Test Strategy

Systematic approach to test planning: pyramid structure (70% unit, 20% integration, 10% E2E), coverage targets (70-80%), and risk-based testing.

Context

You are planning test coverage for a feature. Your role is to:

• Design test pyramid: unit (fast, isolated) > integration (slower, real) > E2E (slowest, brittle)
• Identify high-risk code needing extra testing
• Set realistic coverage targets
• Balance speed (fast feedback) vs confidence (comprehensive testing)
• Avoid the "test everything" trap

Good testing builds confidence, not false security. 100% coverage often means testing trivia instead of business logic.

Domain Context

Based on Martin Fowler, Google Testing, and J.B. Rainsberger:

• Test Pyramid: Most tests at bottom (unit), fewer at middle (integration), few at top (E2E)
• Coverage Target: 70-80% code coverage sufficient; 100% is waste
• Business Value: Test what users care about; not every code path
• High-Risk Code: Auth, payment, data integrity → extra testing
• Speed: Unit tests <100ms; integration <1s; E2E <10s each

When to Use This Skill

• Planning tests for a new feature
• Adding tests to existing codebase
• Auditing test coverage (too little or too much)
• Improving test speed (too slow)
• Preparing for high-risk deployments

Prerequisites

• Understand feature requirements and acceptance criteria
• Know the domain (what's valuable to test vs trivial)
• Have testing tools/frameworks available
• Can run tests locally in <5 seconds

Instructions (Detailed)

1. Define Layers

Unit Tests (70% of tests):

• Single function/method in isolation
• Mock external dependencies
• Run in <1ms each
• Example: test_calculate_discount_rate_returns_0_05_for_vip()

Integration Tests (20% of tests):

• Component interaction with real dependencies
• Database, file system, real (test) services
• Run in <1s each
• Example: test_user_registration_saves_to_database()

E2E Tests (10% of tests):

• Full flow through system (UI → API → database)
• Real browser/client, real servers
• Run in <10s each
• Example: test_checkout_flow_creates_order_and_sends_confirmation()

2. Identify High-Risk Code

Ask: "What breaks the business?"

High-risk areas:

• Authentication/Authorization: Wrong user gets access
• Payment Processing: Charges wrong amount or double-charges
• Data Integrity: Corrupt or lost data
• Critical Flows: Core business logic
• Security: SQL injection, XSS, secrets leaked

Low-risk areas:

• Formatting (display logic)
• Trivial getters/setters
• Framework boilerplate
• Error messages (non-security)

3. Set Coverage Target

Realistic targets:

• High-risk code: 90%+ coverage
• Core logic: 80%+ coverage
• General code: 70-80% coverage
• Framework code: 50% coverage (don't over-test)

Total target: 70-80% for whole codebase.

Don't chase 100%. Diminishing returns: 80% coverage catches 95% of bugs; last 20% coverage catches 4% of bugs but takes 50% of effort.

4. Plan Unit Tests

For each function/method:

• Happy path: Normal input, expected output
• Edge cases: Boundary values (0, 1, -1, max, min)
• Error cases: Invalid input, exceptions

Example (Order::apply_discount):

• Happy path: valid discount code → correct reduction
• Edge case: discount = 100% → total = $0
• Error case: invalid code → raises exception

5. Plan Integration Tests

For each business flow:

• Happy path: Complete flow succeeds
• Dependency failure: Database down, service timeout
• Data consistency: Changes persist correctly

Example (User registration):

• Happy path: register → user saved → email sent
• Database failure: registration fails gracefully
• Duplicate email: rejected with clear error

6. Plan E2E Tests

For critical user journeys only:

• Checkout flow: Add to cart → checkout → payment → confirmation
• Login flow: Enter credentials → authorize → redirect to dashboard
• Report generation: Select criteria → generate → download

Avoid: Every button click, every error message, every combination. E2E tests are brittle and slow.

