A Representational Framework for Scenarios of System Use

Scenarios are becoming widely used in three areas of system development: software engineering, human–computer interaction (HCI), and organisational process design. There are many reasons to use scenarios during system design. The one usually advanced in support of the practice is to aid the processes of validating the developers’ understanding of the customers’ or users’ work practices, organisational goals and structures, and system requirements. All three areas identified above deal with these processes, and not surprisingly this has given rise to a profusion of scenario-based practices and representations. Yet there has been little analysis of why scenarios should be useful, let alone whether they are. Only by having such a framework for understanding what scenarios are, and what they are for, can we begin to evaluate different scenario approaches in specific development contexts. This paper is a contribution toward such a framework. We lay out a space of representational possibilities for scenarios and enumerate a set of values or criteria that are important for different uses of scenarios. We then summarise several salient representations drawn from the software engineering, HCI, and organisational process design communities to clarify how these representational choices contribute to or detract from the goals of the respective practices. Finally, we discuss how scenario representations from one area of design may be useful in others, and we discuss the relationship between these representations and other significant early-design and requirements engineering practices.     

Communicating the variability of a software-product family to customers

Variability is a central concept in software product family development. Variability empowers constructive reuse and facilitates the derivation of different, customer specific products from the product family. If many customer specific requirements can be realised by exploiting the product family variability, the reuse achieved is obviously high. If not, the reuse is low. It is thus important that the variability of the product family is adequately considered when eliciting requirements from the customer. In this paper we sketch the challenges for requirements engineering for product family applications. More precisely we elaborate on the need to communicate the variability of the product family to the customer. We differentiate between variability aspects which are essential for the customer and aspects which are more related to the technical realisation and need thus not be communicated to the customer. Motivated by the successful usage of use cases in single product development we propose use cases as communication medium for the product family variability. We discuss and illustrate which customer relevant variability aspects can be represented with use cases, and for which aspects use cases are not suitable. Moreover we propose extensions to use case diagrams to support an intuitive representation of customer relevant variability aspects. Received: 14 October 2002 / Accepted: 8 January 2003 Published online: 27 February 2003 This work was partially funded by the CAFé project “From Concept to Application in System Family Engineering”; Eureka Σ! 2023 Programme, ITEA Project ip00004 (BMBF, F?rderkennzeichen 01 IS 002 C) and the state Nord-Rhein-Westfalia. This paper is a significant extension of the paper “Modellierung der Variabilit?t einer Produktfamilie”, [15].     

Scenario Analysis in an Automated Tool for Requirements Engineering

 This paper presents an automated tool for scenario-driven requirements engineering where scenario analysis plays the central role. It is shown that a scenario can be described by three views of data flow, entity relationship and state transition models by slight extensions of classic data flow, entity relationship and state transition diagrams. The notions of consistency and completeness of a set of scenarios are formally defined in graph theory terminology and automatically checked by the tool. The tool supports automatic validation of requirements definitions by analysing the consistency between a set of scenarios and requirements models. It also supports automatic synthesis of requirements models from a set of scenarios. Its utility and usefulness are demonstrated by a non-trivial example in the paper. Case studies of the tools are also presented.     

Rationale-Based Use Case Specification

The requirements specification – as outcome of the requirements engineering process – falls short of capturing other useful information generated during this process, such as the justification for selected requirements, trade-offs negotiated by stakeholders and alternative requirements that were discarded. In the context of evolving systems and distributed development, this information is essential. Rationale methods focus on capturing and structuring this missing information. In this paper, we propose an integrated process with dedicated guidance for capturing requirements and their rationale, discuss its tool support and describe the experiences we made during several case studies with students. Although the idea of integrating rationale methods with requirements engineering is not new, few research projects so far have focused on smooth integration, dedicated tool support and detailed guidance for such methods.     

