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1.6 On Designing Complex Systems

The practice of every engineering discipline—be it civil, mechanical, chemical, electrical, or software engineering—involves elements of both science and art. As Petroski eloquently states, "The conception of a design for a new structure can involve as much a leap of the imagination and as much a synthesis of experience and knowledge as any artist is required to bring to his canvas or paper. And once that design is articulated by the engineer as artist, it must be analyzed by the engineer as scientist in as rigorous an application of the scientific method as any scientist must make" [38]. Similarly, Dijkstra observes, "the programming challenge is a large-scale exercise in applied abstraction and thus requires the abilities of the formal mathematician blended with the attitude of the competent engineer" [39].

Engineering as a Science and an Art

The role of the engineer as artist is particularly challenging when the task is to design an entirely new system. Especially in the case of reactive systems and systems for command and control, we are frequently asked to write software for an entirely unique set of requirements, often to be executed on a configuration of target processors constructed specifically for this system. In other cases, such as the creation of frameworks, tools for research in artificial intelligence, or information management systems, we may have a well-defined, stable target environment, but our requirements may stress the software technology in one or more dimensions. For example, we may be asked to craft systems that are faster, have greater capacity, or have radically improved functionality. In all these situations, we try to use proven abstractions and mechanisms (the "stable intermediate forms," in Simon's words) as a foundation on which to build new complex systems. In the presence of a large library of reusable software components, the software engineer must assemble these parts in innovative ways to satisfy the stated and implicit requirements, just as the painter or the musician must push the limits of his or her medium.

The Meaning of Design

In every engineering discipline, design encompasses the disciplined approach we use to invent a solution for some problem, thus providing a path from requirements to implementation. In the context of software engineering, Mostow suggests that the purpose of design is to construct a system that:

  • Satisfies a given (perhaps informal) functional specification
  • Conforms to limitations of the target medium
  • Meets implicit or explicit requirements on performance and resource usage
  • Satisfies implicit or explicit design criteria on the form of the artifact
  • Satisfies restrictions on the design process itself, such as its length or cost, or the tools available for doing the design [40]

As Stroustrup suggests, "the purpose of design is to create a clean and relatively simple internal structure, sometimes also called an architecture.... A design is the end product of the design process" [41]. Design involves balancing a set of competing requirements. The products of design are models that enable us to reason about our structures, make trade-offs when requirements conflict, and in general, provide a blueprint for implementation.

The Importance of Model Building

The building of models has a broad acceptance among all engineering disciplines, largely because model building appeals to the principles of decomposition, abstraction, and hierarchy [42]. Each model within a design describes a specific aspect of the system under consideration. As much as possible, we seek to build new models upon old models in which we already have confidence. Models give us the opportunity to fail under controlled conditions. We evaluate each model in both expected and unusual situations, and then we alter them when they fail to behave as we expect or desire.

We have found that in order to express all the subtleties of a complex system, we must use more than one kind of model. For example, when designing a personal computer, an electrical engineer must take into consideration the component-level view of the system as well as the physical layout of the circuit boards. This component view forms a logical picture of the design of the system, which helps the engineer to reason about the cooperative behavior of the components. The board layout represents the physical packaging of these components, constrained by the board size, available power, and the kinds of components that exist. From this view, the engineer can independently reason about factors such as heat dissipation and manufacturability. The board designer must also consider dynamic as well as static aspects of the system under construction. Thus, the electrical engineer uses diagrams showing the static connections among individual components, as well as timing diagrams that show the behavior of these components over time. The engineer can then employ tools such as oscilloscopes and digital analyzers to validate the correctness of both the static and dynamic models.

The Elements of Software Design Methodologies

Clearly, there is no magic, no "silver bullet" [43] that can unfailingly lead the software engineer down the path from requirements to the implementation of a complex software system. In fact, the design of complex software systems does not lend itself at all to cookbook approaches. Rather, as noted earlier in the fifth attribute of complex systems, the design of such systems involves an incremental and iterative process.

Still, sound design methods do bring some much-needed discipline to the development process. The software engineering community has evolved dozens of different design methodologies, which we can loosely classify into three categories (see the Categories of Analysis and Design Methods sidebar). Despite their differences, all of these have elements in common. Specifically, each includes the following:



The language for expressing each model



The activities leading to the orderly construction of the system's models



The artifacts that eliminate the tedium of model building and enforce rules about the models themselves, so that errors and inconsistencies can be exposed

A sound design method is based on a solid theoretical foundation yet offers degrees of freedom for artistic innovation.

The Models of Object-Oriented Development

Is there a "best" design method? No, there is no absolute answer to this question, which is actually just a veiled way of asking the earlier question: What is the best way to decompose a complex system? To reiterate, we have found great value in building models that are focused on the "things" we find in the problem space, forming what we refer to as an object-oriented decomposition.

Object-oriented analysis and design is the method that leads us to an object-oriented decomposition. By applying object-oriented design, we create software that is resilient to change and written with economy of expression. We achieve a greater level of confidence in the correctness of our software through an intelligent separation of its state space. Ultimately, we reduce the risks inherent in developing complex software systems.

In this chapter, we have made a case for using object-oriented analysis and design to master the complexity associated with developing software systems. Additionally, we have suggested a number of fundamental benefits to be derived from applying this method. Before we present the notation and process of object-oriented design, however, we must study the principles on which object-oriented development is founded, namely, abstraction, encapsulation, modularity, hierarchy, typing, concurrency, and persistence.

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