Finding the High-Level and Low-Level modules in dependencies for applying Dependency Inversion Principe

Question

The Dependency Inversion Principle say:

• High-level modules should not depend on low-level modules. Both should depend on abstractions.
• Abstractions should not depend upon details. Details should depend upon abstractions.

How can I practically find the High-level and the Low-level modules in my applications, is there any clear definition for them?

Answer 1

This is how usually I refer them:

High Level Module --> this module represent more business aspect rather than technical aspect. It can be refered as an abstraction rather than implementation, and usually achieved through interfaces.

Some example maybe: RegisterAccount, PostAnswer, PostQuestion, AddComment, InsertComment.

Since Low Level Module represent more technical aspect rather than the business aspect. Say for example we take the InsertComment HLM. The LLM should be:

1. Open database connection
2. Execute insert statement
3. Close database connection

A High Level Module can be a Low Level Module in another context. Taking another example, AddComment's LLM should be:

1. Validate the comment (e.g. 15 char min) --> this will be another HLM
2. Insert comment to database --> this will be another HLM (InsertComment)
3. Add notification to involved user --> this will be another HLM

The same apply for other HLM as well.