Reading.CleanArchitecture.Services

Key Concepts of Services: Great and Small

1. What Are Services?

• A service is any software component that performs a specific function or provides a set of behaviors to other components.
• Services can exist at various levels within the architecture, ranging from small, narrowly scoped functions to large, multi-faceted systems.

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2. Types of Services

Martin categorizes services based on their scope and purpose:

a. Business Services

• Represent high-level business operations or workflows.
• Often implemented as use cases in the application layer.
• Example: PlaceOrderUseCase orchestrates actions like validating an order, calculating totals, and saving the order.

b. Application Services

• Handle application-specific tasks that don’t belong to the domain layer but are critical to the system’s operation.
• Example: A service that sends confirmation emails or logs application events.

c. System Services

• Provide low-level, infrastructure-specific functionality.
• Example: Database access, file I/O, network communication.

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3. Services in Clean Architecture

• Placement in Layers:
  • Business services (use cases) belong in the application layer.
  • Application services and system services belong in the interface adapter or framework and driver layers.
• Dependency Rule:
  • High-level services (business logic) should not depend on low-level system services. Instead, system services should be abstracted behind interfaces.

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4. Characteristics of Good Services

a. Single Responsibility

• A service should have one and only one reason to change.
• This aligns with the Single Responsibility Principle (SRP).

b. Testability

• Services should be designed so that their behavior can be tested independently of other components.

c. Independence

• Services should not tightly couple themselves to frameworks, databases, or other infrastructure elements.
• Dependencies should be injected rather than directly instantiated.

d. Reusability

• Properly designed services are reusable across different parts of the application or even different applications.

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5. Avoiding Common Pitfalls with Services

a. Overloading Services

• Don’t create “God services” that handle too many responsibilities. For example, a UserService that manages everything from authentication to user data retrieval violates SRP.

b. Overusing Services

• Avoid wrapping everything in a service when it’s unnecessary. Some logic is better encapsulated in entities or value objects.

c. Misplacing Business Logic

• Business logic should reside in the domain layer or application layer, not in system services or controllers.

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6. Examples of Services in Practice

Business Service Example (Use Case):

• A service to handle order placement:javaCopyEdit

public class PlaceOrderService {
    private OrderRepository orderRepository;
    private PaymentGateway paymentGateway;

    public PlaceOrderService(OrderRepository orderRepository, PaymentGateway paymentGateway) {
        this.orderRepository = orderRepository;
        this.paymentGateway = paymentGateway;
    }

    public void placeOrder(Order order) {
        if (order.isValid()) {
            paymentGateway.process(order);
            orderRepository.save(order);
        } else {
            throw new IllegalArgumentException("Invalid order");
        }
    }
}

Application Service Example:

• A service to send confirmation emails:

public class EmailService {
    public void sendOrderConfirmation(String email, Order order) {
        // Implementation for sending email
    }
}

System Service Example:

• A service to handle database interactions

public class SqlOrderRepository implements OrderRepository {
    public void save(Order order) {
        // SQL logic to save the order
    }
}

7. Testing Services

• Unit Tests:
  • Test each service in isolation, mocking dependencies like repositories or external APIs.
• Integration Tests:
  • Test how services interact with external systems (e.g., databases, third-party APIs).
• Example

@Test
public void testPlaceOrder() {
    OrderRepository mockRepository = Mockito.mock(OrderRepository.class);
    PaymentGateway mockPaymentGateway = Mockito.mock(PaymentGateway.class);

    PlaceOrderService service = new PlaceOrderService(mockRepository, mockPaymentGateway);

    Order order = new Order(...);
    service.placeOrder(order);

    Mockito.verify(mockPaymentGateway).process(order);
    Mockito.verify(mockRepository).save(order);
}

Key Takeaways

1. Categorize Services:
   • Differentiate between business, application, and system services, and place them in the appropriate architectural layers.
2. Follow SRP:
   • Ensure that each service has a single responsibility to make it maintainable and testable.
3. Abstract System Services:
   • Use interfaces to abstract low-level system services, ensuring independence of high-level business logic.
4. Avoid Overengineering:
   • Don’t create unnecessary services. Keep logic where it belongs (e.g., domain or application layer).
5. Design for Testability:
   • Use dependency injection to make services easy to test in isolation.