Software Dev Models & Methodologies: Waterfall, Agile, Spiral, V-Model, RAD, Prototyping

Core Concepts: What is a Software Development Methodology?

A software development methodology is a structured framework that guides the planning, design, implementation, testing, and maintenance of software. It defines:

• Process flow (sequential vs. iterative)
• Roles & responsibilities (who does what)
• Deliverables (documents, code, tests)
• Tools & techniques (e.g., UML, Agile boards)

Why do we need methodologies? Without a methodology, projects risk:

• Scope creep (uncontrolled feature additions)
• Missed deadlines (no clear milestones)
• Poor quality (no systematic testing)
• Budget overruns (no cost estimation)

stateDiagram-v2
    [*] --> MethodologyChoice: Start
    MethodologyChoice --> Waterfall: Sequential, rigid
    MethodologyChoice --> Agile: Iterative, flexible
    MethodologyChoice --> Spiral: Risk-driven, iterative
    MethodologyChoice --> VModel: Verification-focused
    MethodologyChoice --> RAD: User-driven, fast
    MethodologyChoice --> Prototyping: User feedback early
    Waterfall --> Design --> Implementation --> Testing --> Maintenance
    Agile --> Sprint1: 2-4 weeks
    Sprint1 --> Review --> Sprint2
    Spiral --> Plan --> RiskAnalysis --> Develop --> Evaluate
    RAD --> JointApplicationDesign --> Prototyping --> Testing
    Prototyping --> BuildPrototype --> UserFeedback --> Refine

1. Waterfall Model: The Classic Sequential Approach

Definition: A linear, phase-based model where each phase must be completed before the next begins. Phases:

1. Requirements
2. Design
3. Implementation
4. Testing
5. Deployment
6. Maintenance

How It Works (Trace Example: NTC’s Online Payment System)

Assume NTC wants to build a system for online bill payments. Here’s how Waterfall applies:

Advantages

• Simple to understand and manage.
• Clear milestones (easy for stakeholders).
• Works well for small, well-defined projects (e.g., a bank’s loan calculator).

Disadvantages

• No going back: If requirements change mid-project, costly.
• Late testing: Bugs found only in the testing phase.
• Not flexible: Poor for dynamic environments (e.g., Daraz’s daily sales).

Phases of the Waterfall Model with arrows showing sequential flow (Image: Peter Kemp / Paul Smith, CC BY 3.0, via Wikimedia Commons)

2. Agile Methodology: Iterative and Flexible

Definition: A non-linear, iterative approach where work is divided into small sprints (2–4 weeks). Key principles (from the Agile Manifesto):

1. Individuals and interactions > Processes and tools.
2. Working software > Comprehensive documentation.
3. Customer collaboration > Contract negotiation.
4. Responding to change > Following a plan.

Core Practices in Agile

Agile Frameworks

Worked Example: Agile vs. Waterfall for eSewa

3. Spiral Model: Risk-Driven Iterations

Definition: Combines Waterfall’s structure with Agile’s iterations, focusing on risk analysis at each cycle. Best for high-risk projects (e.g., medical software, space systems).

4 Phases per Spiral Cycle

flowchart TD
    A["Plan"] --> B["Risk Analysis"]
    B --> C["Engineering"]
    C --> D["Evaluate"]
    D -->|"Repeat"| A

Example: NEPSE Trading System

Advantages

• Handles uncertainty well (e.g., regulatory changes in banking).
• Early risk identification (e.g., Ncell’s 4G rollout risks).
• Flexible (can pivot based on feedback).

Disadvantages

• Complex to manage (requires expert risk analysts).
• Costly (multiple prototypes).

4. V-Model: Verification and Validation

Definition: An extension of Waterfall with testing phases aligned with development phases. Each development phase has a corresponding testing phase.

flowchart TD
    A["Requirements"] --> B["System Design"]
    B --> C["Architecture Design"]
    C --> D["Module Design"]
    D --> E["Implementation"]
    E --> F["Unit Testing"]
    F --> G["Integration Testing"]
    G --> H["System Testing"]
    H --> I["Acceptance Testing"]
    I --> J["Maintenance"]

Example: Daraz Order Fulfillment System

Why Use V-Model?

• Early testing: Bugs found in design phase (not coding).
• Traceability: Each requirement maps to a test case.
• Regulatory compliance: Critical for banks, healthcare (e.g., CIBIL credit score system).

