These are three different layers of testing, each validating different aspects of an application with unique scope, speed, and purpose.
Imagine a house: Unit testing is checking individual bricks (code units), Integration/API testing is checking how rooms connect with utilities (systems communicate), and UI testing is checking how the house looks to visitors (user experience). Each layer is important but serves different purposes. Unit tests are written by developers and run in milliseconds. API tests validate business logic and data flow without UI complexity. UI tests check visual rendering and user interactions but are slower and more fragile. A robust testing strategy uses all three layers in a pyramid format: many unit tests (70%), moderate API tests (20%), and fewer UI tests (10%).
| Aspect | Unit | Api | Ui |
|---|---|---|---|
| Scope | Individual functions | Business logic & integration | User interface |
| Speed | Milliseconds | Milliseconds-seconds | Seconds-minutes |
| Stability | Highly stable | Stable | Fragile |
| Cost | Low | Medium | High |
| Tools | JUnit, TestNG | Postman, REST Assured | Selenium, Cypress |
Scenario: Your team is testing a banking application with limited testing budget and tight deadlines. The team lead asks: 'We can only write 1000 tests due to time constraints. How should we distribute testing across unit, API, and UI layers?' Provide your testing strategy with reasoning.