Domain Services

 

Domain Services (500 Words in Points)

• Definition:
  Domain services are specialized components in software design that encapsulate business logic not naturally belonging to a specific object or entity.
• Role in DDD (Domain-Driven Design):
  In Domain-Driven Design, domain services represent business operations that involve multiple entities or value objects.
• Not Bound to One Entity:
  They handle actions that span across multiple domain objects, instead of being tied to just one.
• Stateless:
  Most domain services are stateless. They don’t hold any persistent data internally.
• Business Logic Holder:
  If a business rule doesn’t clearly belong to a single entity, it goes into a domain service.
• Naming Convention:
  Their names usually end with a verb or describe an action, e.g., InvoiceGenerator, PaymentProcessor.
• Examples of Domain Services:
  • Transferring money between two bank accounts.
  • Calculating shipping fees for an order.
  • Sending confirmation emails after order placement.
• Difference from Application Services:
  Domain services are part of the business layer, while application services coordinate application-level tasks like calling APIs or user interfaces.
• Communication with Entities:
  Domain services often use and manipulate entities to complete business operations.
• Interface Implementation:
  A domain service can be defined by an interface and implemented by a concrete class.
• Injectable:
  Domain services are typically injected using dependency injection in modern frameworks.
• Testable:
  Due to their stateless nature, they are easy to unit test.
• Focus on Behavior:
  They focus on what the system does, not what it stores.
• Part of Ubiquitous Language:
  Their naming and structure should reflect the business language used in the domain.
• Do Not Handle Persistence:
  Domain services do not directly manage data access or databases; that is handled by repositories.
• Example Use Case:
  In an e-commerce platform, calculating discounts based on multiple conditions is often done in a domain service.
• Keeps Entities Clean:
  By putting complex operations into domain services, the design keeps entity classes simpler and focused.
• Single Responsibility Principle:
  Domain services usually do one thing well, aligning with good software design principles.
• Pure Functions:
  In some cases, domain services behave like pure functions, returning output based on input without side effects.
• Usage in Layers:
  Domain services sit in the domain layer of a layered architecture and should not depend on infrastructure or UI code.
• Bridge Between Entities and Rules:
  They act as a bridge to enforce rules across multiple business objects.
• Can Be Internal or Public:
  Depending on the structure, domain services can be used within the domain or exposed to the application layer.
• Improves Reusability:
  Common operations shared between entities can be reused through domain services.
• Good for Complex Logic:
  When logic doesn’t clearly belong to a single entity, domain services are the right place.
• Encapsulation of Logic:
  They encapsulate domain-specific knowledge without tying it to data models.
• Flexible and Extendable:
  They can be extended or modified without affecting the entities themselves.

Domain Services