Here's a visual example of a services layer, where the application is exposing services:
Note that services don't need to be "web" services.
Key Services Layer Components
Here's the key components of a services layer:
- Service Interfaces. The service interface acts as a facade that exposes the business logic to potential consumers. The definition of the set of messages that must be exchanged with a service in order for the service to perform a specific business task is the contract.
- Message Types. Message Types are “message contracts” for communication between service consumers and providers. They wrap business entity components into specific messages for the calling service.
- Designing the Components of an Application or Service (App Arch Guide)
- Architecture (Web Service Software Factory: Modeling Edition)
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