Large-Scale Requirements Analysis Revisited: The need for Understanding the Political Ecology of Requirements Engineering

This paper addresses the political nature of requirements for large systems, and argues that requirements engineering theory and practice must become more engaged with these issues. It argues that large-scale system requirements is constructed through a political decision process, whereby requirements emerge as a set of mappings between consecutive solution spaces justified by a problem space of concern to a set of principals. These solution spaces are complex socio-technical ensembles that often exhibit non-linear behaviour in expansion due to domain complexity and political ambiguity. Stabilisation of solutions into agreed-on specifications occurs only through the exercise of organisational power. Effective requirements engineering in such cases is most effectively seen as a form of heterogeneous engineering in which technical, social, economic and institutional factors are brought together in a current solution space that provides the baseline for construction of proposed new solution spaces.     

Formal Analysis of Memory Requirements

Shared memory provides a convenient programming model for parallel applications. However, such a model is provided on physically distributed memory systems at the expense of efficiency of execution of the applications. For this reason, applications can give minimum consistency requirements on the memory system, thus allowing alternatives to the shared memory model to be used which exploit the underlying machine more efficiently. To be effective, these requirements need to be specified in a precise way and to be amenable to formal analysis. Most approaches to formally specifying consistency conditions on memory systems have been from the viewpoint of the machine rather than from the application domain.  In this paper we show how requirements on memory systems can be given from the viewpoint of the application domain formally in a first-order theory MemReq, to improve the requirements engineering process for such systems. We show the general use of MemReq in expressing major classes of requirements for memory systems and conduct a case study of the use of MemReq in a real-life parallel system out of which the formalism arose.     

Designing Paper Disasters: An Authoring Environment for Developing Training Exercises in Integrated Emergency Management

Discussion-based exercises are a prevalent form of training in emergency management, aimed at improving coordinative decision making between the various agencies involved in disaster response. In each exercise, small multi-agency groups of decision makers discuss potential courses of action within a fictitious disaster scenario presented as a textual narrative supported by visual materials. We present a cognitive engineering analysis of the problem of designing disaster scenarios for effective discussion-based exercises. The analysis was carried out through the development of a pilot authoring environment to establish and address the requirements of a training organisation in the UK. The pilot authoring environment embodies a simple theoretical model of the exercise process in which facts of a disaster scenario afford discussion of pertinent issues which are elicited by considerations fed to trainees. This representational scheme allows the authoring environment to complement and extend authors’ mental models of exercises, and thereby enhance five aspects of authoring: rationalisation; continuity of rationale; evolution; adaptability; and the integration of evaluation feedback.     

A Feasibility Study of Automated Natural Language Requirements Analysis in Market-Driven Development

In market-driven software development there is a strong need for support to handle congestion in the requirements engineering process, which may occur as the demand for short time-to-market is combined with a rapid arrival of new requirements from many different sources. Automated analysis of the continuous flow of incoming requirements provides an opportunity to increase the efficiency of the requirements engineering process. This paper presents empirical evaluations of the benefit of automated similarity analysis of textual requirements, where existing information retrieval techniques are used to statistically measure requirements similarity. The results show that automated analysis of similarity among textual requirements is a promising technique that may provide effective support in identifying relationships between requirements.     

On Abstraction in Scenarios

Scenarios are ways of representing knowledge. They may take many forms, from films of real events through acted scenes to documented procedures. These forms differ in many ways, including how vivid or abstract they are, how accessible they are as specifications, and how effective they are in helping to elicit requirements. Scenarios, especially as Use Cases, are in use or proposed for many aspects of systems engineering. Understanding of the different forms scenarios may take, and then of the costs and benefits of applying these forms in practice, may be valuable.     

A Qualitative Scenario Approach to Managing Evolving Requirements

In this paper, we argue that information systems requirements are inherently dynamic, and that a methodology that caters for such dynamicity must enable the evaluation of requirements, as they evolve, against dynamic contexts. Moreover, information systems contexts are soft, ambiguous, and are thus mainly characterised by qualitative data. We present an analytical technique, based on the grounded theory method for developing qualitative scenarios against which statements of requirements can be evaluated.     

