An Overview of Strategies for Real-Time System Specification
For more than a decade, there has been growing dissatisfaction with conventional strategies for systems development, and an increasing number of proposals for replacing them with new, more formal approaches. The problem is that conventional development methods cannot cope with the size and complexity of the systems we are now able to implement. These system characteristics are possible because of the ever-increasing capacity and capability of computer hardware. Systems having up to a million lines of high-order source code are already commonplace, and others are on the horizon that will require ten or more times that amount.
The inadequacies of the development methods were first apparent in large business and commercial systems, but as digital technology came into use in real-time embedded systems, they too became subject to the same problems. We are past the stage of wondering whether formal development methods are really needed. The question is, Can we keep pace with the capabilities of the technology using the methods we already have? What further extensions and improvements can we make to them? Can we devise better and more comprehensive methods?
A problem with past system specification methods has been that they each tend to address only one aspect of the system, whereas systems actually have many aspects, all of which need to be addressed. The methods described in this book seek to overcome this problem by addressing a number of these aspects in an integrated manner. From the functional requirements point of view, they address the aspects of processing, control, and timing. From the physical requirements point of view, they address the architectural specification of the system, and most importantly, they take account of the hierarchical and iterative nature of systems and of systems development. Figure 1.1 illustrates how the whole universe may be thought of as a hierarchy of systems. Any individual system may itself be expressed as such a hierarchy, and the progression down through its levels involves defining the requirements for the level below, then making decisions on the architecture or design of that level to meet those requirements, then repeating this procedure for the next level down, and so on. This perspective makes clear the adage that one person’s requirements are another person’s design: a cause of considerable confusion in earlier development methods, but an integral part of the methods addressed here.
Figure 1.1. The universal hierarchy of systems.
Before we delve into the details of the methods, we will give a brief historical account of their development. This will help you understand the situations that create the need for such methods, and will give you some insight into what it might take to introduce them into your own organization.
1.1 The Birth of the Requirements Model
The requirements specification model described in this book evolved at a major avionics systems development company. This evolution started in late 1982, and culminated two years later in the successful Federal Aviation Administration certification of a complex real-time, embedded avionics system for a commercial airliner. The system is now in widespread use in the airline industry.
At the time, the development effort was the largest we had ever undertaken. Previously, we had been without formal procedures for integrating the efforts of our system and software developers: We had depended on their individual communications with each other, and on the abilities of a few overworked lead engineers, in order to keep the various subsystems consistent.
There comes a point at which this just won’t do, a point at which the interactions between the subsystems are at least as complex as the subsystems themselves. Faced with this dilemma, and with the very real threat of a major disaster if we did not improve our performance, we investigated the formal development methods that already existed. We found a fairly mature set of methods that had been developed in the 1970s for large mainframe business applications, and that had been successfully applied in that field.
Several of the existing requirements definition methods were capable of representing the information processing parts of our systems, but they could not handle the complex control structures that large real-time systems also typically include. Since these control structures can be represented using finite state machine theory, we decided that we needed to integrate an established requirements definition method with finite state machine theory in a unified structure, and to do it so as to retain the benefits of both.
The requirements definition method that seemed to have the most merit was DeMarco’s structured analysis , so we chose this as the starting point. One of the greatest merits of the DeMarco method is its attention to human readability and understandability through the use of graphics. In addition, it shares with most other requirements definition methods the attributes of information abstraction, built-in self-consistency checking, and the ability to be self-indexing.
Because the project was already underway, we needed a requirements definition method immediately. During just two weeks of brainstorming and trial and error, we found the approach we were looking for—a way to partition a large finite state machine into pieces corresponding to the pieces of a structured analysis—and the requirements model described in this book was born. One week later, a group of development engineers took the first class in the new method. That first teaching effort was not a polished presentation, but the substance was there. As we gained experience in its use, we made some further improvements in our emerging method, but, surprisingly, it has never needed any fundamental changes.
The existing and widely used structured design method [11,19] was applicable (with very little modification) to the design definition of our real-time systems.
Introducing the model
The approach we took to introduce our new methods was contrary to that advocated by methods experts, who advise that methods be introduced on a small, low-visibility project. We did not have (and have never had) such a project, and in fact introduced the method on the largest, most critical project we had ever undertaken. To offset the risks in this situation, we established a Methods Team—a group of six people whose sole role was to make the methods work. The team members prepared and taught classes, provided consultation, solved problems as they arose, made improvements to the methods, and investigated ways to automate them.
If critical projects are the only kind your shop, too, ever has to work on, and you are about to introduce revolutionary new development methods, we recommend this approach of dedicating a team to support them.
It would be misleading to suggest that the requirements and design definition methods alone were responsible for the success of the project. We made a number of other changes at the same time: For example, a large scientific computer was dedicated to the project; each system and software engineer was provided with an interactive terminal; an efficient, modular working environment was installed; we scaled the project using the Putnam model ; and we used a more detailed and comprehensive progress tracking procedure than we had ever used before. The relative contributions of the individual factors are hard to judge, but their combined effect was greatly improved productivity.
One of the earliest decisions made with regard to the requirements model was to make it freely available to anyone wishing to use it, rather than to copyright it, or treat it as proprietary information. Our rationale was first, that our business is avionics, not development methods; second, that it would be to our advantage to have the rest of the industry follow our lead; and further, that it would be particularly advantageous to have automated tool developers embed our method in their tools. As a result of this early decision (and because of the success of the requirements model in its first application), the method has become widespread in the industry. Many diverse companies are using it, and there are currently at least six automated CASE tool developers  supporting it (some more completely than others—we neither endorse nor recommend against these or any other CASE tools; you must evaluate them relative to your own needs and environment). In addition, there are seminars  available giving instruction in the use of both the requirements and the architecture models. This popularity has given rise to constant demands for more available information on the method, and these demands provided our principal motivation for writing this book.