applicationservice

An application service is a component of the application layer in layered or hexagonal architectures. It coordinates the execution of a use case by orchestrating domain objects and infrastructure resources to fulfill a request. It sits between the presentation layer and the domain model and does not primarily implement business rules.

Its responsibilities include input validation, authorization checks, mapping between data transfer objects and domain objects, orchestrating calls to aggregates or domain services, and managing transaction boundaries. It may also apply cross-cutting concerns such as logging, auditing, and error handling, while delegating behavior to the domain layer.

In Domain-Driven Design, the application layer is above the domain layer. Domain entities, value objects, and domain services encapsulate business rules and invariants, while application services implement workflows by coordinating domain operations. They ensure a use case is carried out without leaking domain implementation details to the presentation layer.

Best practices emphasize keeping application services thin and stateless, avoiding embedding complex business rules, and relying on the domain model for behavior. They typically interact with repositories or domain services through the domain layer, preserving clean separation of concerns and enabling easier testing and maintenance.

Example: a PlaceOrderService may validate input, retrieve the customer and product information via repositories, construct an Order aggregate, invoke domain methods to reserve stock and calculate totals, and commit the transaction. The implementation may vary across architectures, but the role remains orchestration of a use case rather than rule enforcement.