5. Rapid Application Development (RAD)

Definition: Fast delivery using prototyping, reusable components, and user feedback. Ideal for short-term projects (e.g., a university exam portal).

Key Techniques

1. Joint Application Design (JAD): Workshops with users to define requirements.
2. Prototyping: Build a throwaway model to validate ideas.
3. Reusable Components: Use libraries (e.g., Bootstrap for UI).

Example: Kathmandu Traffic Management App

Advantages

• Speed: Deliver in weeks/months (vs. years in Waterfall).
• User-centric: Constant feedback (e.g., Pathao’s ride-sharing).

Disadvantages

• High initial cost (prototyping tools).
• Not scalable for large systems (e.g., Ncell’s core network).

6. Prototyping Model: Build → Test → Refine

Definition: Build a working model early, get user feedback, and refine. Used for UI/UX-heavy apps (e.g., eSewa’s new feature).

Types of Prototypes

Example: Khalti’s New Feature (Auto-Pay Bills)

1. Build: Create a prototype showing "Auto-pay electricity bill every month."
2. Test: Show to 50 users → "Add a reminder notification."
3. Refine: Update prototype → "Now includes SMS alerts."

In the Real World

1. eSewa: Agile + Prototyping

• Problem: Needed to add QR code payments quickly.
• Solution: Used Agile sprints (2-week cycles) and high-fidelity prototypes to test with merchants.
• Result: Feature launched in 3 months (vs. 1 year with Waterfall).

2. Daraz: RAD for Flash Sales

• Problem: Daily sales require fast UI updates.
• Solution: RAD model with reusable components (e.g., discount banner templates).
• Result: New sale pages deployed in hours.

3. Ncell: Spiral Model for 5G Rollout

• Problem: High risk of network congestion in Kathmandu.
• Solution: Spiral cycles to test 5G in Thapathali before city-wide rollout.
• Result: Identified interference issues early and fixed them.

4. NTC: Waterfall for Billing System

• Why Waterfall?: Requirements were stable (no frequent changes).
• Outcome: System built in 18 months with no major rework.

Cost Estimation: COCOMO Model

The Constructive Cost Model (COCOMO) estimates effort, cost, and schedule based on lines of code (LOC) and project size.

3 COCOMO Variants

COCOMO II (Modern Version)

COCOMO II has 3 project classes:

1. Organic: Small team, familiar tech (e.g., a bank’s loan app).
2. Semi-Detached: Medium complexity (e.g., Daraz’s logistics tracker).
3. Embedded: High complexity (e.g., Ncell’s core network).

Worked Example: Estimating a Pathao Driver App

Interpretation:

• Team size: 15 devs working for 12 months.
• Cost: If salary = $500/month → $90,000.

Comparison Table: Methodologies

Exam Tip

What Examiners Look For

1. Definitions: Know the key phases of each model (e.g., Waterfall’s 6 phases).
2. Comparisons: Be ready to contrast Agile vs. Waterfall (flexibility, testing, risk).
3. Real-World Links: Connect models to Nepali tech (e.g., "Why would Daraz use RAD?").
4. Diagrams: Draw flowcharts (e.g., Spiral model) or V-Model alignment.
5. COCOMO: Memorize the formula and EAF factors (e.g., team experience = 1.1).
6. Prototyping: Explain paper vs. high-fidelity prototypes with examples.

Common Mistakes to Avoid

• Mixing terms: Don’t say "Agile is Waterfall with iterations."
• Ignoring context: Always justify why a model is chosen (e.g., "Spiral for high-risk projects").
• Skipping math: In COCOMO, show calculations (even if simplified).
• Overlooking tools: Mention Jira for Agile, UML for Waterfall.

Practice Questions (Self-Check)

1. Short Answer:

   • Differentiate cohesion and coupling in modular design.
   • List 3 cost drivers in COCOMO II.

2. Long Answer:

   • Explain how Pathao would use Agile to develop a new feature (e.g., "Auto-fare split for group rides"). Include sprints, backlog, and risks.
   • Draw the V-Model for a bank’s ATM system and label all phases.

3. Scenario-Based:

   • NTC wants to build a real-time traffic monitoring system. Which model would you choose? Justify with advantages/disadvantages of your choice.