Services vs Entities: When Business Logic Doesn't Fit in Domain-Driven Design

In the realm of Domain-Driven Design (DDD), understanding the roles of services and entities is crucial for structuring your application effectively. Both concepts serve distinct purposes, especially when it comes to implementing business logic. This article will clarify the differences between services and entities, and guide you on when to use each.

Understanding Entities

Entities are objects that have a distinct identity that runs through time and different states. They are defined by their attributes and behaviors, and they encapsulate the core business logic related to that identity. For example, in an e-commerce application, a Customer entity might have attributes like customerId, name, and email, along with behaviors such as placeOrder() or updateProfile().

Characteristics of Entities:

  • Identity: Each entity has a unique identifier.
  • Lifecycle: Entities can change state over time.
  • Business Logic: They encapsulate business rules that pertain to their identity.

Understanding Services

Services, on the other hand, are used to encapsulate business logic that does not naturally fit within an entity. They are stateless and typically operate on multiple entities or value objects. Services are ideal for operations that involve complex business rules or interactions between different parts of the domain model.

For instance, a PaymentService might handle the logic for processing payments, which involves interacting with both Order and Customer entities but does not belong to either of them directly.

Characteristics of Services:

  • Stateless: Services do not maintain state between calls.
  • Cross-Entity Logic: They often coordinate actions across multiple entities.
  • Business Operations: Services encapsulate operations that are not tied to a single entity's lifecycle.

When to Use Services vs Entities

The decision to use a service or an entity often comes down to the nature of the business logic you are implementing:

  • Use Entities when the logic is closely tied to the identity and lifecycle of a specific object. If the behavior is intrinsic to the entity and affects its state, it should be part of the entity.
  • Use Services when the logic spans multiple entities or does not belong to any single entity. If the operation is more about coordinating actions or performing tasks that do not alter the state of a specific entity, a service is the appropriate choice.

Conclusion

In Domain-Driven Design, distinguishing between services and entities is essential for maintaining a clean and effective architecture. By understanding their roles, you can better organize your business logic, leading to a more maintainable and scalable application. Always consider the nature of the logic you are implementing to determine the best fit between services and entities.