Spiral Model

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A Spiral Model of Software
Development and Enhancement
Barry W. Boehm, TRW Defense Systems Group

“Stop the life cycle-I want to get off!’’ “Life-cycle Concept Considered Harmful. ” “The waterfall model is dead.” “No, it isn’t, but it should be.” hese statements exemplify the current debate about software Iife-cycle process models. The topic has recently received a great deal of attention. The Defense Science Board Task Force Report on Military Software‘ issued in 1987 highlighted the concern that traditional software process models were discouraging more effective approaches to software development such as prototyping and software reuse. The Computer Society has sponsored tutorials and workshops on software process models that have helped clarify many of the issues and stimulated advances in the field (see “Further reading”). The spiral model presented in this article is one candidate for improving the software process model situation. The major distinguishing feature of the spiral model is that it creates a risk-driven approach to the software process rather than a primarily document-driven or code-driven process. It incorporates many of the strengths of other models and resolves many of their difficulties. This article opens with a short description of software process models and the issues they address. Subsequent sections outline the process steps involved in the May 1988


This evolving riskdriven approach provides a new framework for guiding the software process.

spiral model; illustrate the application of the spiral model to a software project, using the TRW Software Productivity Project as an example; summarize the primary advantages a n d implications involved in using the spiral model and the primary difficulties in using it at its current incomplete level of elaboration; and present resulting conclusions.

criteria for the current stage plus choice criteria and entrance criteria for the next stage. Thus, a process model addresses the following software project questions: (1) What shall we do next? ( 2 ) How long shall we continue to do it? Consequently, a process model differs from a software method (often called a methodology) in that a method’s primary focus is on how to navigate through each phase (determining data, control, or ‘‘uses” hierarchies; partitioning functions; allocating requirements) and how to represent phase products (structure charts; stimulus-response threads; state transition diagrams). Why are software process models important? Primarily because they provide guidance on the order (phases, increments, prototypes, validation tasks, etc.) in which a project should carry out its major tasks. Many software projects, as the next section shows, have come to grief because they pursued their various development and evolution phases in the wrong order.

Background on software process models
The primary functions of a software process model are to determine the order of the stages involved in software development and evolution and to establish the transition criteria for progressing from one stage to the next. These include completion 0018 9162/8X/0500-0061$01 00

Evolution of process models. Before concentrating in depth on the spiral model, we should take a look at a number of others: the code-and-fix model, the stagewise model and the waterfall model, the evolutionary development model, and the transform model.

The code-and-fix model. The basic model used in the earliest days of software 61


Figure 1. The waterfall model of the software life cycle.

development contained two steps: (1) Write some code. (2) Fix the problems in the code. Thus, the order of the steps was to do some coding first and to think about the requirements, design, test, and maintenance later. This model has three pri-

mary difficulties: (a) After a number of fixes, the code became so poorly structured that subsequent fixes were very expensive. This underscored the need for a design...
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