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This diagram shows how DDD structures a system into layers: the user interface, application logic, domain logic with business rules, and infrastructure for technical details. The domain layer is central, representing the core business concepts.
### Before applying DDD - tangled business logic mixed with data access class OrderService: def create_order(self, user_id, items): # Validate items if not items: raise ValueError("No items") # Calculate total total = sum(item['price'] * item['qty'] for item in items) # Save order directly order = {'user_id': user_id, 'items': items, 'total': total} database.save(order) return order ### After applying DDD - clear domain model and separation class OrderItem: def __init__(self, product_id, price, quantity): self.product_id = product_id self.price = price self.quantity = quantity def total_price(self): return self.price * self.quantity class Order: def __init__(self, user_id): self.user_id = user_id self.items = [] def add_item(self, item: OrderItem): self.items.append(item) def total(self): return sum(item.total_price() for item in self.items) class OrderRepository: def save(self, order: Order): # Persist order to database pass class OrderService: def __init__(self, repository: OrderRepository): self.repository = repository def create_order(self, user_id, items_data): if not items_data: raise ValueError("No items") order = Order(user_id) for data in items_data: item = OrderItem(data['product_id'], data['price'], data['quantity']) order.add_item(item) self.repository.save(order) return order
A unique object with an identity that persists over time and changes state.