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Software Development Life Cycle

The software development life cycle organises development into phases; models like Waterfall, V, spiral and Agile differ in how they sequence and iterate those phases to manage risk, change and cost.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Waterfall is sequential; Agile is iterative
  • The spiral model emphasises risk analysis each cycle
  • Prototyping reduces requirements uncertainty

Topic details

Introduction

This Sommerville topic covers process models. You compare Waterfall, the V-model, incremental, spiral and Agile approaches, weigh their suitability against requirement stability and risk, and understand why late defects are so expensive.

Key relations & formulas

Formulas (Indian textbook notation)

  • effortgrowsroughlyassizeαwithα≈1.0−1.2(COCOMO−style)effort grows roughly as size^\alpha with \alpha \approx 1.0-1.2 (COCOMO-style)

Formulas (Indian textbook notation)

  • defect−fixcostrisessharplythelateradefectisfounddefect-fix cost rises sharply the later a defect is found

Formulas (Indian textbook notation)

  • V−model:eachdevelopmentphasehasapairedtestphaseV-model: each development phase has a paired test phase

Notation and sign conventions

Relation 1 —
effortgrowsroughlyassizeαwithα≈1.0−1.2effort grows roughly as size^\alpha with \alpha \approx 1.0-1.2

Formulas (Indian textbook notation)

  • effortgrowsroughlyassizeαwithα≈1.0−1.2(COCOMO−style)effort grows roughly as size^\alpha with \alpha \approx 1.0-1.2 (COCOMO-style)
Write this relation with symbols exactly as in Software Engineering — Roger Pressman before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
defect−fixcostrisessharplythelateradefectisfounddefect-fix cost rises sharply the later a defect is found

Formulas (Indian textbook notation)

  • defect−fixcostrisessharplythelateradefectisfounddefect-fix cost rises sharply the later a defect is found
Write this relation with symbols exactly as in Software Engineering — Roger Pressman before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
V−model:eachdevelopmentphasehasapairedtestphaseV-model: each development phase has a paired test phase

Formulas (Indian textbook notation)

  • V−model:eachdevelopmentphasehasapairedtestphaseV-model: each development phase has a paired test phase
Write this relation with symbols exactly as in Software Engineering — Roger Pressman before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

Every project moves through requirements, design, implementation, testing and maintenance; process models differ in how rigidly and how often those phases run. Waterfall runs them once in sequence, which suits stable, well-understood requirements but handles change poorly. The V-model pairs each development phase with a validation phase to catch defects early — important because the cost of fixing a defect climbs steeply the later it is discovered. Iterative and Agile models embrace change by delivering in small increments with continuous feedback, while the spiral model foregrounds risk, tackling the riskiest elements first in each loop. Choosing a model is really choosing how to cope with uncertainty.

Assumptions and validity limits

State assumptions explicitly before using any relation for software development life cycle — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Software Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Software Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to software development life cycle.
4. Use equation 1:
effortgrowsroughlyassizeαwithα≈1.0−1.2effort grows roughly as size^\alpha with \alpha \approx 1.0-1.2
.
5. Use equation 2:
defect−fixcostrisessharplythelateradefectisfounddefect-fix cost rises sharply the later a defect is found
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Software Development Life Cycle appears in product teams and IT services. In Indian it software curricula this topic is tested because it connects theory to SDLC, requirements, and quality.
GATE and semester exams often combine software development life cycle with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use software development life cycle?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

Students treat Waterfall as always inferior (it fits stable requirements), confuse iterative with incremental delivery, and forget the V-model’s emphasis on early test planning. Ignoring the rising cost of late defects when arguing for a model is a common gap.

Quick revision checklist

Before attempting software development life cycle problems, confirm you can:
1. Waterfall is sequential; Agile is iterative
2. The spiral model emphasises risk analysis each cycle
3. Prototyping reduces requirements uncertainty
Revise the solved examples in Software Engineering — Roger Pressman and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Choosing a process model

Problem

A project has volatile, poorly understood requirements and needs frequent stakeholder feedback. Which model fits and why?

Solution

An Agile/iterative model fits: short increments deliver working software for feedback and absorb changing requirements, whereas Waterfall would lock in requirements too early and handle change poorly.

Conceptual check — Software Development Life Cycle

Problem

In a Software Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of software development life cycle." What should a complete answer include?

📖 Standard books (India)

  • Software Engineering — Roger Pressman

    Read: Syllabus unit

    SDLC, Agile, and software metrics

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