Iterative and Evolutionary Development
- Involves early programming and testing of a partial system, in repeating cycles.
- Relies on short quick development steps (or iterations), feedback and adaptation to clarify the requirements and design so successively enlarging and refining a system.
- Normally assumes that the development starts before all requirements are defined in detail, feedback is used to clarify and improve the evolving specifications.
- Each iteration will include requirements, analysis, design, implementation, and test.
- Iterative feedback and evolution leads towards the desired system. The requirements and design instability lowers over time.
Current research
demonstrates that iterative methods are associated with higher success and
productivity rates, and lower defect levels.
Timeboxing
A key idea is that iterations are timeboxed, or fixed in
length.
- Most iterative methods recommend in iteration length between 2 – 6 weeks.
- If it seems that it will be difficult to meet the deadline, the recommended response is to de-scope
De-scoping: removing tasks or requirements from the
iteration, and including them in a future iteration, rather than slipping the
completion date.
Iterative and Evolutionary Development (also known as iterative and inceremental development; spiral development and evolutionary development)
Build-Feedback-Adapt
Cycles
In complex changing systems, feedback and adaptation are key
ingredients for success:
- Feedback from early development, programmers trying to read specifications, and client demos: to refine the requirements.
- Feedback from tests and developers : to refine the design and models.
- Feedback from the progress of the team tackling early features : to refine the schedule and estimates.
Benefits of Iterative development
- Less project failure, better productivity, and lower defect rates
- Early rather than late mitigation of high risks
- Early visible progress
- Early feedback, user engagement, and adaptation
- Managed complexity: the team is not overwhelmed by “analysis paralysis” or very long and complex steps
- The learning within an iteration can be methodically used to improve the development process itself, iteration by iteration.
The Unified Process is a popular iterative software development process.
Why a new
methodology?
Process-oriented
methods:
- Requirements of a project are completely frozen before the design and development process commences.
- Not always feasible
- Need for flexible, adaptable and agile methods, which allow the developers to make late changes in specifications.
Waterfall (Sequential)
Lifecycle
- Promotes big up-front “speculative” requirements and design steps before programming.
- Historically promoted due to belief or hearsay rather than statistically significant evidence.
- Success/failure studies show that the waterfall has high failure rates.
Why the waterfall
lifecycle fails?
The key false assumption:
- The specifications are predictable and stable and can be correctly defined at the start, with low change rates.
- But change is a constant on software projects.
- A typical software project experienced a 25% change.
