Why Service Layer Interfaces Matter: Boosting Maintainability and Testability

Recently, a discussion in a community raised the question of whether a Service layer interface is required and what its purpose is.

DD's view:

When business logic is simple, the Service layer may appear unnecessary, but as the application evolves and logic becomes complex, this layer becomes very useful.

The main advantages are:

It is better suited for handling complex business logic that may involve operations on multiple tables, message dispatch, API calls, and other comprehensive transactions, which corresponds to the transaction management layer.

It supports reuse for the presentation layer; writing the same logic directly in controllers leads to duplication, making maintenance harder.

It facilitates unit testing; isolating business logic in a Service layer allows straightforward unit tests for the service, while web‑layer tests can focus solely on response formatting.

It enables flexible assembly of business logic; controllers depend on Service interfaces, allowing different implementations to be injected for various needs, making version coexistence easier.

Therefore, designing a Service layer is essential for the maintainability and testability of monolithic applications.

So, do you think a Service layer interface is necessary?