基于DDD架构的依赖注入标准疑问：各层注入主体咨询

Hey Leo, great questions—these are exactly the kinds of decisions that keep DDD implementations clean and maintainable. Let’s break this down layer by layer, sticking to core DDD principles:

1. Application Layer: Should ClientAppService Inject UserApplicationService or UserService? #

Go with injecting UserApplicationService (the application-layer service) first—here’s why:

• Single Responsibility: UserApplicationService should own all user-related application use cases (like fetching user details, validating user status for business workflows). ClientAppService can focus solely on client-specific logic, reusing user-related workflows via the application service instead of mixing concerns.
• Layered Encapsulation: The application layer’s job is to coordinate use cases, not dive into domain-layer details. UserApplicationService already handles calls to UserService (domain layer), repositories, and external services. ClientAppService doesn’t need to know about those underlying dependencies, which keeps coupling low.
• Exception: If you only need a tiny, pure domain logic snippet from UserService (like validating a user ID format) and there’s no existing application-layer wrapper for it, you could inject UserService directly. But try to avoid this long-term—wrap that logic in a UserApplicationService method to keep your application layer consistent.

2. Domain Layer: Should ClientService Inject UserService? #

Absolutely—if the dependency serves domain logic. Domain services exist to handle cross-aggregate or complex business rules that don’t fit neatly into a single entity. For example:

• If creating a client requires verifying that the associated user is in an active state (a domain rule), ClientService should inject UserService to handle that validation.
• If a client’s permissions are derived from the linked user’s role (another domain rule), ClientService needs UserService to access that user-specific domain logic.
• Just make sure this dependency is strictly for domain rules, not application-level tasks (like sending notifications or handling HTTP requests)—those belong in the application layer.

内容的提问来源于stack exchange，提问作者LeoFelipe