Steven Bradley

Using Timed Automata for Response Time Analysis of Distributed Real-Time Systems

Abstract Rate Monotonic Analysis (RMA) is a well-established technique for assessing schedulability of periodic and sporadic tasks which share a processor resource using fixed priority scheduling. Adaptations of this technique have been made to perform Response Time Analysis (RTA), accounting for jitter, blocking, distributed systems and end-to end timing constraints. However, the nature of the analysis means that, while good bounds can be given for uni-processor systems with relatively little interdependency, the response times calculated for more complex systems can be very conservative. An alternative approach to analysing such systems is to build a model which represents the behaviour of the system more dynamically, taking into account the dependency between the tasks. To do this, we introduce a simple language for describing the tasks which comprise a system and the precedence relationships between them. From this a timed hybrid automaton is generated which can be analysed automatically to predict end-to-end response times. Applying this technique in practice yields promising results, with response times lower than those calculated with RTA. However, there is a trade-off to be made between thecomplexity of the hybrid automaton analysis (which suffers from the state explosion problem) and the conservatism of the more standard RTA approach.

Studying the process of software change: an analysis of software evolution

The paper describes the analysis and results of studies in software evolution. Four major applications have been studied, three of which are from the commercial sector. The study concentrates on analysing changes in data complexity, as a direct result of the maintenance processes through comparing versions of the same application. Reverse engineering techniques are employed as a means of analysing the changes to the software. The evolution of each application is then analysed and its evolutionary trends are scrutinised. The attributes of specific types of changes are then discussed, as are the results of the overall trend for each application. In addition, the maintenance policies employed for each of the applications are reviewed. Finally, based on the results of the evolution study, some recommendations are made with regard to good maintenance policy.

Integrating AORTA with model-based data specification languages

AORTA has been proposed as an implementable real-time algebra for concurrent systems where event times, rather than values of data, are critical. In this paper we discuss an extension to AORTA to include a formal data model, allowing integration with a variety of modelbased data specification languages. An example is given using VDM with AORTA to define a time-critical system with important data attributes, and supporting software tools for AORTA and a simple imperative language are described.

A formal design language for real-time systems with data

AORTA has been proposed as an implementable real-time language for concurrent systems where event times, rather than values of data, are critical. In this paper we describe how to use AORTA with a formal data model, allowing integration with a variety of model-based data specification languages. Example definitions are given of time-critical systems with important data attributes. A development technique and supporting software tools for AORTA are also described.

Software evolution in an interdisciplinary environment

At Durham, an interdisciplinary team has implemented the UKs first Web-centred database of adult mental health service provision (we regard interdisciplinary research to be taking place when two or more groups from different disciplines work together to solve a common problem). Like most applications, this is subject to evolution: the health service structure is changing rapidly, while feedback from domain users provides a constant source of improvements. One potential advantage of the database is that it offers potential to track and report on changes in service provision, although this must be implemented in the face of the substantial evolution. The main contribution of this paper is to reflect on the combination of domain drivers and evolution in software; we propose that a Web services solution will meet the needs of the former while minimizing the effort to implement the latter.