The V-Model maps every development phase to a corresponding test phase, forming a V shape. The left leg is Verification (are we building the product correctly?) and the right leg is Validation (are we building the correct product?).
The Mapping
| Verification (Left) | Phase | Validation (Right) |
|---|---|---|
| Business requirements analysis | → | Acceptance testing |
| System requirements | → | System testing |
| Architectural design | → | Integration testing |
| Module design | → | Unit testing |
| Coding | (implementation) |
The critical insight: test planning starts at the same time as requirements. The acceptance test plan is written during requirements analysis, not at the end.
Verification vs Validation — Classic Interview Question
| Verification | Validation | |
|---|---|---|
| Question | ”Did we build it right?" | "Did we build the right thing?” |
| Activity | Reviews, walkthroughs, inspections | Actual testing |
| Artifacts | Design docs, requirement specs | Working software |
| When | Before coding | After coding |
When to Use
Best for safety-critical systems where every requirement must be traceable to a test: medical devices, automotive software, aerospace, nuclear controls. The V-Model creates an audit trail that regulatory bodies require.
Q: What’s the advantage of starting test planning during requirements?
A: It forces testability thinking early. If a requirement can’t be tested, it’s a bad requirement. Catching untestable requirements before coding saves enormous cost.
Q: Is the V-Model Waterfall with extra testing?
A: No — it’s a fundamentally different approach. In Waterfall, testing is a single phase at the end. In V-Model, testing is distributed across parallel phases. The test team is involved from day one.
Q: Why doesn’t every project use the V-Model?
A: It’s expensive. Writing test plans for every requirement, maintaining traceability matrices, and performing reviews at every level requires significant overhead. Low-risk projects don’t need this rigor.
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