Graduate Program KB

Domain-Driven Design (DDD) is an approach to software development that focuses on understanding the business domain and modeling it in software. Event Storming is a collaborative modeling technique that enables teams to explore complex business domains and identify key events, commands, and aggregates. By combining these approaches, a software developer can create a robust domain layer for a bookshop inventory management system.

Here's a detailed explanation of how to use DDD and Event Storming to develop the domain layer:

1. Event Storming session: Gather domain experts, developers, and other stakeholders to conduct an Event Storming session. Use sticky notes or a digital board to capture key events, commands, aggregates, and policies. For the bookshop inventory management system, examples of events might be:

• BookAddedToInventory
• BookRemovedFromInventory
• InventoryAdjusted

2. Identify key domain concepts: From the Event Storming session, identify key domain concepts (entities, value objects, aggregates, and domain events). In the bookshop inventory system, some examples might include:

• Entities:

** Book ** InventoryItem

• Value Objects:

** ISBN ** BookTitle ** BookAuthor

• Aggregates:

** Inventory

• Domain Events:

** BookAddedToInventory ** BookRemovedFromInventory ** InventoryAdjusted

3. Define aggregates: An aggregate is a cluster of domain objects that can be treated as a single unit. In our case, the Inventory aggregate ensures that the business rules around inventory management are enforced.

class Inventory {
  constructor() {
    this.items = new Map();
  }

  addItem(book, quantity) {
    // Business logic for adding items
    this.items.set(book.isbn, { book, quantity });
    // Emit the domain event
    return new BookAddedToInventory(book, quantity);
  }

  removeItem(isbn, quantity) {
    // Business logic for removing items
    const item = this.items.get(isbn);
    if (!item || item.quantity < quantity) {
      throw new Error("Not enough books in inventory");
    }
    item.quantity -= quantity;
    this.items.set(isbn, item);
    // Emit the domain event
    return new BookRemovedFromInventory(isbn, quantity);
  }

  adjustInventory(isbn, newQuantity) {
    // Business logic for adjusting inventory
    if (newQuantity < 0) {
      throw new Error("Quantity cannot be negative");
    }
    const item = this.items.get(isbn);
    if (!item) {
      throw new Error("Book not found in inventory");
    }
    item.quantity = newQuantity;
    this.items.set(isbn, item);
    // Emit the domain event
    return new InventoryAdjusted(isbn, newQuantity);
  }
}

4. Define entities and value objects: Entities are objects with a distinct identity, while value objects are immutable and their identity is based on their attributes. In our case, Book and InventoryItem are entities, while ISBN, BookTitle, and BookAuthor are value objects.

class ISBN {
  constructor(value) {
    // Validate ISBN format and assign value
  }
}

class BookTitle {
  constructor(value) {
    // Assign value
  }
}

class BookAuthor {
  constructor(value) {
    // Assign value
  }
}

class Book {
  constructor(isbn, title, author) {
    this.isbn = new ISBN(isbn);
    this.title = new BookTitle(title);
    this.author = new BookAuthor(author);
  }
}

class InventoryItem {
  constructor(book, quantity) {
    this.book = book;
    this.quantity = quantity;
  }
}

5. Implement domain events:

Domain events are a way to communicate that something important has happened within the domain. In our case, we have three domain events: BookAddedToInventory, BookRemovedFromInventory, and InventoryAdjusted. Implement these events as classes with the necessary data.

class DomainEvent {
  constructor(occurredOn) {
    this.occurredOn = occurredOn || new Date();
  }
}

class BookAddedToInventory extends DomainEvent {
  constructor(book, quantity, occurredOn) {
    super(occurredOn);
    this.book = book;
    this.quantity = quantity;
  }
}

class BookRemovedFromInventory extends DomainEvent {
  constructor(isbn, quantity, occurredOn) {
    super(occurredOn);
    this.isbn = isbn;
    this.quantity = quantity;
  }
}

class InventoryAdjusted extends DomainEvent {
  constructor(isbn, newQuantity, occurredOn) {
    super(occurredOn);
    this.isbn = isbn;
    this.newQuantity = newQuantity;
  }
}

6. Integrate with the application layer:

With the domain layer in place, you can now integrate it with the application layer. This could involve using command handlers to handle user actions and dispatching the resulting domain events to listeners or event handlers.

For example, you could have a command handler for adding a book to the inventory:

class AddBookToInventoryCommandHandler {
  constructor(inventory) {
    this.inventory = inventory;
  }

  handle(command) {
    const book = new Book(command.isbn, command.title, command.author);
    const event = this.inventory.addItem(book, command.quantity);

    // Handle the event, e.g., persist the changes or dispatch to event listeners
    console.log("Event:", event);
  }
}

7. Testing and validation:

Finally, thoroughly test your domain layer to ensure it enforces the correct business rules and provides a solid foundation for your bookshop inventory management system.

8. Conclusion

In conclusion, using Domain-Driven Design and Event Storming, a software developer can create a robust, maintainable, and scalable domain layer for a bookshop inventory management system. The techniques encourage collaboration between domain experts and developers, resulting in a system that accurately reflects the business domain and its rules.