The Spiral Model

The spiral model, originally proposed by Boehm , is an evolutionary software

process model that couples the iterative nature of prototyping with the controlled and

systematic aspects of the linear sequential model. It provides the potential for rapid

development of incremental versions of the software. Using the spiral model, software

is developed in a series of incremental releases. During early iterations, the

incremental release might be a paper model or prototype. During later iterations,

increasingly more complete versions of the engineered system are produced.

A spiral model is divided into a number of framework activities, also called task

regions.6 Typically, there are between three and six task regions. A spiral model

 that contains six task regions:

• Customer communication—tasks required to establish effective communication

between developer and customer.

• Planning—tasks required to define resources, timelines, and other projectrelated

information.

• Risk analysis—tasks required to assess both technical and management

risks.

• Engineering—tasks required to build one or more representations of the

application.

• Construction and release—tasks required to construct, test, install, and

provide user support (e.g., documentation and training).

• Customer evaluation—tasks required to obtain customer feedback based

on evaluation of the software representations created during the engineering

stage and implemented during the installation stage.

Each of the regions is populated by a set of work tasks, called a task set, that are

adapted to the characteristics of the project to be undertaken. For small projects, the

number of work tasks and their formality is low. For larger, more critical projects,

each task region contains more work tasks that are defined to achieve a higher level

of formality

.

As this evolutionary process begins, the software engineering team moves around

the spiral in a clockwise direction, beginning at the center. The first circuit around

the spiral might result in the development of a product specification; subsequent

passes around the spiral might be used to develop a prototype and then progressively

more sophisticated versions of the software. Each pass through the planning region

results in adjustments to the project plan. Cost and schedule are adjusted based on

feedback derived from customer evaluation. In addition, the project manager adjusts

the planned number of iterations required to complete the software.

Unlike classical process models that end when software is delivered, the spiral

model can be adapted to apply throughout the life of the computer software. An alternative

view of the spiral model can be considered by examining the project entry point

axis. Each cube placed along the axis can be used to represent

the starting point for different types of projects. A “concept development

project” starts at the core of the spiral and will continue (multiple iterations occur

along the spiral path that bounds the central shaded region) until concept development

is complete. If the concept is to be developed into an actual product, the process

proceeds through the next cube (new product development project entry point) and

a “new development project” is initiated. The new product will evolve through a number

of iterations around the spiral, following the path that bounds the region that has

somewhat lighter shading than the core. In essence, the spiral, when characterized

in this way, remains operative until the software is retired. There are times when the

process is dormant, but whenever a change is initiated, the process starts at the appropriate

entry point .

The spiral model is a realistic approach to the development of large-scale systems

and software. Because software evolves as the process progresses, the developer and

customer better understand and react to risks at each evolutionary level. The spiral model

uses prototyping as a risk reduction mechanism but, more important, enables the developer

to apply the prototyping approach at any stage in the evolution of the product. It

maintains the systematic stepwise approach suggested by the classic life cycle but incorporates

it into an iterative framework that more realistically reflects the real world. The

spiral model demands a direct consideration of technical risks at all stages of the project

and, if properly applied, should reduce risks before they become problematic.

But like other paradigms, the spiral model is not a panacea. It may be difficult to

convince customers (particularly in contract situations) that the evolutionary approach

is controllable. It demands considerable risk assessment expertise and relies on this

expertise for success. If a major risk is not uncovered and managed, problems will

undoubtedly occur. Finally, the model has not been used as widely as the linear

sequential or prototyping paradigms. It will take a number of years before efficacy of

this important paradigm can be determined with absolute certainty.