A simple service using WebClient

The purpose of this project is to show-case, with a simple example, how to use WebClient (Spring Framework's reactive implementation of a http client) to communicate to an external service in an asynchronous manner.

While keeping it simple, the service using WebClient implements retries, error handling and other features, to show how to peek into the reactive sequence of events along the process of sending a request by making use of reactive operators.

For more information, refer to Spring documentation about WebClient. WebClient is part of Spring Webflux, Spring's reactive-stack web framework.

API

The service exposes an API that communicates with WIREMOCK Cloud, where answers to requests sent to this service, and redirected to WIREMOCK, can be stubbed. Next figure shows the setup:

The API offers two endpoints:

• /test/ok
• /test/fail

Internally, the service performs the translation between these two endpoints and those exposed by WIREMOCK Cloud instance, prior sending a http request to WIREMOCK Cloud.

Wiremock Cloud

The Wiremock Cloud instance should be configured in this way, since the endpoints are hard-coded:

• /mock/ok, to reply with http status code 200 and any jason payload in the body, or without body if you prefer.
• /mock/fail, to replay with any faulty status code you want to test. Every time you want to test how the service behaves on different http status codes, the WIREMOCK stub for this endpoint has to be modified accordingly.

See Getting Started section on WIREMOCK Cloud page.

Tests

Tests are intended to demo three major aspects when dealing with reactive code:

1. The differences between block() and subscribe() or, in other words: "to block, or not to block", because that is the question.
2. Triggering the reactive flow with subscribe(), and ways to provide callbacks in the subscription to handle both successful and unsuccessful completion of the reactive flow.
3. Common errors and strategies to test asynchronous code.

Regarding the topic of strategies to test asynchronous code, refer to Awaitility library, that implements a DSL that allows you to express expectations of an asynchronous system in a concise and easy to read manner:

@Test
public void updatesCustomerStatus() {
    // Publish an asynchronous message to a broker (e.g. RabbitMQ):
    messageBroker.publishMessage(updateCustomerStatusMessage);

    // Awaitility lets you wait until the asynchronous operation completes:
    await().atMost(5, SECONDS).until(customerStatusIsUpdated());
    ...
}

Tests can be executed with mvn test.