Integration and E2E Testing Principles
References
E2E test design: See 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:
- 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)
- State carries across steps — data produced or actions taken in one step affect what the next step accepts or displays
- The journey has a completion point — a final state the user or caller reaches (e.g., confirmation page, saved record, API success response, completed workflow)
User-Facing vs Service-Internal Journeys
Multi-step journeys are classified for reserved-slot eligibility:
| Classification | Condition | Reserved Slot Eligibility | Example |
|---|---|---|---|
| User-facing | A human user directly triggers and observes the steps (via UI, CLI, or direct API interaction) | Eligible — defaults to fixture-e2e reserved slot. Add a service-integration-e2e reserved slot only when the journey's correctness depends on real cross-service behavior | Web checkout flow, CLI setup wizard, mobile onboarding |
| Service-internal | Steps are triggered by backend services without direct user interaction | Not eligible for reserved slot — use integration tests. Service-integration-e2e through normal ROI > 50 path is still valid when full-system verification is warranted | Async job pipeline, service-to-service saga, scheduled batch processing |
This classification applies only to the reserved-slot rule and the E2E Gap Check. Other selection follows lane-specific ROI rules above.
Use this definition when evaluating E2E test candidates and E2E gap detection.
Test Skeleton Specification
Required Comment Patterns
Each test MUST include the following annotations:
AC: [Original acceptance criteria text]
Behavior: [Trigger] → [Process] → [Observable Result]
@category: core-functionality | integration | edge-case | fixture-e2e | service-integration-e2e
@lane: integration | fixture-e2e | service-integration-e2e
@dependency: none | [component names] | full-system
@complexity: low | medium | high
ROI: [score]
@lane selection rule:
integration— Component interaction in-process, no browser (e.g., RTL+MSW for React/TS, in-process module/handler integration in any language)fixture-e2e— Browser-level UI verification with mocked backend / fixture-driven stateservice-integration-e2e— Browser-level or end-to-end verification against running local s