Shared Kernel in Domain Driven Design

Shared Kernel: A Shared Kernel in Domain-Driven Design (DDD) is a set of domain concepts and code that is shared by multiple bounded contexts. A bounded context is a conceptual boundary within a software system that separates different aspects of the domain. By sharing a kernel, bounded contexts can use the same domain concepts and code, which helps to ensure consistency and avoid duplication.

Shared kernels can be implemented in a variety of ways. One common approach is to create a separate library project that contains the shared code. The bounded contexts can then reference this library project.

Here are simple analogys:

• Products and Orders in a Business Context: Imagine a software system that has two bounded contexts: one for managing products and one for managing orders. Both of these bounded contexts need to represent the concept of a customer. Instead of defining a separate customer class in each bounded context, the two bounded contexts can share a single customer class from a shared kernel.

// Shared Kernel: Customer class
public class Customer {
    private String customerId;
    private String name;
    private String email;

    // Constructors, getters, and setters
}

// In Products Bounded Context
public class ProductManagement {
    private Map<String, Product> products;
    private Map<String, Customer> customers;  // Reusing Customer from the shared kernel

    // Methods related to managing products
}

// In Orders Bounded Context
public class OrderManagement {
    private Map<String, Order> orders;
    private Map<String, Customer> customers;  // Reusing Customer from the shared kernel

    // Methods related to managing orders
}

• Library Books Catalog: Imagine a university with two separate departments: one for literature and another for history. Both departments maintain their own library catalogs. Instead of duplicating information about the books and their details in each catalog, they decide to create a shared catalog system. This shared catalog contains common information like book titles, authors, and publication years. Both departments can refer to and use this shared catalog, ensuring consistency and avoiding redundant data.

// Shared Kernel: Book class
public class Book {
    private String title;
    private String author;
    private int publicationYear;

    // Constructors, getters, and setters
}

// In Literature Department
public class LiteratureCatalog {
    private List<Book> books;

    // Methods related to managing literature catalog
}

// In History Department
public class HistoryCatalog {
    private List<Book> books;

    // Methods related to managing history catalog
}

• Social Media Profiles: Consider a social media platform with distinct modules for user profiles and messaging. Each module needs to represent a user's profile information, including details like name, profile picture, and bio. Instead of maintaining separate profile structures in each module, they opt for a shared user profile structure. This shared profile ensures that when a user updates their information, it reflects consistently across both the profile and messaging modules, preventing inconsistencies and providing a unified user experience.

// Shared Kernel: UserProfile class
public class UserProfile {
    private String username;
    private String profilePictureUrl;
    private String bio;

    // Constructors, getters, and setters
}

// In User Profile Module
public class UserProfileModule {
    private Map<String, UserProfile> userProfiles;

    // Methods related to managing user profiles
}

// In Messaging Module
public class MessagingModule {
    // Methods related to messaging, using UserProfile from the shared kernel
}

Note: These examples are simplified and intended for illustration purposes. In real-world scenarios, you might have additional considerations, such as handling data consistency across contexts and managing dependencies.