication are important functions of traditional data management systems. Using a volatile communication infrastructure, replication plays an even more important role than in fixed wired systems in order to increase availability of data. Keeping replica consistent, however, is more involved with unreliable communication

Storing RDF as a Graph

RDF is the first W3C standard for enriching information resources of the Web with detailed meta data. The semantics of RDF data is defined using a RDF schema. The most expressive language for querying RDF is RQL, which enables querying of semantics. In order to support RQL, a RDF storage system has to map the RDF graph model onto its storage structure. Several storage systems for RDF data have been developed, which store the RDF data as triples in a relational database. To evaluate an RQL query on those triple structures, the graph model has to be rebuilt from the triples. In this paper, we presented a new approach to store RDF data as a graph in a object-oriented database. Our approach avoids the costly rebuilding of the graph and efficiently queries the storage structure directly. The advantages of our approach have been shown by performance test on our prototype implementation OO-Store

Flexible workflows to support transactional service composition in mobile environments

Service oriented computing provides suitable means to technically support distributed collaboration of heterogeneous devices, for example those present in mobile environments. E.g., many applications are built on composite Web- Services. However, when executing these applications in dynamic environments, failures of participating entities have to be optimistically coped with, in order to avoid inconsistent system states and thereby provide suitable correctness guarantees. Transactional coordination for services so far lacks the possibility to adapt failure handling to the current execution context, e.g. dynamically bound services at runtime. In this paper, we employ transactional service properties to ensure reliable, i.e., correct execution of workflows by still respecting the autonomy of participants. We propose algorithms to verifiy and alter the structure of the composition at runtime, thus adapting the control flow to the current execution context to ensure correct execution

On Plasma Oscillations in Strong Electric Fields

We describe the creation and evolution of electron-positron pairs in a strong electric field as well as the pairs annihilation into photons. The formalism is based on generalized Vlasov equations, which are numerically integrated. We recover previous results about the oscillations of the charges, discuss the electric field screening and the relaxation of the system to a thermal equilibrium configuration. The timescale of the thermalization is estimated to be $\sim 10^{3}-10^{4} \hbar /m_{e}c^{2}$.Comment: 10 pages, 2 figures, to appear on Phys. Lett. B, fixed a typo mistake in the reference lis

Abstraction and Decomposition in Open GIS

ion and Decomposition in Open GIS Agn`es Voisard Heinz Schweppe y TR-97-006 March 1997 Abstract With the emergence of distributed computing and the increasing trend towards the reuse of geographic data, a new generation of geographic information systems (GIS) is currently being specified. The key characteristics of these open GIS are modularity and extensibility, and they are composed of existing software systems such as database management systems, traditional GIS, statistics packages and simulation models. They can be defined in terms of generic frameworks which facilitate both information exchange between participating systems and the addition of new functionalities. Even though the idea of defining open GISs is not new, it is crucial that the steps necessary to design such a complex system be clearly decomposed. In this paper, we propose a layer decomposition for the design of an open GIS. Each layer corresponds to a different level of abstraction, starting with the applicatio..

Database systems as middleware- event, notifications, messages

Database systems have been traditionally used as integration tools. Data integration was the primary goal of first generation systems. After relational technology had become a mature base for application development, new architectures where developed for the tight integration of data, procedures and all kinds of processing. This phase of DBS research and development was dominated by object-oriented database management and specific architectures like Starburst [HaCH 1990], which had a strong impact on current object-relational technology. The ubiquitous computer network has added another facet to the usage of DBS as an integration tool. In distributed environment, middleware aims at making distribution transparent. Corba or RMI are well-known examples. The call-oriented style of communication has been complemented by message-oriented, event-driven interaction of independent programs. System processes use this type of communication for decades. However, it is not well known as a mechanism on the application level - despite the fact that it has been employed for quite some time, e.g. in workflow systems [LeRo 2000]. The event-driven message passing paradigm becomes more and more important for highly distributed applications. Many kinds of interactions between applications follow a common pattern: n inde-pendent providers submit their output as messages, which in turn will be consumed asynchronously by m consumer applications. As opposed to call-level interaction, providers and consumers may or may not know each others identity. In a stock ticker application for example, there is no reason why providers should know the identity of consumers