layered-testing

name	layered-testing
category	testing
description	Design and implement tests at multiple layers following the project's layered testing strategy.

When to Use This Skill

• When creating tests for any new feature or work item
• When establishing test infrastructure for a project
• When evaluating test coverage of existing code

Layered Testing Approach

Unit Tests

• Purpose: Verify isolated logic and pure functions
• Scope: Single function or class
• Speed: Must be fast (< 100ms per test)
• Isolation: Mock external dependencies, test logic only
• When: Always — every piece of logic gets unit tests

Integration Tests

• Purpose: Validate component boundaries and contracts
• Scope: Module interactions, API contracts, database queries
• Speed: Can be slower but should use in-memory alternatives where possible
• Isolation: Real components, mocked external services
• When: When components interact across boundaries

E2E Tests

• Purpose: Confirm critical user flows work end-to-end
• Scope: Full system paths from input to output
• Speed: Accept slower execution for confidence
• Isolation: Real system, may use test fixtures
• When: For critical user-facing flows

Explorative Tests

• Purpose: Discover edge cases through property-based or fuzz testing
• Scope: Domain-specific risk areas (parsing, crypto, data transforms)
• Speed: May run many iterations
• When: When the domain has complex input spaces

Test Quality Checklist

1. Tests validate behavior, not implementation details
2. Test names read like specifications
3. Arrange-Act-Assert structure
4. Descriptive assertions (not just truthy/falsy)
5. Independent tests (no shared mutable state)
6. Deterministic (no flaky tests)
7. Edge cases covered (empty, null, boundary values)
8. Error scenarios tested explicitly