Spiral Model

The WIN-WIN Spiral Model

In this model the developer and the customer both together strive for a “win-win” result. The customer wins by getting the system or product that satisfies the majority of the customer needs and the developer wins by working on realistic and achievable goals, budgets and deadlines. Rather than a single customer communication activity the following activities are defined:

• Identification of the Key Stakeholders in the organization.
• Determination of the Key Stakeholders “Win conditions” - a crucial step.
• Negotiating of the stake holders win conditions into a set of win-win conditions for all including the developers, management, customers and the various other stake holders.

In addition to the negotiations, the WINWIN spiral model also introduces three process milestones (anchor points) which help completion of one cycle around the spiral and provides the decision milestones. The three process milestones are:

• 1. Life Cycle Objective (LCO) – defines a set of activity for each major software engineering activity. Eg. Defining the top-level system/product requirements.
• Life Cycle Architecture (LCA) – defines the objectives that must be met as the system and software architecture is defined. Eg. The software team can demonstrate that they have evaluated the applicability of the software and also considered the impact on architectural decisions.

3. Initial Operational Capability (IOC) – defines the set of objectives

Advantages:

• Faster software production facilitated through collaborative involvement of the relevant stake holders.
• Cheaper software via rework and maintenance reductions

The Spiral Model

The Spiral Model is an evolutionary software process model that couples the iterative nature of prototyping with the controlled and systematic aspects of the Linear Sequential Model. Using the Spiral Model the software is developed in a series of incremental releases. Unlike the Iteration Model where in the first product is a core product, in the Spiral Model the early iterations could result in a paper model or a prototype. However, during later iterations more complex functionalities could be added.

A Spiral Model, combines the iterative nature of prototyping with the controlled and systematic aspects of the Waterfall Model, therein providing the potential for rapid development of incremental versions of the software. A Spiral Model is divided into a number of framework activities, also called task regions. These task regions could vary from 3-6 in number and they are:

• Customer Communication - tasks required to establish effective communication between the developer and customer.

• Planning - tasks required to define resources, timelines and other project related information /items.

• Risk Analysis - tasks required to assess the technical and management risks.

• Engineering - tasks required to build one or more representation of the application.

• Construction & Release - tasks required to construct, test and support (eg. Documentation and training)

• Customer evaluation - tasks required to obtain periodic customer feedback so that there are no last minute surprises.

Advantages of the Spiral Model

• Realistic approach to the development because the software evolves as the process progresses. In addition, the developer and the client better understand and react to risks at each evolutionary level.

• The model uses prototyping as a risk reduction mechanism and allows for the development of prototypes at any stage of the evolutionary development.

• It maintains a systematic stepwise approach, like the classic waterfall model, and also incorporates into it an iterative framework that more reflect the real world.

Disadvantages of the Spiral Model

• One should possess considerable risk-assessment expertise
• It has not been employed as much proven models (e.g. the Waterfall Model) and hence may prove difficult to ‘sell’ to the client.