Software Development Life Cycle (SDLC) + Waterfall vs Agile

SDLC — Phases

The Software Development Life Cycle is a structured process for developing software. Its phases:

1. Communication / Requirement Gathering — collecting user & system requirements, feasibility study.
2. Planning — project planning, estimation, scheduling, risk analysis.
3. Modeling / Analysis & Design — analyzing requirements; producing design (architecture, data design, interface design).
4. Construction / Coding — implementation of the design in code (unit testing during coding).
5. Testing — verification & validation, defect fixing (integration, system, acceptance).
6. Deployment / Delivery — installation, release, user training.
7. Maintenance — corrective, adaptive, perfective, preventive changes after delivery.

Any SDLC model arranges these phases in a particular order with particular feedback.

Waterfall Model

• Linear-sequential: each phase must be completed before the next begins; no overlap; document-driven.
• Phases: Requirements → Design → Implementation → Testing → Deployment → Maintenance (with strict phase-exit gates).
• Advantages: simple, disciplined, well-documented; works when requirements are fixed and clear; easy to manage milestones.
• Disadvantages: no feedback loops; late discovery of requirement/design errors is costly; not suitable for uncertain/volatile requirements.

Agile Model

• Iterative-incremental: the product is built in short sprints/iterations (1–4 weeks), each delivering a working increment.
• Principles: customer collaboration over contract negotiation, working software over comprehensive documentation, responding to change over following a plan; continuous feedback, self-organizing teams, daily stand-ups, frequent releases (Scrum, XP, Kanban).
• Advantages: adaptable to changing requirements; early and continuous delivery of value; high customer involvement; risks visible early.
• Disadvantages: less emphasis on documentation; requires close/full-time customer availability; hard to scale/estimate precisely for very large distributed projects.

Comparison table

Aspect	Waterfall	Agile
Approach	Linear sequential	Iterative & incremental
Requirements	Fixed/known upfront	Evolving; change welcomed
Customer involvement	At milestones	Continuous throughout
Delivery	Single release at end	Frequent working increments
Documentation	Heavy/documented	Minimal ("just enough")
Feedback	Late (after phase)	Each iteration
Best for	Stable, well-understood projects	Dynamic, evolving requirements
Risk handling	High risk if delayed discovery	Risks surfaced early per sprint