API Testing Types

A comprehensive visual guide showing the different levels and types of API testing in software development.

Testing Pyramid

The testing pyramid illustrates the recommended distribution of different test types:

1. Unit Tests (Base Layer)

• Purpose: Test individual functions/methods in isolation
• Characteristics:
  • Fastest to execute
  • Most numerous
  • Lowest cost
  • Test single units of code
• Tools: xUnit, NUnit, MSTest, Jest, JUnit

2. Integration Tests (Middle Layer)

• Purpose: Test interactions between components
• Characteristics:
  • Test module combinations
  • Database integration
  • External service mocking
  • Medium execution speed
• Tools: TestContainers, WireMock, Moq

3. End-to-End Tests (Top Layer)

• Purpose: Test complete user workflows
• Characteristics:
  • Slowest to execute
  • Most expensive
  • Fewest in number
  • Test entire system
• Tools: Selenium, Cypress, Playwright

API-Specific Testing Types

Functional Testing

• Validates API endpoints return correct responses
• Tests business logic implementation
• Verifies data transformations

Contract Testing

• Ensures API adheres to specified contracts
• Consumer-driven contract testing
• Prevents breaking changes
• Tools: Pact, Spring Cloud Contract

Load Testing

• Measures performance under expected load
• Identifies bottlenecks
• Validates scalability
• Tools: k6, JMeter, Gatling

Security Testing

• Authentication/Authorization testing
• Input validation
• SQL injection testing
• XSS vulnerability scanning
• Tools: OWASP ZAP, Burp Suite

Smoke Testing

• Quick validation of critical paths
• Run after deployments
• Verify system is operational

Regression Testing

• Ensures new changes don’t break existing functionality
• Automated test suites
• Run before releases

Best Practices

1. Follow the Testing Pyramid: More unit tests, fewer E2E tests
2. Test in Isolation: Use mocks for external dependencies
3. Automate: Integrate tests into CI/CD pipeline
4. Test Edge Cases: Include boundary conditions
5. Maintain Test Data: Use consistent, reproducible test data
6. Document Tests: Clear test names and assertions

C# Example - Testing Layers

// Unit Test
[Fact]
public void CalculateTotal_WithValidItems_ReturnsCorrectSum()
{
    var calculator = new OrderCalculator();
    var result = calculator.CalculateTotal(items);
    Assert.Equal(150.00m, result);
}

// Integration Test
[Fact]
public async Task GetOrder_WithValidId_ReturnsOrder()
{
    await using var application = new WebApplicationFactory<Program>();
    var client = application.CreateClient();

    var response = await client.GetAsync("/api/orders/1");

    response.EnsureSuccessStatusCode();
}

// E2E Test (using Playwright)
[Fact]
public async Task UserCanCompleteCheckout()
{
    await Page.GotoAsync("/products");
    await Page.ClickAsync("[data-testid='add-to-cart']");
    await Page.ClickAsync("[data-testid='checkout']");
    await Expect(Page.Locator(".confirmation")).ToBeVisibleAsync();
}

Sources

• Arhitectura/api testing types.gif