01) Introduction to the Separation of Concerns Design Principle
Basically separation of concerns is the practice of putting logical boundaries on your code. Putting these logical boundaries on your code helps make your code easier to understand, easier to maintain and much more flexible when it needs to be altered (and every code base ever has to be altered all the time).
In the Salesforce Ecosystem there are three major areas of concern we need to separate our code into. They are the following:
The Service Layer:
The Service Layer should house 100% of your non-object specific business logic (object specific logic is most often handled by the domain layer). This is, the logic that is specific to your organizations specific business rules. Say for instance you have a part of your Salesforce App that focuses on Opportunity Sales Projections and the Opportunity Sales Projection App looks at the Oppotunity, Quote, Product and Account objects. You might make an OpportunitySalesProjection_Service that houses methods that have business logic that is specific to your Opportunity Sales Projection App.
The Domain Layer:
The Selector Layer: