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Learn how to effectively mock the `Eureka` service in your Spring integration tests using `WireMock` and client-side load balancing techniques.
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Mocking Eureka in Spring Integration Tests with WireMock

When developing microservices using Spring Cloud, integrating with service discovery tools like Eureka is common practice. However, while writing integration tests, you might face challenges due to the way services interact through Eureka. Specifically, if you have a web service (let's call it Service A) that makes a call to another service (Service B) registered with Eureka, it can be tricky to effectively mock the requests to Service B. This guide explores how to mock such calls in integration tests using WireMock and a client-side load balancing technique.

The Problem

You have a web service A that calls Service B using OpenFeign, and Service B is registered with Eureka. Here's a snippet showing how the OpenFeign client is configured:

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In the testing environment, when trying to call the /session/get endpoint, you encounter the following error:

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The main challenge arises because the OpenFeign client is trying to resolve the service name (SESSION-MANAGER) in Eureka, but, for testing purposes, you might not have Eureka running, leading to unresolved host exceptions.

The Solution

To address this issue, you can utilize a combination of WireMock and Spring's LoadBalancerClient. Here's how to effectively mock the Eureka service calls in your integration tests.

Step 1: Create a Client-Side Load Balancer

You need to create a custom LoadBalancerClient that can direct requests to a preconfigured HTTP service (your mock service). This is done by defining a bean that simulates the registered service instances.

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In this snippet, replace LOAD_BALANCER_PORT with the port where the WireMock server will be running.

Step 2: Use the LoadBalancerClient in Your Tests

Now, you can annotate your integration test class to utilize the custom load balancer configuration.

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Step 3: Configure WireMock Stubs

Next, you need to configure WireMock to handle the incoming requests to the /session/get endpoint. Here's an example of how to set up a stub for the mock service.

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This configuration allows WireMock to respond to requests that are processed by OpenFeign during your tests.

Conclusion

Mocking Eureka interactions in Spring integration tests can seem daunting at first, but by leveraging a client-side load balancer and WireMock, you can create a controlled environment for your tests. This setup avoids making real network calls and allows you to focus on validating your application's logic without the overhead of external dependencies.

By implementing the above steps, you’ll ensure that your integration tests remain stable and reliable, enabling efficient and effective testing practices in your Spring microservices architecture.