Decoupling Domain Objects: Simplifying System Architecture

Post Date: 31 Mar 2023

Overview

When you design an object-oriented system from top to bottom, sometimes the objects that represent the “domain” (what the system is about) don’t match the objects that represent the “entities” (what the system stores). To solve this problem, you can use a technique called “decoupling” to separate the layers of objects.

One way to do this is to use “composite” objects. These are objects that contain other objects, and they allow each layer of abstraction (domain and view) to have its own composite object. It might seem like a lot of work to set this up, but most integrated development environments (IDEs) now have tools to make it easier to create the necessary code.

Design Layers

Service Layer, Persistence Layer, Domain Layer, View Layer

Service Layer

A service layer is a design pattern used in software architecture that provides a layer of abstraction between the presentation (user interface) layer and the business logic (data storage) layer. The service layer contains the application’s core logic and provides a set of methods or functions that can be used by the presentation layer to perform specific tasks or operations.

The purpose of the service layer is to separate the presentation layer from the data storage layer, providing a clean and organized architecture that is easy to maintain and modify. The service layer also helps to improve the testability of the application by allowing for unit testing of the business logic without the need for the presentation layer.

Overall, the service layer pattern promotes loose coupling, separation of concerns, and flexibility in software development.

The BlogManager example below provides and interface and an implementation:

public interface BlogManager {
    Blog load(String name) throws EntityNotFoundException;
}

public class BlogManagerImpl implements BlogManager {
    // Fictitious EntityManager
    EntityManager em;

    public Blog load(String name) throws EntityNotFoundException {
        Blog blog = null;
        BlogData data = em.query("select b from BlogData b where b.name = ?", name);
        return new Blog(data);
    }
}

Persistence Layer

The persistence layer is a component in software architecture that is responsible for storing and retrieving data from a permanent storage system, such as a database, file system, or message queue. It provides an interface between the application’s business logic layer and the underlying data storage layer.

The persistence layer is typically implemented using an object-relational mapping (ORM) tool, such as Hibernate or Entity Framework, which maps the application’s domain objects to the database tables and performs CRUD (create, read, update, delete) operations on them.

The purpose of the persistence layer is to abstract the underlying data storage layer from the application’s business logic layer, providing a standardized interface for data access that is independent of the specific storage technology used. This allows for easier maintenance and modification of the application, as well as increased scalability and flexibility.

Overall, the persistence layer is a critical component in modern software architecture that enables the separation of concerns and the implementation of clean, modular, and maintainable code.

Here’s an example of a Persistent Object using JPA

@Persistent
public class BlogData {

   private String name;

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }
}

Domain Layer

The domain layer is a component in software architecture that represents the core logic and behavior of a software application, independent of any specific implementation details such as user interfaces, databases, or external systems. It contains the business rules, entities, and operations that are specific to the problem domain being addressed by the application.

The domain layer is often implemented using object-oriented programming (OOP) techniques, such as classes, interfaces, and inheritance, to model the entities and relationships in the problem domain. It encapsulates the application’s domain-specific knowledge and provides a clear separation of concerns between the domain logic and the infrastructure and presentation layers.

The purpose of the domain layer is to provide a reusable, maintainable, and testable representation of the business logic that can be used across different applications and technologies. It also helps to minimize the impact of changes in the external environment, such as changes in databases or user interfaces, by providing a stable and consistent interface to the rest of the application.

Overall, the domain layer is a critical component of software architecture that enables the development of flexible, scalable, and maintainable applications. It promotes separation of concerns, loose coupling, and high cohesion, leading to better quality code and faster development cycles.

Here is a Domain Layer Object Blog

public class Blog {
    private BlogData delegate;

    public Blog(BlogData delegate) {
        this.delegate = delegate;
    }

    // Example delegate methods
    public getName() {
        return delegate.getName();
    }

    public BlogData getData() {
        return delegate;
    }
}

View Layer

The view layer is a component in software architecture that is responsible for rendering the user interface of a software application. It is the layer that the end user interacts with directly and is responsible for displaying data and receiving input from the user.

The view layer is typically implemented using a combination of HTML, CSS, and JavaScript in web applications, or using native UI frameworks in desktop or mobile applications. It provides a visual representation of the application’s data and functionality, allowing users to interact with the application in a meaningful way.

