Overview of Software Development Models: Waterfall, Iterative, Spiral, and Agile

1. Waterfall Model

Proposed by Winston Royce in 1970, the Waterfall model defines software development as a sequence of fixed phases—planning, requirements analysis, design, coding, testing, and maintenance—flowing downward like a waterfall.

Core idea: separate analysis/design from implementation to enable clear division of labor; however, the model produces heavy documentation, follows a strictly linear path that can delay projects, and does not adapt well to changing user requirements.

2. Iterative Model (RUP)

The Rational Unified Process (RUP) recommends an iterative cycle where each iteration produces a potentially shippable product increment, resembling a small-scale waterfall.

Advantages: reduces incremental risk, lowers market‑entry risk, accelerates overall development speed, and adapts more easily to changing requirements.

Disadvantage: early‑stage changes demand high competence from developers and project managers.

3. Spiral Model

Introduced by Barry Boehm in 1988, the Spiral model combines the Waterfall approach with rapid prototyping and emphasizes risk analysis, making it suitable for large, complex systems.

Advantages: flexible design allowing changes at any stage, simpler cost estimation through incremental builds, and continuous customer involvement ensuring project controllability.

Disadvantages: risk analysis may be hard for customers to accept, can erode profit if overly extensive, and requires staff skilled in identifying and evaluating risks.

4. Agile Development

Based on iterative principles, Agile emerged around 1990 and includes methodologies such as XP, Scrum, FDD, DSDM, Crystal, ASD, Kanban, and Lean, emphasizing close collaboration between development and business teams.

Advantages: rapid, continuous delivery of valuable software; focus on people and interaction over processes and tools; frequent releases (weeks instead of months); face‑to‑face communication; close daily cooperation between business and development; emphasis on technical excellence and good design; high adaptability to changing environments.

Disadvantages: insufficient emphasis on design and documentation; difficulty estimating effort for large projects early on; risk of project drift when customer requirements are unclear.