Coupling Use Cases and Class Models as a Means for Validation and Verification of Requirements Specifications

In many applications, especially from the business domain, the requirements specification mainly deals with use cases and class models. Unfortunately, these models are based on different modelling techniques and aim at different levels of abstraction, such that serious consistency and completeness problems are induced. To overcome these deficiencies, we refine activity graphs to meet the needs for a suitable modelling element for use case behaviour. The refinement in particular supports the proper coupling of use cases via activity graphs and the class model. The granularity and semantics of our approach allow for a seamless, traceable transition of use cases to the class model and for the verification of the class model against the use case model. The validation of the use case model and parts of the class model is supported as well. Experience from several applications has shown that the investment in specification, validation and verification not only pays off during system and acceptance testing but also significantly improves the quality of the final product.       

Delay-jitter control in multimedia applications

The growing needs of multimedia communications are leading to new developments in providing real-time communication with guarantees. Several extensions have been proposed for different layers of the Open Systems Interconnection Reference Model to accomodate these needs. In this paper, we study methods for guaranteeing delay jitter bounds for high-speed networks in the network and application layers of this model. The method proposed for the network layer provides distributed jitter control. The method proposed for the application layer allows the destination application to control delay jitter. We use a simulation to compare the effects on delay jitter in each method for various scenarios, such as constant bit rate, cross traffic, and bursty data. In addition, the buffer space requirements for accommodating real-time channels are monitored at each node in the network.     

Requirements Engineering, Soft Systems Methodology and Workforce Empowerment

The fidelity and practicality of using soft systems methodology (SSM) to empower the workforce such that its members can make a fuller contribution to the requirements engineering process is critically analysed. The detailed analysis is carried out by using a (critical) philosophical approach to develop an interpretation of (some key aspects of) requirements engineering practice in actual information systems development situations, utilising a number of practical requirements engineering studies. This analysis is built upon to explain the relationship between requirements engineering, SSM and workforce empowerment. It is concluded that, by maintaining critically focused attention on the economic context, it is theoretically possible to engineer requirements for information systems that would actually empower the workforce. However, the likelihood of using SSM successfully for this purpose is low, as the economic context in which requirements engineering takes place is largely ignored by the SSM advocates.     

Formal verification of conflict detection algorithms

Safety assessment of new air traffic management systems is a main issue for civil aviation authorities. Standard techniques such as testing and simulation have serious limitations in new systems that are significantly more autonomous than the older ones. In this paper, we present an innovative approach for establishing the correctness of conflict detection systems. Fundamental to our approach is the concept of trajectory, and how we represent a continuous physical trajectory by a continuous path in the x-y plane constrained by physical laws and operational requirements. From the model of trajectories, we extract, and formally prove, high-level properties that can serve as a framework to analyze conflict scenarios. We use the AILS (Airborne Information for Lateral Spacing) alerting algorithm as a case study of our approach. Published online: 19 November 2002     

An Industrial Case Study on Distributed Prioritisation in Market-Driven Requirements Engineering for Packaged Software

When developing packaged software, which is sold ‘off-the-shelf’ on a worldwide marketplace, it is essential to collect needs and opportunities from different market segments and use this information in the prioritisation of requirements for the next software release. This paper presents an industrial case study where a distributed prioritisation process is proposed, observed and evaluated. The stakeholders in the requirements prioritisation process include marketing offices distributed around the world. A major objective of the distributed prioritisation is to gather and highlight the differences and similarities in the requirement priorities of the different market segments. The evaluation through questionnaires shows that the stakeholders found the process useful. The paper also presents novel approaches to visualise the priority distribution among stakeholders, together with measures on disagreement and satisfaction. Product management found the proposed charts valuable as decision support when selecting requirements for the next release, as they revealed unforeseen differences among stakeholder priorities. Conclusions on stakeholder tactics are provided and issues of further research are identified, including ways of addressing identified challenges.     

