Here's a breakdown of their differences:
Service
Purpose:
Services are used to encapsulate domain logic that doesn't naturally fit within an entity or value object. They are responsible for operations that involve multiple entities or aggregate roots.
They provide a way to coordinate operations, enforce business rules, and ensure the correct sequencing of tasks.
Characteristics:
Stateless: Ideally, services should be stateless and should not hold any state themselves. Any state required for their operations should be managed by entities or passed in as parameters.
Focused on behavior: Services are primarily focused on implementing domain logic and business rules.
Types:
Application Services: These are part of the application layer and handle use cases and orchestrate tasks across the domain layer. They do not contain domain logic but call domain services and entities.
Domain Services: These reside within the domain layer and contain domain-specific business logic that doesn't belong to a specific entity or value object.
Examples:
Calculating a discount on an order based on complex business rules.
Transferring money between bank accounts, where the logic involves multiple aggregate roots (accounts).
Repository
Purpose:
Repositories are used to abstract the persistence of entities and aggregates. They act as a collection-like interface for accessing and storing entities.
They encapsulate data access logic, providing a clean separation between the domain model and the data access layer.
Characteristics:
Interface for data access: Repositories provide methods to add, remove, update, and retrieve entities or aggregates.
Focus on persistence: Their primary responsibility is to manage the lifecycle of entities within the persistence context.
Types:
Typically, there is one repository per aggregate root. For example, an
OrderRepositoryfor managingOrderaggregates.They can be implemented using various technologies (e.g., ORM frameworks, direct SQL, NoSQL databases) but expose a consistent interface to the domain layer.
Examples:
Finding an order by its ID.
Saving a newly created customer to the database.
Querying for a list of products that match certain criteria.
Key Differences
Focus:
Services focus on domain logic and business rules.
Repositories focus on data access and persistence.
State:
Services are generally stateless.
Repositories manage the state of entities within the persistence layer.
Responsibility:
Services coordinate actions and enforce business rules across multiple entities or aggregates.
Repositories manage the lifecycle of entities, including retrieving, storing, and deleting them.
By clearly separating these concerns, DDD helps maintain a clean and modular architecture, where each component has a well-defined role and responsibility.