Testing Services in Hexagonal Architecture

In software architecture, ensuring effective communication between different components of a system is vital. One effective technique for verifying these connections is through interface mocking. This involves creating mock implementations of interfaces to simulate the behavior of different components. By using these mocks, we can test the services independently, ensuring that the messages they send and receive adhere to a predefined standard, commonly referred to as a "contract". This approach allows us to validate the interactions and functionality of each service without needing the actual implementations or infrastructure.

In Hexagonal Architecture, we adhere to a practice of creating a distinct interface or contract for each service that may have one or more implementations. These implementations are then allocated to different layers. For instance, consider a UserRepository contract which may have multiple implementations such as SQLiteUserRepository, PostGreSQLUserRepository, or JSONFSUserRepository, depending on the scalability needs of our system. The interface or contract belongs to the domain or application layer, while the actual implementations reside in the infrastructure layer.

By employing interface mocking, we streamline the process of testing the domain and application layers without the complexity of setting up a real infrastructure environment.

Let's say we have the following application service from the previous article Hexagonal Architecture:

final readonly class UserFinder  
    public function __construct(  
        private UserRepository $repository  
    ) {  

    public function byId(int $id): User  
        if (!$user = $this->repository->find($id)) {  
            throw new UserNotFound("User $id not found");  
        return $user;  

    public function all(): array
        return $this->repository->all();

The service above depends on a UserRepository contract exposing the following methods:

interface UserRepository  
    public function find(int $id): ?User;

    public function all(): array

    public function save(User $user): User;

Initially, when testing the UserFinder service, one might assume that a functioning database is necessary. This would involve deploying the schema, seeding the database with test data, creating an instance of a UserRepository service implementation, and then injecting it into the new UserFinder instance for testing.

However, this process can be overly complex for testing just one application service. Fortunately, we can simplify it by conducting interface mocking on the UserRepository interface. This approach allows us to test the code in the domain and application layers without the need for any actual infrastructure services.

To ensure proper testing of the domain and application services, we must rigorously validate the API exposed by the contract we are testing. This means thoroughly testing all the inputs and outputs specified by the contract.

Let's explore a quick example using the Pest and Mockery frameworks. First, we need to install both frameworks via composer:

composer require pestphp/pest --dev --with-all-dependencies
composer require mockery/mockery --dev 

Next, we need to initialize Pest:

vendor/bin/pest --init

Now, let's create our first unit test in tests/Unit/UserFinderTest.php. The first use case we will test is finding a user by their unique ID, which involves testing the byId function. This function calls the find method of the UserRepository contract. According to the contract, this method can return either a valid instance of User (success) or null if a user with the specified ID is not found (fail). With this in mind, let's write the test.

describe('UserFinder', function () {

    test('should find a `User` by give ID.', function () {  
        $repository = Mockery::mock(UserRepository::class);  
            ->andReturnUsing(fn(int $id) => new User($id, 'Vincent Vega'));  

        $service = new UserFinder($repository);  



Let's review what we have done:

  1. We created a mock of the UserRepository contract.
  2. We specified that the mock should expect a call to the find method with an int argument.
  3. We defined that the find method must be called exactly once during this test case.
  4. We provided a function that will execute when the find method is called. This function returns a new instance of User, which matches the expected output defined by the contract for a successful case.
  5. We instantiated a new UserFinder, our subject under test.
  6. We executed the byId method of UserFinder and asserted the response. Since we are testing the successful scenario, the returned value must be an instance of User.

Since our test subject is the UserFinder, we must assert what this service exposes. By mocking the UserRepository, we can control the behavior of UserFinder during the test. In this first test, we want the UserRepository to always return a User instance, simulating a scenario where a matching user record exists in a database. As we can see, we don't need a running database to execute the test; we only need to adhere to the defined contract properly.

We have already tested the success case, so now let's test the failure case. According to the UserRepository contract, if a user is not found by the given ID, the return value must be null.

describe('UserFinder', function () {  


    test('should throw `UserNotFound` exception when a `User` is not found by the given ID.', function () {  
        $repository = Mockery::mock(UserRepository::class);  
            ->andReturnUsing(fn(int $id) => null);  

        $service = new UserFinder($repository);  
    })->throws(UserNotFound::class, "User 42 not found");


This new test has some differences; let's review the changes:

  1. The return value of the function is now null, simulating that the database does not have the requested user record.
  2. Instead of directly asserting the return value of the service, we assert that an exception must be raised. Specifically, we expect a UserNotFound exception.

This test case verifies the failure scenario of the UserFinder. By changing the behavior of the UserRepository and adhering to the contract, we can test the entire functionality without any real implementation of a UserRepository.

Let's continue adding more tests to our UserFinder. Now it's time to cover the all method. This method calls the all method of the UserRepository. According to the contract, this method always returns an array.

describe('UserFinder', function () {

    // ...

    test('should return all `User`s', function () {  
        $repository = Mockery::mock(UserRepository::class);  
            ->andReturnUsing(fn() => []);  

        $service = new UserFinder($repository);  


In this case, we instruct the mock to:

  1. Not expect any arguments for the all method.
  2. Return an empty array when the all method is called.
  3. Finally, we assert that the value returned from the UserFinder is indeed an array.

At this point, we have thoroughly tested the UserFinder service. Next, let's move on to the UserCreator service (tests/Unit/UserCreatorTest.php), which also depends on the UserRepository. For this service, we need to test the save method. This method takes a User without an ID as an argument and returns a User with an ID.

describe('UserCreator', function () {  

    test('should create a new `User` from the given data.', function () {  
        $repository = Mockery::mock(UserRepository::class);  
            ->andReturnUsing(fn(User $user) => new User(1, $user->name));  

        $creator = new UserCreator($repository);  
        $user = $creator->create('Vincent Vega');  

        expect($user->name)->toBe('Vincent Vega')  


In the code above, we did the following:

  1. Created a mock of the UserRepository contract.
  2. Specified that the mock should expect a call to the save method with a User argument and that it should return a User.
  3. Defined that the save method must be called exactly once during the test case.
  4. Ensured the returned User has a valid int ID.
  5. Instantiated the UserCreator.
  6. Executed the save method and asserted the response. Since we are testing the successful scenario, the returned value must be an instance of User with a valid int ID.

Check the code example in the repository Hexagonal Architecture Example in PHP.


Interface mocking in Hexagonal Architecture helps ensure that different parts of your application work together correctly without needing a complex setup. By focusing on the contracts or interfaces, we can test the core logic without setting up real infrastructure like databases.

In this article, we showed how to use interface mocking to check the UserFinder and UserCreator services. By creating mocks of the UserRepository, we controlled how these services behaved during tests. This allowed us to verify that they respond correctly in different situations without needing a real database.

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