Software lifecycle models explain how software is planned, built, tested, delivered and maintained. PSC Computer Engineer questions often ask model comparison, when to use waterfall/prototype/agile/spiral and how risk drives process choice.
Engineering Definitions
SDLC
Standard definition: A structured process for planning, developing, testing, deploying and maintaining software.
Exam meaning: Software लाई idea देखि maintenance सम्म व्यवस्थित बनाउने lifecycle।
Waterfall model
Standard definition: A sequential lifecycle model where each phase is completed before the next begins.
Exam meaning: Requirement, design, coding, testing, deployment क्रमशः जाने linear model।
Spiral model
Standard definition: A risk-driven iterative model that combines prototyping and systematic development with risk analysis in each cycle.
Exam meaning: हरेक cycle मा objective, risk analysis, engineering र evaluation गर्ने model।
Prototype
Standard definition: An early working model used to clarify requirements and user feedback.
Exam meaning: Requirement बुझ्न बनाइने early sample system।
Concept Teaching
No lifecycle model is universally best. Stable requirements fit waterfall, uncertain user interfaces benefit from prototyping, changing products fit agile/incremental, and high-risk systems fit spiral because risk is identified and reduced early.
Common SDLC Phases
Most models reorder or repeat these phases.
- Feasibility study checks technical, economic, operational and schedule feasibility.
- Requirements define what the system must do.
- Design defines architecture, database, interfaces and components.
- Implementation converts design to code.
- Testing finds defects and validates requirements.
- Deployment releases system to users.
- Maintenance fixes, adapts and improves the system.
Lifecycle Model Comparison
This table is a high-yield subjective answer.
| Model | Best use | Limitation |
|---|---|---|
| Waterfall | Stable requirements and regulated documentation | Poor flexibility for change |
| V-model | Strong verification/testing alignment | Still rigid |
| Prototype | Unclear UI/requirements | Can create unrealistic expectations |
| Incremental | Deliver in parts | Needs architecture planning |
| Agile | Changing requirements and close user feedback | Needs discipline and active customer |
| Spiral | Large high-risk projects | Complex and costly |
Risk-Driven Process
Risk-driven process chooses activities based on project uncertainties.
- Identify technical, requirement, cost, schedule and people risks.
- Prioritize risks by probability and impact.
- Prototype or experiment to reduce high technical risk.
- Review risk after each iteration.
- Spiral model explicitly places risk analysis in each loop.
Verification and Validation
V&V is a frequent exam distinction.
- Verification asks: are we building the product right?
- Validation asks: are we building the right product?
- Reviews and inspections support verification.
- User acceptance testing supports validation.
- Both are needed across lifecycle.
Engineering Mechanism
- Project begins with feasibility and scope.
- Requirements are collected and validated.
- Model determines sequencing or iteration.
- Design/coding/testing happen once or repeatedly depending model.
- Risk and feedback adjust plan.
- Maintenance continues after deployment.
Diagrams / Models To Draw
- Draw waterfall phase flow.
- Draw V-model with development and test levels.
- Draw spiral quadrants: objectives, risk analysis, engineering, evaluation.
- Draw incremental releases timeline.
Formulas, Algorithms and Rules
- Risk exposure = probability x impact.
- Spiral loop = objectives + risk analysis + development + customer evaluation.
- Verification = product against specification.
- Validation = product against user need.
| Concept | Purpose | Exam trap |
|---|---|---|
| Waterfall | Sequential control | Not good for volatile requirements |
| Prototype | Requirement clarification | Not final-quality system |
| Agile | Iterative customer feedback | Not absence of documentation |
| Spiral | Risk reduction | Not simple waterfall with loops |
| Verification | Specification correctness | Different from validation |
| Validation | User need correctness | Needs user perspective |
Exam Point
- Compare models by requirement stability, risk and customer involvement.
- For spiral, always mention risk analysis.
- Use V&V distinction precisely.
- Do not say agile means no planning.
- Mention maintenance as part of lifecycle.
Worked Example
For an e-governance payment system with uncertain security and integration risks, spiral is better than pure waterfall because each iteration can analyze payment gateway risk, prototype integration, review security and then proceed.
Subjective Answer Pattern
- Define SDLC.
- List phases.
- Compare lifecycle models.
- Explain risk-driven process and spiral.
- Add V&V distinction.
- Conclude with model selection criteria.
Common Engineering Mistakes
- Choosing waterfall for highly changing requirements without caveat.
- Forgetting maintenance phase.
- Confusing prototype with final system.
- Ignoring risk in spiral model.
- Mixing verification and validation.
MCQ Revision
- Which model is risk-driven?
- Waterfall is best for what requirements?
- Verification asks which question?
- Validation asks which question?
- What is risk exposure?
- Which model uses customer feedback frequently?
Final Summary
- SDLC structures software development and maintenance.
- Model selection depends on requirement stability, risk and feedback need.
- Spiral is explicitly risk-driven.
- Agile and incremental deliver iteratively.
- Verification and validation answer different quality questions.