Technical Design:Information system design

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9.1 Introduction

This chapter is concerned with building a bridge between the problem situation requirements, as reflected in the UML information models, and an information system implementation. It is apposite to view technical design as a bridge, rather than a transition, since the analogy of a bridge suggests a bi-directional movement between information system requirements and implementation in software.

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9.2 Information system design

The metaphor of time's arrow suggests a one-way path from requirements to design to implementation. Feedback loops and arrows can be added to give the impression of iteration, but this is a weak depiction of the complex relationship between requirements and design. The idea of mediation in IS development means that the social and technical aspects are mangled together and whether we like it or not, technical design should not and cannot be reduced to an exercise in techno-rationality. The technical design process must be considered within the context of organizational analysis and work design, and not simply as a scientistic refinement of the requirements specification. However, despite this caveat, in order to illustrate technical design in a clear way we will work within a techno-rational paradigm in this chapter.

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Analysis is often defined as the 'what' of IS development, being concerned with what the system can do, e.g., find available seats, process a ticket sale transaction, rather than how it achieves this end. In moving to design we are more concerned with 'how' the ends will be achieved in software terms. In practice the boundaries between 'what' and 'how' are blurred and analysis and design are difficult to disentangle. With traditional approaches to IS development, such as SSADM (Structured Systems Analysis and Design Method), there are clear cut-off points where the deliverables from one stage are signed-off, ready to move to the next stage where new notations and techniques are introduced. For many developers, the appeal of OO and UML is that the developer is not forced into making a hard, once-and-for-all, break between analysis and design. With UML the principal diagrams are elaborated in greater detail until they become sufficiently detailed to become an executable  specification, or for a human programmer to implement them by hand in program code. In the UML approach design is not a one-off activity. It is an iterative process of elaboration of detail, moving back and forwards between a logical, implementation-independent design and a physical, implementation-specific design (figure 9.2). The design process also entails a movement between a high level system or architectural design and the detailed design of software components, such as the individual classes and operations (figure 9.2).

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