Software engineering is] the establishment and use of sound engineering principles in order to obtain economically software that is reliable and works efficiently on real machines. The IEEE [IEE93a] has developed a more comprehensive definition when it states: Software Engineering: (1) The application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software; that is, the application of engineering to software. (2) The study of approaches as in (1). A quality focus Software engineering is a layered technology. Referring to Figure 1.3, any engineering approach (including software engineering) must rest on an organizational commitment to quality. Total quality management, Six Sigma, and similar philosophies foster a continuous process improvement culture, and it is this culture that ultimately leads to the development of increasingly more effective approaches to software engineering. The bedrock that supports software engineering is a quality focus.The foundation for software engineering is the process layer. The software engineering process is the glue that holds the technology layers together and enables rational and timely development of computer software. Process defines a framework that must be established for effective delivery of software engineering technology. The software process forms the basis for management control of software projects and establishes the context in which technical methods are applied, work products (models, documents, data, reports, forms, etc.) are produced, milestones are established, quality is ensured, and change is properly managed. Software engineering methods provide the technical how-to’s for building software. Methods encompass a broad array of tasks that include communication, requirements analysis, design modeling, program construction, testing, and support. Software engineering methods rely on a set of basic principles that govern each area of the technology and include modeling activities and other descriptive techniques. Software engineering tools provide automated or semiautomated support for the process and the methods. When tools are integrated so that information created by one tool can be used by another, a system for the support of software development, called computer-aided software engineering, is established.
The Software Process A process framework establishes the foundation for a complete software engineering process by identifying a small number of framework activities that are applicable to all software projects, regardless of their size or complexity. In addition, the process framework encompasses a set of umbrella activities that are applicable across the entire software process. A generic process framework for software engineering encompasses five activities: Communication. Before any technical work can commence, it is critically important to communicate and collaborate with the customer (and other stakeholders The intent is to understand stakeholders’ objectives for the project and to gather requirements that help define software features and functions. Planning. Any complicated journey can be simplified if a map exists. A software project is a complicated journey, and the planning activity creates a “map” that helps guide the team as it makes the journey. The map—called a software project plan—defines the software engineering work by describing the technical tasks to be conducted, the risks that are likely, the resources that will be required, the work products to be produced, and a work schedule. Modeling. Whether you’re a landscaper, a bridge builder, an aeronautical engineer, a carpenter, or an architect, you work with models every day. You create a “sketch” of the thing so that you’ll understand the big picture—what it will look like architecturally, how the constituent parts fit together, and many other characteristics. If required, you refine the sketch into greater and greater detail in an effort to better understand the problem and how you’re going to solve it. A software engineer does the same thing by creating models to better understand software requirements and the design that will achieve those requirements. Construction. This activity combines code generation (either manual or automated) and the testing that is required to uncover errors in the code. Deployment. The software (as a complete entity or as a partially completed increment) is delivered to the customer who evaluates the delivered product and provides feedback based on the evaluation. For many software projects, framework activities are applied iteratively as a project progresses. That is, communication, planning, modeling, construction, and deployment are applied repeatedly through a number of project iterations. Each project iteration produces a software increment that provides stakeholders with a subset of overall software features and functionality. As each increment is produced, the software becomes more and more complete. Software engineering process framework activities are complemented by a number of umbrella activities. In general, umbrella activities are applied throughout a software project and help a software team manage and control progress, quality, change, and risk. Typical umbrella activities include: Software project tracking and control—allows the software team to assess progress against the project plan and take any necessary action to maintain the schedule.
Risk management—assesses risks that may affect the outcome of the project or the quality of the product. Software quality assurance—defines and conducts the activities required to ensure software quality. Technical reviews—assesses software engineering work products in an effort to uncover and remove errors before they are propagated to the next activity. Measurement—defines and collects process, project, and product measures that assist the team in delivering software that meets stakeholders’ needs; can be used in conjunction with all other framework and umbrella activities. Software configuration management—manages the effects of change throughout the software process. Reusability management—defines criteria for work product reuse (including software components) and establishes mechanisms to achieve reusable components. Work product preparation and production—encompasses the activities required to create work products such as models, documents, logs, forms, and lists. Prescriptive process model The Waterfall Model There are times when the requirements for a problem are well understood—when work flows from communication through deployment in a reasonably linear fashion. This situation is sometimes encountered when well-defined adaptations or enhancements to an existing system must be made (e.g., an adaptation to accounting software that has been mandated because of changes to government regulations). It may also occur in a limited number of new development efforts, but only when requirements are well defined and reasonably stable. The waterfall model, sometimes called the classic life cycle, suggests a systematic, sequential approach to software development that begins with customer specification of requirements and progresses through planning, modeling, construction, and deployment, culminating in ongoing support of the completed software (Figure 2.3). A variation in the representation of the waterfall model is called the V-model. Represented in Figure 2.4, the V-model [Buc99] depicts the relationship of quality
assurance actions to the actions associated with communication, modeling, and early construction activities. As a software team moves down the left side of the Vbasic problem requirements are refined into progressively more detailed and technical representations of the problem and its solution. Once code has been generated, the team moves up the right side of the V, essentially performing a series of tests (quality assurance actions) that validate each of the models created as the team moved down the left side. In reality, there is no fundamental difference between the classic life cycle and the V-model. The V-model provides a way of visualizing how verification and validation actions are applied to earlier engineering work. The waterfall model is the oldest paradigm for software engineering. Howeverover the past three decades, criticism of this process model has caused even ardent supporters to question its efficacy [Han95]. Among the problems that are sometimes encountered when the waterfall model is applied are: 1. Real projects rarely follow the sequential flow that the model proposes. Although the linear model can accommodate iteration, it does so indirectly. As a result, changes can cause confusion as the project team proceeds. It is often difficult for the customer to state all requirements explicitly. The waterfall model requires this and has difficulty accommodating the natural uncertainty that exists at the beginning of many projects. The customer must have patience. A working version of the program(s) will not be available until late in the project time span. A major blunder, if undetected