The spiral model of software development and enhancement, developed by Boehm (1988), is based on experience with various refinements of the waterfall model as applied to large government software projects. Relying heavily on prototyping and risk management, it is much more flexible than the waterfall model. The most comprehensive application of the model is the development of the TRW Software Productivity System (TRW-SPS) as described by Boehm. The spiral concept and the risk management focus have gained acceptance in software engineering and project management in recent years .
Figure 2.3 shows Boehm's spiral model. The underlying concept of the model is that each portion of the product and each level of elaboration involves the same sequence of steps (cycle). Starting at the center of the spiral, one can see that each development phase (concept of operation, software requirements, product design, detailed design, and implementation) involves one cycle of the spiral. The radial dimension in Figure 2.3 represents the cumulative cost incurred in accomplishing the steps. The angular dimension represents the progress made in completing each cycle of the spiral. As indicated by the quadrants in the figure, the first step of each cycle of the spiral is to identify the objectives of the portion of the product being elaborated, the alternative means of implementation of this portion of the product, and the constraints imposed on the application of the alternatives. The next step is to evaluate the alternatives relative to the objectives and constraints, to identify the associated risks, and to resolve them. Risk analysis and the risk-driven approach, therefore, are key characteristics of the spiral model, in contrast to the document-driven approach of the waterfall model.
Figure 2.3. Spiral Model of the Software Process
From "A Spiral Model of Software Development and Enhancement," by B. W. Boehm. IEEE Computer (May): 61 “72. 1988 IEEE. Reprinted with permission.
In this risk-driven approach, prototyping is an important tool. Usually prototyping is applied to the elements of the system or the alternatives that present the higher risks. Unsatisfactory prototypes can be thrown away; when an operational prototype is in place, implementation can begin. In addition to prototyping, the spiral model uses simulations, models, and benchmarks in order to reach the best alternative. Finally, as indicated in the illustration, an important feature of the spiral model, as with other models, is that each cycle ends with a review involving the key members or organizations concerned with the product.
For software projects with incremental development or with components to be developed by separate organizations or individuals, a series of spiral cycles can be used, one for each increment or component. A third dimension could be added to Figure 2.3 to represent the model better.
Boehm (1988) provides a candid discussion of the advantages and disadvantages of the spiral model. Its advantages are as follows :
On the other hand, difficulties with the spiral model include the following:
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