In DDD, a service is a part of the domain layer that implements domain logic or a use case that doesn’t naturally fit into a particular entity or value object. It typically coordinates interactions between multiple entities or aggregates. A service represents a cohesive and stateless operation that has an input and an output. It doesn’t have a persistent state of its own and doesn’t need to be instantiated. Services often have names that reflect their domain purpose, such as `OrderProcessingService` or `PaymentService`.

There are certain scenarios where using services may not be appropriate.

Firstly, if the business logic can be associated with a single entity or value object, it may be better to encapsulate the logic within that entity or value object. This can help to improve the cohesion of the code and make it more understandable.

Secondly, if the business logic involves multiple entities or aggregates that are tightly coupled, it may be better to re-evaluate the design of the aggregates themselves. In such cases, it may be more appropriate to split the aggregates or re-define their boundaries to reduce coupling and improve maintainability.

Finally, if the business logic involves external systems or infrastructure, it may be better to use an anti-corruption layer or adapter pattern to isolate the domain model from the external dependencies. This can help to reduce coupling and improve the flexibility of the system.

Example

Here’s an example of a service in Java where more than one aggregate is involved:

public class OrderService {
    private final OrderRepository orderRepository;
    private final ProductRepository productRepository;

    public void createOrder(Order order) {
        for (OrderItem item: order.getItems()) {
            Product product = productRepository.findById(item.getProductId());
            product.reduceStockBy(item.getQuantity());
            productRepository.save(product);
        }
        orderRepository.save(order);
    }

    // Other methods...
}

In this example, the OrderService is responsible for creating orders, and it interacts with two aggregates: Order and Product. The Order aggregate represents an order placed by a customer, while the Product aggregate represents a product that can be ordered.

The createOrder method updates the stock of each product in the order by calling the save method on the ProductRepository, and then saves the order itself by calling the save method on the OrderRepository.