integration-e2e-testing

# Integration and E2E Testing Principles

## References

**E2E test design**: See [references/e2e-design.md](references/e2e-design.md) for UI Spec-driven E2E test candidate selection and browser test architecture. The reference uses Playwright as the default browser harness; substitute the project's standard when different.

## Test Type Definition and Limits

| Test Type | Purpose | Scope | External Deps | Limit per Feature | Implementation Timing |
|-----------|---------|-------|---------------|-------------------|----------------------|
| Integration | Verify component interactions in-process | Partial system integration (in-process modules; for UI components, the framework's in-process renderer e.g., RTL+MSW for React/TS) | Mocked or in-process | MAX 3 | Created alongside implementation |
| fixture-e2e | Verify UI behavior in a browser with deterministic fixtures | Full UI flow with mocked backend / fixture-driven state | Mocked / fixture only — no live services | MAX 3 | Created alongside the UI feature |
| service-integration-e2e | Verify critical user journeys against a running local stack | Full system across services | Live local services or stubs | MAX 1-2 | Executed only in the final phase |

**Lane selection (E2E only)**:
- Default lane for user-facing UI journeys is **fixture-e2e** — it runs a real browser against deterministic fixtures, catches the bugs that unit/integration tests miss (button no-op, state never updates, navigation breaks), and runs in CI without infrastructure setup
- Add **service-integration-e2e** only when the journey's correctness depends on real cross-service behavior (data persistence, transactional consistency, external service contracts) that cannot be faked safely

The two E2E lanes are budgeted independently — having a fixture-e2e for a journey does not consume the service-integration-e2e budget and vice versa.

## Behavior-First Principle

### Include (High ROI)
- Business logic correctness (calculations, state transitions, data transformations)
- Data integrity and persistence behavior
- User-visible functionality completeness
- Error handling behavior (what user sees/experiences)

### Redirect to Other Test Types
- External service connections → Verify via contract/interface tests
- Performance metrics → Verify via dedicated load testing
- Implementation details → Verify observable behavior instead
- UI layout specifics → Verify information availability instead

**Principle**: Test = User-observable behavior verifiable in isolated CI environment

## ROI Calculation

ROI is used to **rank candidates within the same test type** (integration candidates against each other, E2E candidates against each other). Cross-type comparison is unnecessary because integration and E2E budgets are selected independently.

```
ROI Score = Business Value × User Frequency + Legal Requirement × 10 + Defect Detection
              (range: 0–120)
```

Higher ROI Score = higher priority within its test type. No normalization or capping is applied — the raw score is used directly for ranking. Deduplication is a separate step that removes candidates entirely; it does not modify scores.

### ROI Thresholds by Lane

The two E2E lanes have very different ownership costs and use independent thresholds.

| Lane | ROI threshold | Rationale |
|------|---------------|-----------|
| fixture-e2e | ROI ≥ 20 (beyond reserved slot) | Cost is comparable to integration tests once the harness exists; the floor avoids filling MAX 3 with low-signal tests when fewer would suffice |
| service-integration-e2e | ROI > 50 (beyond reserved slot) | Creation, execution, and maintenance cost is 3-10× higher than integration; reserve for journeys whose value cannot be proven any other way |

Reserved slot rules (see Multi-Step User Journey Definition below) apply per lane and override the threshold (the reserved candidate is emitted regardless of its ROI score). Below-floor candidates beyond the reserved slot are not emitted, leaving budget intentionally unfilled rather than padding with low-value tests.

### ROI Calculation Examples

| Scenario | BV | Freq | Legal | Defect | ROI Score | Test Type | Selection Outcome |
|----------|----|------|-------|--------|-----------|-----------|-------------------|
| Core checkout UI flow | 10 | 9 | true | 9 | 109 | fixture-e2e | Selected (reserved slot: user-facing multi-step journey, browser-level verification with fixtures) |
| Core checkout against live payment service | 10 | 9 | true | 9 | 109 | service-integration-e2e | Selected (real-service correctness above ROI threshold) |
| Dismiss button updates UI state | 6 | 7 | false | 8 | 50 | fixture-e2e | Selected (rank 2 of 3 fixture-e2e budget) |
| Payment error message display | 5 | 4 | false | 7 | 27 | fixture-e2e | Selected (rank 3 of 3 fixture-e2e budget) |
| Optional filter toggle | 3 | 4 | false | 2 | 14 | fixture-e2e | Not selected (rank 4, budget full) |
| Payment retry against real provider | 8 | 3 | false | 7 | 31 | service-integration-e2e | Below ROI threshold (31 < 50), not selected |
| DB persistence check | 8 | 8 | false | 8 | 72 | Integration | Selected (rank 1 of 3) |
| Pure data transformation | 5 | 3 | false | 4 | 19 | Integration | Selected (rank 2 of 3) |

## Multi-Step User Journey Definition

A feature qualifies as containing a **multi-step user journey** when ALL of the following are true:

1. **2+ distinct interaction boundaries** are traversed in sequence to complete a user goal. What counts as a boundary depends on the system type:
   - Web: distinct routes/pages
   - Mobile native: distinct screens/views
   - CLI: distinct command invocations or interactive prompts
   - API: distinct API calls forming a transaction (e.g., create → confirm → finalize)
2. **State carries across steps** — data produced or actions taken in one step affect what the next step accepts or displays
3. **The journey has a completion point** — a final state the user or caller reaches (e.g., confirmation pa