Spiral Model - Short Note

Proposed by: Barry Boehm in 1986. It is an iterative, risk-driven software development model where the project is developed in a series of spiral cycles (repeated loops), each of which passes through four phases.

Four Phases of Each Spiral Cycle

1. Planning - Determine objectives, alternatives and constraints for that cycle; finalize deliverables and the schedule.
2. Risk Analysis - Evaluate alternatives, identify and resolve risks (prototypes are built for risky areas). This is the distinguishing feature of the model.
3. Engineering - Development and testing of the product for the current increment.
4. Customer Evaluation - The customer reviews the output, gives feedback, and approves the next cycle.

Key Characteristics

• Combines waterfall + prototyping + incremental development, with emphasis on risk handling.
• Each loop produces a prototype of the system; the final prototype is the finished system.
• Strong documentation control and support for reuse.
• Requirements can change between cycles, so it is very flexible.

Advantages

• Risk is analyzed and resolved early, so costly failures are avoided.
• Good for large, complex, high-risk projects.
• Customers see working results early and can give feedback.
• Accommodates requirement changes easily.

Disadvantages

• Complex and expensive to manage (risk analysis needs expertise).
• Hard to estimate cost and schedule.
• Not suitable for small or low-risk projects.
• Risk prototypes may not cover all modules.

When to Use

Projects that are large, critical and risk-prone, where risk must be controlled at every phase - e.g. defence, aerospace and large enterprise systems.