Waterfall Model

The Waterfall Model is the earliest SDLC model, proposed by Winston Royce in 1970 (ironically, Royce himself criticized it — he intended it only as a straw man for iterative models). It follows strict linear-sequential phases where each phase must be complete before the next begins.

The Six Phases (in order)

1. Requirement Analysis — gather and document all requirements in the SRS. No phase can start until this is signed off.
2. System Design — create HLD and LLD based on the frozen requirements.
3. Implementation — write code per the design. No design changes mid-coding.
4. Testing — verify the code against requirements. If a bug traces back to a design flaw, you’re in trouble — you have to go all the way back.
5. Deployment — deliver to production.
6. Maintenance — fix post-release issues.

When to Use Waterfall

Good for	Bad for
Fixed, well-understood requirements	Evolving requirements
Regulated industries (FDA, DO-178C)	Startups discovering product-market fit
Short, simple projects	Long projects where requirements drift
Contractual fixed-price projects	Projects needing early user feedback

The Fundamental Problem

The user sees the software only at phase 5 (deployment). If the requirements were wrong, you waste months building the wrong product. This is called “late feedback” — the worst kind because it’s expensive to fix.