Spiral model

Spiral model (Boehm, 1988)

Spiral model is a software development process combining elements of both design and prototyping-in-stages, in an effort to combine advantages of top-down and bottom-up concepts. The spiral model was first defined by Barry Boehm in his 1986 paper "A Spiral Model of Software Development and Enhancement". This model was not the first model to discuss iterative development, but it was the first model to explain why the iteration matters. The spiral model emphasizes the necessity to go back and iterate earlier stages a number of times as the project progresses. It's a risk-driven process model generator that allows for iterations across the product life cycle.

Overview

The spiral model is intended for large, expensive, and complicated projects. The model itself is a risk-driven controlled prototyping approach that allows for incremental refinement of the product through each iteration around the spiral. Each loop of the spiral represents a phase of the software process. The four main phases are: planning, risk analysis, engineering and evaluation. The project passes through these phases in iterations (called Spirals in this model).

Phases of the Spiral Model

Planning Phase

This phase starts with gathering business requirements in the baseline spiral. In the subsequent spirals as the product matures, identification of system requirements and unit requirements are done in this phase. This also includes understanding the system requirements by continuous communication between the customer and the system analyst. At the end of the spiral, the product is deployed.

Risk Analysis Phase

A detailed risk analysis is performed for each iteration. This includes identifying, estimating, and monitoring technical feasibility and management risks, such as schedule slippage and cost overrun. After analyzing the risk, the project team must decide how to handle each risk. Strategies include avoiding the risk, minimizing its impact, or accepting the consequences.

Engineering Phase

The engineering phase is where the actual development and testing of the software products take place. Based on the design documents developed during the planning phase, the software product is engineered as a series of incremental releases, through which the project team can identify and correct defects and mismatches to the requirements.

Evaluation Phase

In this phase, the customer evaluates the output of the project to date before the project continues to the next spiral. Feedback from the customer is gathered and interpreted to make the next cycle of the spiral more efficient and to better meet the customer's needs.

Advantages

• Flexibility in requirements allows for changes and refinements to be made based on feedback.
• Risk management is an integral part of the model, which helps in identifying and managing potential risks at an early stage.
• It supports customer involvement throughout the development process, which can lead to more satisfactory software delivery.

Disadvantages

• It can be a costly model to use due to the constant iterations and risk management activities.
• Requires significant documentation at each phase of the software development cycle.
• The model depends heavily on risk assessment expertise; without it, the project can easily spiral out of control.

Applications

The spiral model is best used for large projects which involve continuous enhancements and high-risk features requiring significant analysis and risk management. It is particularly useful for systems that require frequent releases to accommodate new technologies or changes in user requirements.