Let's start with the domain layer. A domain layer typically consists of a domain model. So, what is a domain model? A domain model is a representation of the data storage types required by the business logic. It describes the various domain objects (entities); their attributes, roles, and relationships; plus the constraints that govern the problem domain. Take a look at the following domain model diagram for order processing to get a quick idea about the domain model:

Sample domain model

Each block in the preceding diagram represents a business entity, and the lines represent the associations between the entities. Based on the preceding domain model diagram, we can understand that, in an order processing domain, a customer can have many orders, each order can have many order items, and each order item represents a single product.

During coding, the domain model will be converted into corresponding domain objects and associations by a developer. A domain object is a logical...