Software Process Models (2017)

CSS CS Optional — Paper-I, Section-B, Topic V.

🔥 Frequency: appeared in 4 of last 7 papers (2021, 2023, 2024, 2025).

🔑 The Models

• Waterfall: sequential — requirements → design → code → test → deploy → maintain. Simple, but late changes are expensive.
• Incremental: usable slices delivered one after another → see own page.
• RAD: component-based, time-boxed → see own page.
• Prototyping:
  • Throwaway — discard prototype, rebuild from scratch.
  • Evolutionary — refine the prototype into final system → see Evolutionary Model page.
• Spiral (Boehm): iterative + risk-driven. Four quadrants per loop:
  1. Determine objectives/alternatives/constraints
  2. Evaluate alternatives, resolve risks
  3. Develop & verify next-level product
  4. Plan next iteration
  • Best for: large, high-risk, poorly-understood projects.
• V-Model: Waterfall + paired verification stage per phase (requirements↔acceptance testing, design↔system testing).

📊 Quick Comparison

🔥 Past Papers

• 2021 Q7(b) — Spiral model in detail (7 marks).
• 2025 MCQ — "iterative cycles + continuous development" → Agile.

⚠️ MCQ Trap

• "Iterative + risk-driven" → Spiral
• "Iterative + customer-collaborative, short sprints" → Agile

📜 Past-Paper Questions & MCQs (CSS 2016–2026)

2017 · Paper-I · Q6(a) · 8 marks

Q. The design methodologies of programs include Big Bang, Code and Fix, Waterfall and the Spiral model. Consider a test (quiz) application and describe its development following each approach.

Ans.

• Big Bang: no formal planning or requirements. Developers start coding with vague ideas and hope the quiz app takes shape. It works only for tiny projects and has a high risk of failure.
• Code and Fix: write some code, test it, find defects, fix them, and repeat. The quiz app grows without design, so the code becomes messy and hard to maintain.
• Waterfall: collect all quiz requirements, design the database and screens, code, test, then deploy, in strict order. It is well documented but cannot handle changed requirements late in the process.
• Spiral: develop in iterations, each with four steps: set objectives, analyse and reduce risks, develop and test, and plan the next loop. The quiz app is built as a prototype, then a version with timers and scoring, then a full version, with risk checks each time.
• Summary: Spiral suits risky or changing projects, Waterfall suits stable requirements, and Big Bang and Code-and-Fix are only for tiny projects.

2021 · Paper-I · Q7(b) · 7 marks

Q. What is a software process model? Explain the spiral model in detail.

Ans.

• A software process model is an abstract description of the activities (requirements, design, coding, testing, maintenance) and their order for building software. Examples: waterfall, incremental, evolutionary, spiral, agile.
• Spiral model (Barry Boehm): a risk-driven model that combines iterative development with waterfall's control. The project moves around a spiral, and each loop has four quadrants:
  • 1. Determine objectives: set goals, alternatives and constraints.
  • 2. Identify and resolve risks: analyse risks and build prototypes to reduce them.
  • 3. Develop and test: build and verify that loop's deliverable.
  • 4. Plan the next iteration: review with the customer and plan the next loop.
• Radius of the spiral shows cost so far, and the angle shows progress.
• Pros: good risk management, suits large and changing projects, early customer feedback.
• Cons: costly, needs risk-analysis skill, and is complex to manage on small projects.