Testing a Spring Boot REST API against a Consumer-Driven Contract with Pact

Testing a Spring Boot REST API against a Consumer-Driven Contract with Pact

7 minute read (1416 words)

Consumer-driven contract tests are a technique to test integration points between API providers and API consumers without the hassle of end-to-end tests (read it up in a recent blog post). A common use case for consumer-driven contract tests is testing interfaces between services in a microservice architecture. In the Java ecosystem, Spring Boot is a widely used technology for implementing microservices. Pact is a framework that facilitates consumer-driven contract tests. So let’s have a look at how to test a REST API provided by a Spring Boot application against a contract previously defined by the API consumer.

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Code Example

This article is accompanied by working example code on github.

In this Article

Instead of testing API consumer and provider in an end-to-end manner, with consumer-driven contract tests we split up the test of our API into two parts:

  • a consumer test testing against a mock provider and
  • a provider test testing against a mock consumer

This article focuses on the provider side. A consumer of our API has created a contract in advance and we want to verify that the REST API provided by our Spring Boot Service matches the expectations of that contract.

In this article we will:

  • have a look at the API contract created in advance by an API consumer
  • create a Spring MVC controller providing the desired REST API
  • verify that the controller against the contract within a JUnit test
  • modify our test to load the contract file from a Pact Broker

For an overview of the big picture of consumer-driven contract testing, have a look at this article.

The Pact

Since we’re using the Pact framework as facilitator for our consumer-driven contract tests, contracts are called “pacts”. We’ll use the following pact that was created by an Angular consumer in another article:

{
  "consumer": {
    "name": "ui"
  },
  "provider": {
    "name": "userservice"
  },
  "interactions": [
    {
      "description": "a request to POST a person",
      "providerState": "provider accepts a new person",
      "request": {
        "method": "POST",
        "path": "/user-service/users",
        "headers": {
          "Content-Type": "application/json"
        },
        "body": {
          "firstName": "Arthur",
          "lastName": "Dent"
        }
      },
      "response": {
        "status": 201,
        "headers": {
          "Content-Type": "application/json"
        },
        "body": {
          "id": 42
        },
        "matchingRules": {
          "$.body": {
            "match": "type"
          }
        }
      }
    }
  ],
  "metadata": {
    "pactSpecification": {
      "version": "2.0.0"
    }
  }
}

As you can see, the pact contains a single POST request to /user-service/users with a user object as payload and an associated response that is expected to have the status code 201 and should contain the ID of the created user. A request / response pair like this is called an interaction.

The Spring Controller

It’s pretty easy to create a Spring controller that should obey that contract:

@RestController
public class UserController {

  private UserRepository userRepository;

  @Autowired
  public UserController(UserRepository userRepository) {
    this.userRepository = userRepository;
  }

  @PostMapping(path = "/user-service/users")
  public ResponseEntity<IdObject> createUser(@RequestBody @Valid User user) {
    User savedUser = this.userRepository.save(user);
    return ResponseEntity
      .status(201)
      .body(new IdObject(savedUser.getId()));
  }
}

IdObject is a simple bean that has the single field id. The UserRepository is a standard Spring Data repository that saves and loads User objects to and from a database.

The Provider Test

The controller works, we can test it by manually sending requests against it using Postman, for example. But now, we want to verify that it actually obeys the contract specified above. This verification should be done in every build, so doing this in a JUnit tests seems a natural fit.

Pact Dependencies

To create that JUnit test, we need to add the following dependencies to our project:

dependencies {
  testCompile("au.com.dius:pact-jvm-provider-junit5_2.12:3.5.20")
  // Spring Boot dependencies omitted
}

This will transitively pull the JUnit 5 dependency as well.

Set up the JUnit Test

Next, we create a JUnit test that:

  • starts up our Spring Boot application that provides the REST API (our contract provider)
  • starts up a mock consumer that sends all requests from our pact to that API
  • fails if the response does not match the response from the pact
@ExtendWith(SpringExtension.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.DEFINED_PORT, 
        properties = "server.port=8080")
@Provider("userservice")
@PactFolder("../pact-angular/pacts")
public class UserControllerProviderTest {

  @MockBean
  private UserRepository userRepository;
  
  @BeforeEach
  void setupTestTarget(PactVerificationContext context) {
    context.setTarget(new HttpTestTarget("localhost", 8080, "/"));
  }
  
  @TestTemplate
  @ExtendWith(PactVerificationInvocationContextProvider.class)
  void pactVerificationTestTemplate(PactVerificationContext context) {
    context.verifyInteraction();
  }

  @State({"provider accepts a new person"})
  public void toCreatePersonState() {
    User user = new User();
    user.setId(42L);
    user.setFirstName("Arthur");
    user.setLastName("Dent");
    when(userRepository.findById(eq(42L))).thenReturn(Optional.of(user));
    when(userRepository.save(any(User.class))).thenReturn(user);
  }

}

The test uses the standard SpringExtension together with @SpringBootTest to start up our Spring Boot application. We’re configuring it to start on a fixed port 8080.

With @PactFolder we tell Pact where to look for pact files that serve as the base for our contract test. Note that there are other options for loading pact files such as the @PactBroker annotation.

The annotation @Provider("userservice") tells Pact that we’re testing the provider called “userservice”. Pact will automatically filter the interactions from the loaded pact files so that only those interaction with this provider are being tested.

Since Pact creates a mock consumer for us that “replays” all requests from the pact files, it needs to know where to send those requests. In the @BeforeEach annotated method, we define the target for those requests by calling PactVerificationContext#setTarget(). This should target the Spring Boot application we started with @SpringBootTest so the ports must match.

@MockBean is another standard annotation from Spring Boot that - in our case - replaces the real UserRepository with a Mockito mock. We do this so that we do not have to initialize the database and any other dependencies our controller may have. With our consumer-driven contract test, we want to test that consumer and provider can talk to each other - we do not want to test the business logic behind the API. That’s what unit tests are for.

Next, we create a method annotated with @State that puts our Spring Boot application into a defined state that is suitable to respond to the mock consumer’s requests. In our case, the pact file defines a single providerState named provider accepts a new person. In this method, we set up our mock repository so that it returns a suitable User object that fits the object expected in the contract.

Finally, we make use of JUnit 5’s @TestTemplate feature in combination with PactVerificationInvocationContextProvider that allows Pact to dynamically create one test for each interaction found in the pact files. For each interaction from the pact file, context.verifyInteraction() will be called. This will automatically call the correct @State method and then fire the request defined in the interaction verify the result against the pact.

The test should output something like this in the log:

Verifying a pact between ui and userservice
  Given provider accepts a new person
  a request to POST a person
    returns a response which
      has status code 201 (OK)
      includes headers
        "Content-Type" with value "application/json" (OK)
      has a matching body (OK)

Load the Contract from a Pact Broker

Consumer-Driven contracts loose their value if you have multiple versions of the same contract file in the consumer and provider codebase. We need a single source of truth for the contract files.

For this reason, the Pact team has developed a web application called Pact Broker which serves as a repository for pact files.

Our test from above can be modified to load the pact file directly from a Pact Broker instead of a local folder by using the @PactBroker annotation instead of the @PactFolder annotation:


@PactBroker(host = "host", port = "80", protocol = "https",
        authentication = @PactBrokerAuth(username = "username", password = "password"))
public class UserControllerProviderTest {
  ...
}

Conclusion

In this article, we created a JUnit test that verified a REST API against a contract previously created by a consumer of that API. This test can now run in every CI build and we can sleep well knowing that consumer and provider still speak the same language.

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