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 OrderRepository for managing Order aggregates.
- 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.

Hieu Tran Duc

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