SDLC Models Compared: Waterfall vs Spiral vs Agile Scrum with TU Project Case Studies
Author: Bhuban Subedi | Subject: Software Engineering (CACS301) | Semester: Fifth Semester
Building enterprise software without a disciplined architectural methodology is a recipe for missed deadlines, budget overruns, and catastrophic system failures.
The Software Development Life Cycle (SDLC) provides a structured framework outlining each phase of software creation—from initial feasibility study and requirements elicitation to system design, coding, testing, release, and maintenance.
In the Tribhuvan University (TU) BCA Fifth Semester Software Engineering (CACS301) board examination, questions comparing Waterfall vs Agile Scrum, explaining the Risk-Driven Spiral Model, or selecting an SDLC for a university capstone project appear as guaranteed 10-mark long questions.
In this guide, we will break down all 6 SDLC phases, compare the major process models, and see how Agile Scrum is applied in modern software workflows.
1. The 6 Universal Phases of SDLC
Every software process model, regardless of whether it is traditional or modern, encompasses these 6 fundamental phases:
+-------------------------------------------------------------------------------+
| THE 6 CORE PHASES OF SDLC |
| |
| [ 1. Planning & Requirement Analysis ] --> Feasibility, SRS Documentation |
| | |
| v |
| [ 2. System Architecture & Design ] --> DFD, ERD, Class & Use Case Models|
| | |
| v |
| [ 3. Implementation & Coding ] --> Writing clean, modular source code|
| | |
| v |
| [ 4. Testing & Quality Assurance ] --> Unit, Integration & System Tests |
| | |
| v |
| [ 5. Deployment & Release ] --> Cloud hosting, CI/CD pipelines |
| | |
| v |
| [ 6. Operations & Maintenance ] --> Bug patches & feature updates |
+-------------------------------------------------------------------------------+
2. Classical Waterfall Model (Linear Sequential)
The Waterfall Model was the earliest formalized process model. It follows a strictly sequential, phase-by-phase progression where a phase begins only after the preceding phase is 100% complete and signed off.
[ Requirements ]
\
v
[ System Design ]
\
v
[ Implementation ]
\
v
[ Testing & QA ]
\
v
[ Deployment & Maint. ]
Pros:
- Simple to understand, manage, and schedule with clear milestones.
- Rigid stage gates ensure comprehensive documentation at every step.
Major Cons:
- Zero Flexibility for Requirement Changes: Once design starts, going back to alter requirements is prohibitively expensive.
- Late Working Software: Working software is not visible until the very end of the cycle.
- High Risk of Failure: If user requirements were misunderstood during step 1, the flaw is only discovered after full deployment.
3. The Spiral Model (Risk-Driven Process)
Proposed by Barry Boehm, the Spiral Model is a meta-model combining iterative development with systematic Risk Analysis at each cycle.
+---------------------------+
| I. Determine Objectives, |
| Alternatives & Bounds |
+-------------+-------------+
|
v
+---------------------------+ +---------------+ +---------------------------+
| IV. Plan Next Phases | <--| (Spiral Loops)|--> | II. Identify and Resolve |
| | +---------------+ | Risks (Prototyping) |
+---------------------------+ +---------------------------+
|
v
+-------------+-------------+
| III. Develop, Verify and |
| Test Next-Level Code |
+---------------------------+
When to Use Spiral:
- Mission-critical, high-budget enterprise systems (e.g., aerospace control systems, core banking engines) where risk assessment is paramount.
4. Agile Scrum Framework (Iterative & Incremental)
Agile is a mindset guided by the Agile Manifesto (prioritizing working software and customer collaboration over comprehensive documentation and rigid plans). Scrum is the most popular Agile implementation framework.
Product Backlog --> Sprint Planning --> 2-4 Week Sprint --> Working Product
(User Stories) (Sprint Backlog) (Daily Standups) Increment
|
v
Sprint Retrospective
Core Scrum Roles & Artifacts:
- Product Owner (PO): Defines user requirements and maintains the prioritized Product Backlog.
- Scrum Master: Facilitates daily ceremonies, removes technical blockers, and shields the team from external distractions.
- Developers / Engineering Team: Cross-functional team executing sprint tasks.
- Sprint (1 to 4 Weeks): A fixed timebox during which a usable, tested product increment is built.
- Daily Standup (15 Mins): Daily synchronization meeting answering: What did I do yesterday? What will I do today? Are there any blockers?
5. Comprehensive Comparison Matrix
| Parameter | Waterfall Model | Spiral Model | Agile Scrum Framework |
|---|---|---|---|
| Development Approach | Linear Sequential | Iterative & Risk-Driven | Iterative & Incremental |
| Flexibility to Changes | Rigid / Very Low | Moderate | Extremely High |
| Risk Management | Low (addressed at end) | Highest (Built-in) | High (reviewed every Sprint) |
| Working Software Delivery | Only at the end | Incremental after cycles | Delivered every 2-4 weeks |
| Customer Involvement | Only at Start and End | Regular reviews | Continuous collaboration |
| Cost & Overhead | Low management cost | High (Risk experts needed) | Moderate |
Frequently Asked Questions (FAQ)
Q1: What SDLC model should I select for my TU BCA Final Project?
For academic projects where requirements evolve and you must demonstrate continuous progress to your supervisor, the Agile / Iterative Prototyping model is strongly recommended. It allows you to build a working MVP (Minimum Viable Product) early and refine features over subsequent iterations.
Q2: What is an SRS (Software Requirements Specification) document?
An SRS is a formal document that completely describes what the proposed software system will do, outlining all Functional Requirements (user capabilities) and Non-Functional Requirements (security, latency, scalability, platform compatibility).
