Encapsulates business logic and orchestrates interactions between presentation and data access layers.

Encapsulate business logic and provide a clear API for upper layers.

When designing layered architectures to separate concerns and promote maintainability.

Typical in layered enterprise applications (e.g., Java EE, .NET, Spring).

controller → request → service layer → business logic → repository

• service interface
• dependency injection
• repository
• DTO
• transaction management

• Application service (more domain‑centric) – adds clearer intent
• Manager class (generic) – can become a God object

Putting data‑access code in the service layer → violates separation of concerns; Making service methods static without injection → hard to test; Overloading service with UI logic → creates tight coupling

Repository layer – handles persistence; Controller layer – handles HTTP routing; Facade pattern – provides a simplified interface over multiple services

Coming from Python: may place business logic directly in view functions → leads to tangled code → keep logic in dedicated service classes

• Layered architecture
• Presentation layer
• Data access layer

Do not use when business logic is trivial – a service adds unnecessary indirection; incurs slight runtime overhead due to extra method calls; boundary condition – ensure services are stateless to avoid hidden shared state

Enables clear separation of concerns, easier unit testing, and flexible swapping of implementations

A service layer is like a restaurant kitchen: it receives orders from the waitstaff (controllers) and prepares the dishes (business results) before handing them back

Service layer should remain thin, orchestrating domain objects rather than containing complex algorithms

Domain layer or application service layer