Test-Driven Development (TDD)

Best Practice: Test-Driven Development (TDD)

Test-Driven Development, codified by Kent Beck, is a discipline in which tests drive design. You work in short cycles known as red-green-refactor: write a small failing test (red), write just enough code to make it pass (green), then improve the design while keeping tests passing (refactor). TDD matters because it produces an executable specification, keeps code testable by construction, and gives a tight feedback loop that catches regressions instantly. The result is usually simpler designs, fewer defects, and a comprehensive test suite that accumulates as a by-product of normal work.

Step-by-Step Implementation Guidance

1. Pick the smallest next behavior the code should have.
2. Write a test that asserts that behavior and watch it fail for the right reason.
3. Write the minimal production code needed to pass the test.
4. Run all tests to confirm green.
5. Refactor both test and production code to remove duplication and clarify intent.
6. Commit in small increments so each step is reversible.
7. Repeat, growing functionality one verified behavior at a time.

Common Mistakes Teams Make When Ignoring This Practice

• Writing tests after the fact that merely confirm existing behavior, including bugs.
• Taking steps too large, so a failing test no longer points to one cause.
• Skipping the refactor step, letting design debt accumulate.
• Testing private implementation details that make refactoring painful.
• Mocking everything until tests no longer exercise real logic.

Tools and Techniques That Support This Practice

• Unit frameworks: JUnit, pytest, Jest, Vitest, RSpec, Go testing.
• Fast test runners and watch modes for instant feedback.
• Test doubles libraries: Mockito, unittest.mock, Sinon.
• Coverage tools to spot untested paths: JaCoCo, Coverage.py.
• Continuous integration to run the full suite on every push.

How This Practice Applies to Different Migration Types

• Cloud Migration: Use TDD to drive thin abstractions over cloud services so logic stays portable and tested.
• Database Migration: Write tests for data-access behavior first, then swap the underlying engine with confidence.
• SaaS Migration: TDD the integration adapters so the new provider's behavior is pinned by tests.
• Codebase Migration: Characterize existing behavior with tests, then refactor incrementally under a green bar.

Checklist

• Each new behavior starts with a failing test.
• Production code is written only to pass the current test.
• The full suite is green before moving on.
• Refactoring happens every cycle, not occasionally.
• Tests assert behavior, not private internals.
• Changes are committed in small, reversible steps.
• CI runs all tests on every push.