Defining the Domain layer with Use Cases

So far, we've talked about the Presentation layer (with UI and presentation logic) and the Model layer (with data logic). Yet, apart from these two layers, most of the time, applications also encapsulate a different type of logic, different from UI, presentation, or data logic.

To identify this type of logic, we must first acknowledge that most applications have a dedicated business scope – for example, a food delivery application could have the business scope of taking orders and generating revenue for the stakeholder. The stakeholder is the entity interested in the business, such as the company that owns the restaurant chain.

Such applications can contain business rules imposed by the stakeholders that can vary from minimum order amounts, custom availability ranges for certain restaurants, or predefined time frames for different delivery charges; the list could go on. We can refer to such business rules that are dictated...