7. Define Test Data Strategy

• Fixtures: Reusable test data (predefined users, products)
• Factories: Generate test data on-the-fly
• Mocks: Fake dependencies
• Test Databases: Real database with test schema

8. Document Coverage Goals

Create a matrix showing coverage targets:

Area	Unit	Integration	E2E	Coverage Target
User registration	10	3	1	90%
Payment processing	15	5	1	95%
Reporting	5	2	1	70%

Output Format

When planning tests, deliver:

1. Test Pyramid: Counts for unit/integration/E2E
2. Coverage Targets: By risk area
3. Unit Test Plan: What functions, what cases
4. Integration Test Plan: What flows, what dependencies
5. E2E Test Plan: Which user journeys
6. Test Data: How to set up test data
7. Success Criteria: What passing tests prove

Worked Example: E-Commerce Order Feature

Feature: Users can create orders with discount codes, apply taxes, and process payments.

Test Pyramid

E2E (10%): 2 tests
- Checkout flow end-to-end
- Order with discount code

Integration (20%): 8 tests
- Order creation saves to DB
- Payment gateway integration
- Email notifications sent

Unit (70%): 28 tests
- Discount calculation
- Tax calculation
- Validation (email, quantity, amount)
- Payment amount calculation

Coverage Targets

Area	Target	Rationale
Discount logic	95%	High-risk: affects revenue
Tax logic	90%	High-risk: affects revenue
Payment integration	95%	High-risk: money involved
Order persistence	85%	Medium-risk: data integrity
Email notifications	60%	Low-risk: cosmetic
Error handling	80%	Medium-risk: user experience

Overall target: 80% code coverage

Unit Tests (28 tests)

Discount Calculation:

• test_calculate_discount_returns_0_05_for_vip_code()
• test_calculate_discount_returns_0_10_for_premium_code()
• test_calculate_discount_raises_for_invalid_code()
• test_calculate_discount_returns_0_for_no_code()
• test_calculate_discount_handles_max_discount()

Tax Calculation:

• test_calculate_tax_returns_correct_rate()
• test_calculate_tax_zero_for_tax_exempt_item()
• test_calculate_tax_accumulates_for_multiple_items()
• test_calculate_tax_rounds_correctly()

Validation:

• test_validate_email_rejects_invalid_format()
• test_validate_quantity_rejects_zero()
• test_validate_quantity_rejects_negative()
• test_validate_amount_rejects_zero()
• test_validate_amount_rejects_negative()

...13 more unit tests

Integration Tests (8 tests)

• test_create_order_saves_to_database()
• test_order_persistence_retrieves_correct_data()
• test_payment_gateway_processes_valid_payment()
• test_payment_gateway_rejects_invalid_card()
• test_payment_failure_rolls_back_order()
• test_email_service_sends_order_confirmation()
• test_email_service_sends_payment_receipt()
• test_concurrent_orders_dont_interfere()

E2E Tests (2 tests)

• test_complete_checkout_flow() — Cart → Payment → Confirmation
• test_checkout_with_discount_code() — Apply code → discounted price → success

Decision Framework

• If mutation score is low: Add more unit tests (logic isn't well-tested)
• If tests are slow: Move integration tests to unit (mock dependencies)
• If coverage is >85%: Stop chasing coverage; focus on risk-based testing
• If feature touches payment/auth: Increase coverage to 90%+
• If flaky tests: Too much E2E; move to integration/unit tests

Anti-Patterns

• 100% Coverage Goal: Wastes effort on trivial code; stops finding real bugs
• Only E2E Tests: Slow, brittle, hard to debug. Prefer unit tests
• Testing Implementation, Not Behavior: Tests break when refactoring, not when behavior changes
• No High-Risk Focus: Spend effort on what matters (payments, auth); less on formatting
• Massive Test Data Setup: Tests should be self-contained; use factories
• Ignoring Flakiness: Flaky tests are worse than none (erode trust)

Quality Checklist

• Pyramid structure: 70% unit, 20% integration, 10% E2E
• Coverage targets: 70-80% overall, 90%+ for high-risk
• High-risk code identified (auth, payment, data)
• Each layer tested: happy path + edge cases + errors
• Tests are deterministic (no flakiness)
• Test data is repeatable and isolated
• Tests document expected behavior
• Performance: unit <100ms, integration <1s, E2E <10s
• No over-testing of trivial code
• Success criteria defined (what passing tests prove)

Further Reading

• Martin Fowler, Test Pyramid (blog post)
• J.B. Rainsberger, Integrated Tests Are a Scam (conference talk)
• Google, Testing on the Toilet (testing best practices)
• Kent Beck, Test-Driven Development: By Example
• Roy Osherove, The Art of Software Testing