The purpose of the view layer is to provide a clear and intuitive interface for the end user, while minimizing the coupling between the presentation layer and the business logic layer. It is often designed using principles of user experience (UX) and user interface (UI) design to ensure that the interface is easy to use, aesthetically pleasing, and consistent with the user’s expectations.

Overall, the view layer is a critical component of software architecture that enables the development of applications that are intuitive, user-friendly, and visually appealing. It promotes separation of concerns, allowing developers to focus on the specific requirements of each layer without interfering with the functionality of other layers.

Here’s an example of a View Object BlogView

public class BlogView {

    private Blog delegate;

    public BlogView(Blog delegate) {
        this.delegate = delegate;
    }

    public String getName() {
        return delegate.getName();
    }

    // custom UI method

    public boolean isActive() {
        // custom code (UI Logic)
    }
}

Controller Layer

The controller layer is a component in software architecture that is responsible for managing the flow of data between the view layer (user interface) and the business logic layer. It receives input from the user via the view layer, processes the input, and interacts with the business logic layer to retrieve or update data as needed.

In web applications, the controller layer is typically implemented using a server-side framework, such as Spring MVC or Ruby on Rails. It provides a set of endpoints or URLs that map to specific actions or methods in the application, allowing the user to interact with the application via HTTP requests.

The purpose of the controller layer is to provide a centralized point of control for the application, allowing developers to separate the concerns of user interface and business logic. It also provides a layer of abstraction between the view layer and the business logic layer, making it easier to modify or replace either layer without affecting the other.

Overall, the controller layer is a critical component of software architecture that enables the development of flexible, scalable, and maintainable applications. It promotes separation of concerns, loose coupling, and high cohesion, leading to better quality code and faster development cycles.

Advantages

The separation of layers in software architecture provides several advantages, including:

Modularity: Separating the different components of an application into layers makes it easier to modify or replace one layer without affecting the others. This promotes modularity and makes the application more flexible and adaptable to changing requirements.
Reusability: By separating the application into layers, developers can reuse components across different projects or applications, leading to faster development cycles and reduced costs.
Scalability: Separating the application into layers makes it easier to scale each layer independently, allowing for better performance and higher availability.
Maintainability: Separating the application into layers promotes the separation of concerns, making it easier to maintain and modify the codebase. Each layer can be maintained independently, reducing the risk of introducing bugs or breaking functionality.
Testing: Separating the application into layers makes it easier to test each layer independently, reducing the risk of introducing bugs or breaking functionality.
Overall, the separation of layers in software architecture leads to cleaner, more modular, and more maintainable code, as well as faster development cycles and reduced costs.

Caveats (Pitfalls)

While the separation of layers in software architecture provides many benefits, there are also some potential caveats to consider, including:

Over-engineering: Separating the application into too many layers can lead to over-engineering, making the codebase overly complex and difficult to maintain.
Performance: Separating the application into too many layers can also have a negative impact on performance, as each layer adds overhead and latency to the application.
Communication: Separating the application into layers requires clear communication and coordination between the different teams or individuals responsible for each layer. Without proper communication, there is a risk of introducing inconsistencies or misalignments between layers.
Complexity: Separating the application into layers can add complexity to the application architecture, making it more difficult to understand and maintain.
Cost: Separating the application into layers can require additional development and testing resources, leading to increased costs.
Overall, while the separation of layers is generally a best practice in software architecture, it is important to balance the benefits against the potential caveats and ensure that the architecture is appropriate for the specific requirements and constraints of the application.

In Conclusion

In object-oriented system design, it’s common that the objects representing the “domain” don’t match the objects representing the “entities.” To solve this issue, the technique of “decoupling” is used to separate the layers of objects. One way to do this is through the use of “composite” objects, which contain other objects, allowing each layer of abstraction to have its own composite object. By separating the layers, we can achieve better separation of concerns, loose coupling, and flexibility, leading to better quality code and faster development cycles. The service, persistence, domain, and view layers are the essential components of software architecture, and each plays a critical role in creating flexible, scalable, and maintainable applications. By using design patterns and techniques like decoupling, software developers can create well-organized and clean code that’s easy to maintain and modify.

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