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Waterfall model

Overview​

The waterfall model sequences SDLC phases so each stage largely completes before the next begins: requirements → design → implementation → verification → maintenance. It works best when requirements are stable and well understood upfront (e.g. regulated hardware-software projects); it struggles with volatile product discovery.

Key concepts​

  • Phase gates — Formal approvals between stages.
  • Documentation-heavy — Contracts and specs anchor scope.
  • Late integration risk — Long gaps before end-to-end validation.
  • Change cost — Rework is expensive after design sign-off.
  • When to use — Fixed-scope procurements, compliance checkpoints.

Waterfall sequence​

Sample: phase exit criteria (illustrative)​

  • Requirements — Signed SRS with traceable acceptance tests drafted.
  • Design — Architecture review passed; interfaces frozen for build.
  • Implementation — Code complete against frozen design; unit tests ≥ agreed coverage.

References​