Domain-Driven Design by Example

Domain-Driven Design (DDD) is an approach to software development that emphasizes understanding and modeling the business domain of an application. By focusing on the domain, DDD aims to create software that better reflects the needs and goals of the business, and that is more maintainable and adaptable over time.

To illustrate how DDD works, let’s consider an example. Imagine that you are building an e-commerce website that sells books. To apply DDD to this project, you would start by identifying the key concepts and entities in the domain:

– Books: The main product being sold on the website.
– Customers: The people who buy the books.
– Orders: The records of purchases made by customers.
– Inventory: The stock of books available for sale.
– Shipping: The process of delivering books to customers.

Next, you would create a model of these entities and their relationships, using domain-specific language and terminology. This model would serve as a common language for developers, business stakeholders, and domain experts to communicate about the system.

For example, you might create a Book entity that includes properties like title, author, and price, and methods like “reserve” and “release” to manage inventory. You might also create a Customer entity with properties like name and email address, and methods like “placeOrder” to create a new Order record.

By focusing on the domain and creating a domain model, you can ensure that the software you build reflects the real-world concepts and processes of the business. This can lead to more maintainable code, better alignment with business goals, and fewer surprises or misunderstandings during development.

In addition to creating a domain model, DDD also emphasizes the use of patterns and techniques to separate concerns and create a more modular, flexible architecture. For example, you might use the Repository pattern to encapsulate data access logic and make it easier to swap out different storage implementations. Or you might use the Command-Query Responsibility Segregation (CQRS) pattern to separate read and write operations and optimize performance.

Overall, Domain-Driven Design is a powerful approach to software development that can help teams build better software by understanding and modeling the business domain. By focusing on the domain, creating a domain model, and applying patterns and techniques, you can create software that better reflects the real-world needs and goals of your business.

public class Book {
    private String title;
    private String author;
    private BigDecimal price;
    private int inventory;

    public Book(String title, String author, BigDecimal price, int inventory) {
        this.title = title;
        this.author = author;
        this.price = price;
        this.inventory = inventory;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public int getInventory() {
        return inventory;
    }

    public void reserve() {
        if (inventory > 0) {
            inventory--;
        } else {
            throw new IllegalStateException("Cannot reserve a book with no inventory");
        }
    }

    public void release() {
        inventory++;
    }
}

public class Customer {
    private String name;
    private String email;
    private List<Order> orders;

    public Customer(String name, String email) {
        this.name = name;
        this.email = email;
        this.orders = new ArrayList<>();
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public List<Order> getOrders() {
        return orders;
    }

    public void placeOrder(List<Book> books) {
        Order order = new Order(this, books);
        orders.add(order);
    }
}

public class Order {
    private Customer customer;
    private List<Book> books;

    public Order(Customer customer, List<Book> books) {
        this.customer = customer;
        this.books = books;
    }

    public Customer getCustomer() {
        return customer;
    }

    public List<Book> getBooks() {
        return books;
    }

    public BigDecimal getTotalPrice() {
        BigDecimal total = BigDecimal.ZERO;
        for (Book book : books) {
            total = total.add(book.getPrice());
        }
        return total;
    }
}