Unit Tests vs Integration Tests

How to Test Software Well

A simple rule of thumb for effective testing:

• Maximize unit tests — cheap, fast, and ideal for catching logical errors.
• Minimize integration tests — slow, expensive, and best for validating contracts and state changes.

Write maximal unit tests to catch all sorts of logical failures. Write just enough minimal integration tests to ensure valid contracts and correct cross-system (incl. stateful) behaviour.

What Unit Tests Are For

Unit tests verify all things within the code and business logic. This includes:

• validations
• code rules
• mocked database calls
• algorithms
• any logic that can fail without external systems

Any logical failure can and should be caught by the right unit test. These tests are deterministic when built correctly with mocks, and they run fast. They are the preferred tool for validating the core behaviour of your code.

What Integration Tests Are For

Integration tests are best suited for testing moving parts such as:

• live application → database interactions
• contract validation
• cross-system data flow
• state changes across components
• network behaviour
• probable performance or latency issues

It does not make sense to test all combinations of a variable inside a loop using an integration test. Logical correctness belongs in unit tests. Integration tests confirm that the machinery works together under real conditions.

Rule of Thumb

• Unit Tests: fast, cheap, numerous, covering all business logic via mocks.
• Integration Tests: slow, expensive, few, covering cross-system concerns only.

Maximize unit tests for logic. Minimize integration tests for contracts and state.

What Works Best in Practice

The best approach is a combination of both:

• High-coverage unit tests using mocks to verify all business logic.
• A small set of simple, effective integration tests to cover the complete flow across dependent systems like databases and caches.

Unit tests validate logic. Integration tests validate systems working together.