Testing Fundamentals

What Is Integration Testing?

Integration testing verifies that individual software modules or components work correctly when combined, focusing on the interfaces and interactions between them rather than their internal logic.

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In Depth

Unit tests verify that individual functions work. Integration tests verify that those functions work together. The most common integration points are: API calls between services, database read/write operations, message queue producers and consumers, authentication flows, and third-party service integrations.

There are several approaches to integration testing. Top-down testing starts with high-level modules and uses stubs for lower-level dependencies. Bottom-up testing starts with low-level modules and uses drivers to simulate higher-level callers. Big-bang testing integrates everything at once — fast but makes failures hard to isolate.

In practice, most modern teams write integration tests that focus on specific boundaries: API endpoint tests that hit a real database, service-to-service contract tests, and authentication flow tests. These tests are faster than E2E tests but more realistic than unit tests with mocked dependencies.

Tools for integration testing include Playwright (API testing mode), pytest with database fixtures, Jest with Supertest for Node.js APIs, and contract testing tools like Pact.

Why Interviewers Ask About This

Integration testing is where many production bugs are caught. Interviewers ask about it to assess whether you understand the boundaries between system components and how to test them efficiently.

Example Scenario

A payment service passes all unit tests, but integration testing reveals that the payment provider API returns dates in a different format than expected. The integration test catches this before deployment, preventing failed transactions in production.

Interview Tip

Distinguish integration tests from unit tests (which mock dependencies) and E2E tests (which test the full stack). Give concrete examples of integration points you have tested and explain your strategy for managing test data and external dependencies.