James P. Black

A framework for distributed debugging

The authors provide a general picture of current research in distributed debugging. Rather than an exhaustive survey of the area, they present a view of the issues and solutions based on a proposed framework for distributed debugging systems. They concentrate on runtime debugging. However, they stress that static debugging and runtime debugging complement each other and that neither should be overlooked.<<ETX>>

WAP traffic: description and comparison to WWW traffic

The characteristics of the data traffic generated by the use of micro-browser-enabled PCS phones to gain access to the Web is of particular interest to cellular network operators. Questions such as the frequency and length of browser sessions, and the specific characteristic of the traffic generated, need to be answer by researchers. These answers are valuable in network capacity planning as more subscribers use their cellular phones to interact with the Web. In this paper, we analyse network traces generated by a Mobile Browser application. We observe daily and weekly cycles and, some evidence of self-similarity in the network traffic produced by the application. We show that the activity factor for data generated by this application is lower than voice activity factor. This fact is significant for the design of wireless networks because it is possible to multiplex a large number of data sources onto existing channels with the appropriate protocols. We also compare and contrast the Mobile Browser traffic characteristics with results for WWW traffic published in the literature.

Poet: Target-System Independent Visualizations of Complex Distributed-Application Executions

Designing and implementing a visual debugger for distributed programs is a significant challenge. Distributed applications are often large and frequently exhibit a high degree of complexity. Consequently, a debugger must address problems of complexity and scale in at least two ways. First, appropriate user interfaces should allow a user to manage the vast amount of information typically obtained from distributed executions. Second, the tool itself, in handling this information, should be implemented efficiently, providing a user with reasonable response times for interactive use. Our research efforts, concentrating on these problems, have led to the development of Poet, a tool for the collection and presentation of event-based traces of distributed executions. Poet makes as few assumptions as possible about characteristics that must be possessed by all target environments. Information describing each target environment is placed in configuration files, allowing a single set of Poet executables to be used for all target environments. Comparing Poets performance to XPVM, the standard visualization tool for PVM executions, reveals that this target-system independence does not impose a performance penalty.

Using automatic process clustering for design recovery and distributed debugging

Distributed applications written in Hermes typically consist of a large number of sequential processes. The use of a hierarchy of process clusters can facilitate the debugging of such applications. Ideally, such a hierarchy should be derived automatically. This paper discusses two approaches to automatic process clustering, one analyzing runtime information with a statistical approach and one utilizing additional semantic information. Tools realizing these approaches were developed and a quantitative measure to evaluate process clusters is proposed. The results obtained under both approaches are compared, and indicate that the additional semantic information improves the cluster hierarchies derived. We demonstrate the value of automatic process clustering with an example. It is shown how appropriate process clusters reduce the complexity of the understanding process, facilitating program maintenance activities such as debugging. >

Vector time and causality among abstract events in distributed computations

Abstract. An important problem in analyzing distributed computations is the amount of information. In event-based models, even for simple applications, the number of events is large and the causal structure is complex. Event abstraction can be used to reduce the apparent complexity of a distributed computation. This paper discusses one important aspect of event abstraction: causality among abstract events. Following Lamport [24], two causality relations are defined on abstract events, called weak and strong precedence. A general theoretical framework based on logical vector time is developed in which several meaningful timestamps for abstract events are derived. These timestamps can be used to efficiently determine causal relationships between arbitrary abstract events. The class of convex abstract events is identified as a subclass of abstract events that is general enough to be widely applicable and restricted enough to simplify timestamping schemes used for characterizing weak precedence. We explain why such a simplification seems not possible for strong precedence.

Discovering Services Is Not Enough

Faced with multiple electronic devices, service providers, and ISPs, tomorrows mobile users might find it challenging to identify available services and acquire the rights to use them. The authors tackle this problem, combining the acquisition of role-based credentials with servicediscovery.

Maintaining quality of service for adaptive mobile map clients

In this paper we present the implementation of an adaptive mobile map client. We used a client-proxy-server model that enables the map client to activate and deactivate a set of filters managed by the proxy. We developed three map generalization filters each of which reduces the amount of data received by the map client using a different technique. A set of experiments and their results are discussed along with some heuristics that we believe could help optimize the use of the system.

Adaptive groupware for wireless networks

Current groupware systems do not work well over wireless networks, as a wireless connection is of variable and often poor quality. Our research suggests a number of changes that allow groupware applications to make efficient use of a wireless channel. One fundamental change is providing adaptability, which allows applications to take advantage of favourable network conditions and to react to difficult conditions as they occur. Our experiments with a multi user text editor revealed several strategies for adaptive groupware. Applying these strategies demonstrated a bandwidth reduction in one scenario of over 90%.

A case study in fault tolerant software

The addition of redundancy to data structures can be used to improve the ability of a software system to detect and correct errors, and to continue to operate according to its specifications. A case study is presented which indicates how such redundancy can be deployed and exploited at reasonable cost to improve software fault tolerance. Experimental results are reported for the small data base system considered.

Data-centric support of a smart walker in a ubiquitous-computing environment

As our population ages, many researchers have investigated how assistive devices may be used to prolong independent living, thus reducing health-care costs. Some have suggested augmenting walkers (also called rollators) with sensors and actuators to assist elderly users. Such systems may also collect usage behavior and make it available to medical professionals for diagnosis or additional research. Ubiquitous computing provides an opportunity to embed computing and networking technologies in our everyday lives. It is natural to incorporate smart walkers into such environments. Some uses of a walker, such as environment customization and cognitive monitoring, also depend on a smart environment. Existing intelligent walkers usually aim at facilitating user mobility and focus on control. We propose that to support a smart walker in a ubiquitous-computing environment, the system should also focus on data, i.e., it should facilitate data collection, aggregation, and dissemination between the walker and the system. This paper describes a smart walker, and discusses how to incorporate it into a ubiquitous-computing environment using a data-centric approach with Jabber technologies. It also argues that Jabber-based middleware is suitable for general ubiquitous computing. Some applications show how the walker may be used with the middleware support.