A module is a logical grouping of related bounded contexts, domain entities, value objects, and other artifacts within a domain model. It provides a way to organize and structure the domain model into smaller, more manageable parts, which can help improve maintainability, scalability, and understandability. A module typically represents a specific domain subdomain or domain layer, and encapsulates its own domain logic and functionality. Within a module, domain concepts should be expressed using a consistent ubiquitous language and should be designed to be highly cohesive and loosely coupled to other modules in the system.

Example

Here’s an example of how to create a module in Java using DDD principles.

Let’s say we’re building an e-commerce application, and we want to create a module to handle the shopping cart functionality. First, we’ll create a module package and some sub-packages to organize our code:

com.example.ecommerce
    - module
        - application
        - domain
        - infrastructure

In the domain package, we’ll define our ShoppingCart entity and any related value objects or aggregates:

package com.example.ecommerce.module.domain;

import java.util.List;

public class ShoppingCart {
    private List<CartItem> items;

    public void add(CartItem cartItem) {
        // Implementation omitted
    }

    public void remove(CartItem cartItem) {
        // Implementation omitted
    }

    // Other code omitted
}

In the application package, we’ll define the use cases for the shopping cart module, which might include adding items to the cart, removing items from the cart, and checking out:

package com.example.ecommerce.module.application;

import com.example.ecommerce.module.domain.CartItem;
import com.example.ecommerce.module.domain.ShoppingCart;

public class ShoppingCartService {
    public void addItemToCart(ShoppingCart cart, CartItem item) {
        cart.addCartItem(item);
    }

    public void removeItemFromCart(ShoppingCart cart, CartItem item) {
        cart.removeCartItem(item);
    }

    public void checkout(ShoppingCart cart) {
        // Implementation omitted
    }
}

Finally, in the infrastructure package, we might define any implementation-specific details, such as database or API connections:

package com.example.ecommerce.module.infrastructure;

import com.example.ecommerce.module.domain.ShoppingCart;

public class ShoppingCartRepository {
    public ShoppingCart getById(String id) {
        // Implementation omitted
    }

    public void save(ShoppingCart cart) {
        // Implementation omitted
    }
}

By organizing our code into modules with distinct responsibilities, we can more easily reason about the behavior and interactions of our system, and make changes to one part of the system without affecting others.