Universal Access in the Information Society: Methods, Tools, and Interaction Technologies

Accessibility and high quality of interaction with products, applications, and services by anyone, anywhere, and at any time are fundamental requirements for universal access in the emerging Information Society. This paper discusses these requirements, and their relation to the concept of automated adaptation of user interfaces. An example application is presented, showing how adaptation can be used to accommodate the requirements of different user categories and contexts of use. This application is then used as a vehicle for discussing a new engineering paradigm appropriate for the development of adaptation-based user interfaces. Finally, the paper investigates issues concerning the interaction technologies required for universal access. Published online: 23 May 2001     

Tracing Requirements Errors to Problems in the Requirements Engineering Process

A case study of requirements engineering practice is reported. The application, a decision support system for the Greek Ministry of Health, was investigated by studying the process of requirements analysis through to design and implementation. A usability analysis was then conducted on the designed system with the users. Several usability problems were discovered, and interviews uncovered further problems with the system that could be attributed to failure in requirements engineering (RE). Even though requirements were explicitly stated and the system was an evolution from an existing legacy system, functionality was defective and usability was poor. The client’s prime concern for redeveloping the system was to improve usability; unfortunately communications problems in the RE process meant that the developers did not appreciate this. The implications for RE methods and understanding the RE process are discussed.     

Continuity of interaction in nomadic interfaces through migration and dynamic utilization of I/O resources

The concept of ubiquitous computing reflects an infrastructure in which users are engaged in mobile interaction sessions within environments composed of dynamically varying computational resources. In this paradigm, applications are required to continuously follow end users and provide high-quality interaction while migrating among different computing devices and dynamically utilizing the available input/output (I/O) resources of each device. In the context of such interaction scenarios, the principle of continuity, which emphasizes the uninterrupted sequence of dialogue activities, is put forward as a key design goal. This paper presents an application experiment demonstrating interface migration, distributed I/O control, and dynamic I/O reconfiguration. The adopted dialogue design approach is discussed, along with the identified interaction design requirements, from the perspective of dialogue continuity. Finally, the employed software engineering strategy is presented, elaborating on the way dialogue mobility, distribution, and dynamic I/O control have been accomplished. Published online: 11 June 2002     

Time Aspects in Requirements Engineering: Or ‘Every Cloud Has A Silver Lining’

Nowadays, time is critical in most system engineering projects. The ability to deliver systems in short time determines the success of the system supplier. For customers, the quicker the system delivery time, the better are their chances to get some business advantages in their ever-changing business environments. As a consequence, an increasing number of projects are subjected to tight deadlines in all project phases, including requirements elicitation. A project with plenty of time for developing a requirements specification is hard to find. In this paper, experiences from one such project are reflected. Based on these experiences, time aspects in requirements engineering are discussed; i.e., what could be done better in requirements engineering if there were more time and what can easily be missed in requirements engineering under a tight deadline.     

Managing database server performance to meet QoS requirements in electronic commerce systems

The performance of electronic commerce systems has a major impact on their acceptability to users. Different users also demand different levels of performance from the system, that is, they will have different Quality of Service (QoS) requirements. Electronic commerce systems are the integration of several different types of servers and each server must contribute to meeting the QoS demands of the users. In this paper we focus on the role, and the performance, of a database server within an electronic commerce system. We examine the characteristics of the workload placed on a database server by an electronic commerce system and suggest a range of QoS requirements for the database server based on this analysis of the workload. We argue that a database server must be able to dynamically reallocate its resources in order to meet the QoS requirements of different transactions as the workload changes. We describe Quartermaster, which is a system to support dynamic goal-oriented resource management in database management systems, and discuss how it can be used to help meet the QoS requirements of the electronic commerce database server. We provide an example of the use of Quartermaster that illustrates how the dynamic reallocation of memory resources can be used to meet the QoS requirements of a set of transactions similar to transactions found in an electronic commerce workload. We briefly describe the memory reallocation algorithms used by Quartermaster and present experiments to show the impact of the reallocations on the performance of the transactions. Published online: 22 August 2001