Testing Strategy (Platform & Tooling Agnostic)

Overview

Establish a pragmatic, layered testing strategy that maximises signal, supports incremental adoption, and ensures failures are diagnosable with minimal noise. Extend functional testing with architecture testing and public API/contract governance so solutions remain maintainable over time.

Testing Skills Decision Tree

Use this decision tree to select the appropriate testing skill:

Start Here
    │
    ├── Defining overall testing strategy?
    │   └── YES → testing-strategy-agnostic (this skill)
    │
    ├── Using .NET?
    │   │
    │   ├── YES → testing-strategy-dotnet (for .NET conventions)
    │   │         │
    │   │         ├── Using .NET Aspire distributed apps?
    │   │         │   └── YES → aspire-integration-testing
    │   │         │
    │   │         └── Need real database/queue/cache in tests?
    │   │             └── YES → testcontainers-integration-tests
    │   │
    │   └── NO → Use this skill with language-specific tooling
    │
    └── Need architecture boundary testing?
        └── YES → architecture-testing

Testing Skills Comparison

Invocation Flow

For a complete .NET testing implementation:

1. •testing-strategy-agnostic - Understand principles (if new to testing strategy)
2. •testing-strategy-dotnet - Apply .NET-specific conventions
3. •testcontainers-integration-tests OR aspire-integration-testing - For integration tests
4. •architecture-testing - For boundary enforcement

Do NOT Use This Skill When

• •You need .NET-specific tooling guidance (use testing-strategy-dotnet)
• •You're implementing Aspire-specific tests (use aspire-integration-testing)
• •You need container-based infrastructure for tests (use testcontainers-integration-tests)

When to Use

• •Defining or modernising a test strategy.
• •Designing CI quality gates (tiers, coverage, architecture constraints, compatibility checks).
• •Reviewing changes that impact solution structure, public interfaces, or integration contracts.
• •Introducing or tightening E2E/system testing and their operational criteria.

Core Workflow

1. •Select appropriate testing skill using the decision tree (this skill for strategy, others for implementation)
2. •Define test tiers: unit tests (isolated, fast), system tests (real wiring, stubbed externals), E2E tests (minimal, high-value)
3. •Establish architecture testing rules for layering, dependencies, and conventions
4. •Define contract versioning and public API governance policies
5. •Configure observability requirements with payload logging constraints
6. •Create quality gates with appropriate thresholds for each tier
7. •Document acceptance criteria using provided templates

Core Principles

• •Layered test pyramid (Unit → System → E2E).
• •Incremental enforcement focused on changed code (and changed contracts/APIs).
• •Repeatability and isolation (test-owned state only).
• •Strict data safety (never mutate non-test-owned data).
• •Observability is testable (diagnosable failures, controlled noise).
• •Architecture is enforceable (structure and dependency rules are verified continuously).
• •Contracts and public APIs are governed (versioning discipline and compatibility checks).

Test Tiers

Unit Tests

• •Validate class- and method-level behaviour.
• •Fully isolated from external I/O.
• •Deterministic and fast.

System Tests

• •Validate component behaviour with real internal wiring.
• •Stub/mock external dependencies only.
• •Validate functional outcomes and operational diagnosability.

E2E Tests

• •Validate end-to-end user journeys.
• •Minimal, high-value scenarios.
• •Strong isolation and cleanup guarantees.

Architecture Testing (Hard Requirements)

Architecture tests enforce solution structure and prevent architectural drift.

What to enforce

• •Layering rules (e.g., UI depends on Application; Application depends on Domain; Domain depends on nothing).
• •Allowed dependencies at package/module boundaries.
• •No cyclic dependencies between modules.
• •Namespace/folder conventions (e.g., vertical slices, bounded contexts, feature folders).
• •Forbidden frameworks in the wrong layer (e.g., no persistence types in Domain).
• •Test project conventions (naming, colocation, allowed references).

Where to place architecture tests

• •Prefer a dedicated test suite (or a dedicated project) that runs as part of PR gates.
• •Keep rules small, explicit, and business-aligned (avoid overly abstract purity constraints).

Incremental enforcement

• •Apply strict architecture rules to new modules and modified boundaries first.
• •Fail fast on new violations, optionally tolerate legacy violations with a tracked baseline until touched.

Contract Versioning & Public Interfaces (Hard Requirements)

Concepts

• •Integration contracts: APIs/events/messages shared between components.
• •Published library public surface: exported types/members that downstream consumers compile against.

