Application Services orchestrate Domain Operations

Application Services are abstractions that act as intermediaries between the external world (APIs, console commands, message brokers) and the Domain Model (Domain Services, Aggregates). Use cases of an application are generally placed and exposed in the Services Layer.

Application Services handle requests from the outside world and orchestrate an operation. They are the primary mechanisms through which the application interacts and responds to the external world.

They perform the routine tasks that happen with most operations like:

• Perform checks or asserts about the request against the current state of the domain
• Fetch data from the database through repositories
• Update the domain model, either by calling a Domain Service or an Aggregates Method
• Persist changes with Unit of Work

Placing orchestration logic in Application Services simplifies the API layer, which then has to focus only on session management, parameter parsing, response status codes, and JSON.

Application logic is also better testable in the absence of concerns like interacting with the external world. Tests are written at a higher level than the Domain Layer, facilitating more application and workflow tests while reducing the need for integration tests.