In domain-driven design (DDD), the repository (Repository) serves as a container for aggregates and is responsible for obtaining, storing, and retrieving aggregates. Each aggregate is closely related to its corresponding resource library. However, the definition of the resource library interface should be located at the domain layer, but it is implemented at the infrastructure layer, which causes a problem in the traditional layered architecture.
First, let's take a look at traditional layered architecture. In this architecture, each layer can only couple to the layer immediately below it. In other words, the user interface layer can only be coupled with the application layer, the application layer can only be coupled with the domain layer, and the domain layer can only be coupled with the infrastructure layer. The advantage of this architecture is that it can reduce the interdependence between layers, making each layer more independent, and it is also conducive to the maintainability and reusability of the code.
However, in DDD, the resource library acts as a component of the domain layer and is responsible for obtaining and persisting aggregates. This means that the definition of the repository interface should be at the domain level. However, in actual implementation, the implementation of the resource library interface needs to rely on the persistence mechanism of the infrastructure layer. This leads to a problem: if the implementation of the resource library interface is placed on the infrastructure layer, it will cause the infrastructure layer to rely upward on the domain layer, which violates the principle of layered architecture.
To solve this problem, we can use the Dependency Inversion Principle (DIP). According to the dependency inversion principle, high-level modules should not depend on low-level modules, but should rely on abstractions. Abstractions should not depend on details, but details should depend on abstractions. Regarding the resource library issue, we can create an abstract resource library interface defined in the domain layer, and implement this interface in the infrastructure layer. In this way, both the domain layer and the infrastructure layer depend on this abstract repository interface, rather than each other.
Specifically, we can define a resource library interface at the domain layer, which defines methods for obtaining and persisting aggregates. Then, implement this interface in the infrastructure layer so that the infrastructure layer depends on this interface instead of the domain layer. At the same time, the domain layer only relies on this interface, not the specific implementation details of the infrastructure layer. In this way, we can achieve decoupling of the domain layer and the infrastructure layer, so that the two can be developed and evolved independently.
In addition, the dependency inversion principle can also help us reduce code duplication and complexity. If we follow a traditional layered architecture, we need to define our own interfaces and implementations at each level, which results in a lot of duplicated code and complexity. Using the principle of dependency inversion, we only need to define the interface in the high-level module and then implement this interface in the low-level module to avoid this duplication and complexity.
Although the dependency inversion principle sometimes breaks traditional layered architecture principles, when used in the right circumstances, it can make our architecture more flexible, maintainable, and scalable. By relying on abstractions for specific implementation details, we can reduce the coupling of the system and improve the reusability and maintainability of the code. At the same time, using the dependency inversion principle can also help us better organize code and manage dependencies, making the code clearer and easier to understand.
To sum up, although the dependency inversion principle sometimes breaks the traditional layered architecture principle, when used in appropriate scenarios, it can greatly improve the maintainability and scalability of the system. Therefore, in practical applications, we should flexibly use the dependency inversion principle and other design principles to design and optimize software architecture according to specific circumstances.