Rules

• •Contracts must be versioned explicitly and follow a clear compatibility policy.
• •Breaking changes require:
  • •a new major version (or a new contract version),
  • •clear migration guidance,
  • •and a compatibility window where applicable.
• •PRs must include:
  • •contract change notes (what changed, why),
  • •and evidence of compatibility checks (automated where feasible).

Recommended automated checks

• •API compatibility checks comparing current output to a baseline.
• •Public API surface snapshots (generated lists) to detect accidental exposure.
• •Consumer-driven contract tests where multiple consumers exist (especially for events).
• •Schema linting and backward-compat validation for messages (where applicable).

Observability Requirements (System & E2E)

Minimum Telemetry Bar ("Just Enough")

• •Correlation / trace identifiers
• •Structured logs with operation name and error classification
• •Dependency visibility (name, outcome, failure classification)
• •Actionable diagnostics without payload noise

Payload Logging Constraints (Hard Rule)

• •Full request/response payloads MUST be logged only at Debug or Trace levels.
• •Info/Warn/Error/Critical logs MUST NOT include full payloads.
• •Payload logging must be redacted and gated behind environment-specific log level configuration.

Noise Controls

• •Avoid repeated identical error logs across retries; prefer one summary error with context.
• •Successful scenarios should not emit unexpected Error/Critical logs.

Acceptance Criteria Templates

System Tests

• •Functional behaviour validated.
• •Failures produce structured logs with correlation IDs and error classification.
• •Successful scenarios emit no unexpected Error/Critical logs (unless explicitly expected).
• •Full payloads are absent from Info+ logs; payload detail appears only with Debug/Trace enabled.
• •Where relevant, verifies the component's contract behaviour and version handling.

E2E Tests

• •End-to-end traceability (correlation/trace) across the journey.
• •Diagnosable failures via logs/traces with controlled noise.
• •Never mutates data not created by the test; cleanup is reliable/idempotent.
• •Full payloads are absent from Info+ logs; payload detail appears only with Debug/Trace enabled.
• •Where relevant, validates cross-component contract compatibility and version negotiation/fallback.

Review Heuristics

• •Lowest-cost tier used for the behaviour being proven.
• •Architecture rules: does the change preserve layering and allowed dependencies?
• •Contract discipline: is the contract/public interface change intentional, versioned, and compatible?
• •Observability: are failures diagnosable without payload dumps?
• •Payload discipline: full payloads restricted to Debug/Trace only.

Minimal Baseline Strategy Template

For small repositories or MVP projects, use this streamlined testing strategy:

Small Repo Testing Strategy

# Testing Strategy: [Project Name]

## Scope

[1-2 sentence project description]

## Test Tiers

### Unit Tests (Required)

- **Coverage target**: 70% on business logic
- **Focus**: Domain models, validators, pure functions
- **Exclusions**: Controllers, database access, external integrations

### Integration Tests (Required for APIs)

- **Coverage**: All public API endpoints
- **Approach**: In-memory database or TestContainers
- **Focus**: Request/response contracts, error handling

### E2E Tests (Optional for MVP)

- **Scope**: Critical user journey only (e.g., signup → core action)
- **Frequency**: Run on merge to main, not on every PR

## Quality Gates

| Gate                     | Threshold | Enforcement         |
| ------------------------ | --------- | ------------------- |
| Unit test pass           | 100%      | Block merge         |
| Coverage (changed files) | 70%       | Block merge         |
| Integration tests        | 100% pass | Block merge         |
| E2E tests                | 100% pass | Block merge to main |

## Execution

```bash
# Unit tests (fast, run on every commit)
npm test -- --coverage

# Integration tests (run on PR)
npm run test:integration

# E2E tests (run on merge to main)
npm run test:e2e
```

Evidence Template

When documenting test coverage:

## Test Evidence

- Unit tests: [X/Y passing] ([coverage report link])
- Integration tests: [X/Y passing]
- Changed files coverage: [X]%

When to Upgrade from Minimal

Upgrade to full strategy when ANY of these occur:

• •Team size exceeds 3 developers
• •More than 2 integration points (external APIs, databases)
• •Production incidents related to untested scenarios
• •Code complexity metrics indicate high cyclomatic complexity
• •Contract versioning becomes necessary

Red Flags - STOP

These statements indicate testing strategy anti-patterns: