The Domain Layer Is Where the Model Lives
LAYERED ARCHITECTURE is used in most systems today, under various layering schemes. Many styles of development can also benefit from layering. However, domain-driven design requires only one particular layer to exist.
The domain model is a set of concepts. The "domain layer" is the manifestation of that model and all directly related design elements. The design and implementation of business logic constitute the domain layer. In a MODEL-DRIVEN DESIGN, the software constructs of the domain layer mirror the model concepts.
It is not practical to achieve that correspondence when the domain logic is mixed with other concerns of the program. Isolating the domain implementation is a prerequisite for domain-driven design.
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