Modern applications rarely run as one tightly connected system. They depend on APIs, databases, microservices, message queues and third-party services. That makes integration testing more important because many failures happen between components rather than inside them.
A practical integration testing guide should therefore cover how these modern systems communicate, fail and recover.
API Integration Testing
API integration testing checks whether connected services exchange the right data and handle responses correctly.
Important checks include:
- Request and response schemas
- Status codes
- Authentication
- Headers
- Required fields
- Invalid input
- Timeouts
- Duplicate requests
For example, an order service might send payment information to a payment API. The test should confirm that the correct amount, currency and customer details are sent, and that the application handles approved, declined and failed responses properly.
A successful 200 response is not always enough. The test should also verify whether the expected data was saved and whether the correct downstream actions happened.
Microservices Integration Testing
Microservices create additional risk because different services can be developed and deployed independently.
One service might update its response format while another still expects the old version. Both services may work correctly on their own, but the integration between them breaks.
Integration tests for microservices should check:
- Service-to-service communication
- API compatibility
- Authentication
- Dependency availability
- Timeouts
- Partial failures
- Version changes Contract testing can also help by checking whether a service still follows the interface expected by its consumers.
Event-Driven Integration Testing
Event-driven systems work differently because the result may not appear immediately.
A typical flow might be:
Order Service → Message Queue → Inventory Service
The test should not simply wait for a fixed number of seconds. It should wait for the expected condition or event.
Important scenarios include:
- Delayed messages
- Duplicate events
- Events arriving out of order
- Failed retries
- Dead-letter queues
- Idempotency
For example, if the same payment event is delivered twice, the system should not create two orders or charge the customer again.
The goal of integration testing in modern systems is not just to prove that services can connect. It is to verify that data remains correct, interfaces stay compatible, failures are handled safely and asynchronous workflows reach the expected state.
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