Why is it a good idea for “lower” application layers not to be aware of “higher” ones?

The principle of having "lower" application layers not be aware of "higher" ones is a fundamental concept in software architecture, particularly in layered or tiered architectures. This principle is rooted in several key benefits that contribute to the overall maintainability, scalability, and flexibility of a software system. Here are the main reasons why this architectural approach is considered a good practice:

1. Encapsulation and Modularity: By keeping lower layers unaware of higher layers, each layer can be developed, maintained, and updated independently. This encapsulation ensures that changes in one layer, especially higher-level business logic or user interface changes, do not necessitate changes in lower layers like the data access layer or the database itself[1][6][13].

2. Reusability: Lower layers, such as services or data access layers, can be reused by different parts of the application or even across different applications. If these layers were aware of and dependent on higher layers, their reusability would be significantly limited. This approach allows for a more modular design where components can be plugged in and out as needed[1][6].

3. Simplification of Dependencies: Dependencies are simplified because each layer only depends on the layer directly below it. This reduces the complexity of the system, making it easier to understand, debug, and test. When lower layers are not dependent on higher layers, the risk of circular dependencies is eliminated, which can otherwise lead to tightly coupled code that is hard to manage and evolve[15][16].

4. Flexibility and Adaptability: The system becomes more flexible and easier to adapt to new requirements. For example, if the presentation layer needs to be changed from a web interface to a desktop application, the business logic and data access layers do not need to be altered if they are not aware of the presentation layer. This separation of concerns allows for parts of the system to evolve independently[1][6][15].

5. Improved Testability: Testing becomes more straightforward because each layer can be tested in isolation. Mocks or stubs can be used to simulate the layers below the one being tested, focusing on the functionality of a single layer without the need for the entire application stack. This isolation improves the quality of tests and the speed at which testing can be conducted[1][6].

6. Scalability: It's easier to scale the application because layers can be scaled independently based on their specific load and perform...