What is the difference between testing a microservices architecture and testing a monolithic architecture?

Testing microservices architecture differs from testing a monolith primarily in the level of distribution and component interaction.

In a monolith, all functions and modules are within a single application, which simplifies:

• Unit testing: you can test functions and classes directly.
• Integration testing: components are tightly coupled and run together.

In microservices:

• Each service is a separate application with its own lifecycle.
• Unit tests remain local to each service.
• Integration testing is complicated due to network calls, asynchrony, and the need to mock or deploy real services.
• It is necessary to test interactions between services (contract testing).
• Testing the resilience to network failures, delays, and individual service failures (fault tolerance testing) is important.

Thus, testing microservices requires more attention to integration, communication, and distributed aspects, whereas in a monolith, the focus is more on internal logic and component cohesion.