ddd-domain-events 0.1.0

Domain Driven Design:

Inspired by Udi Dahan’s fabulous Domain Events implementation in particular, and by Domain Driven Design best practices in general.

The DDD Domain Events package makes it easy to:

• Register to Domain Events from the Application Layer

• Raise Domain Events from the Domain Layer so they can be handled in the Application Layer

The Domain Events are local to the execution thread (via Python’s threading.local) and hence are thread specific.

Installing ddd-domain-events

pip install ddd-domain-events

Using DDD Domain Events

Using Domain Events in an existing Application Service can easily be done by using Python’s with-statement.

For example:

Somewhere in the Application Layer…

with DomainEvents() as domain_events:
    high_price_volume_callback = DomainEventCallable(OrderEvent.HIGH_VOLUME_PRICE, cls.notify_top_management),

    domain_events.register_event(callback)

    order = Order()

    order.add_order_items(order_items)

** Somewhere in the Domain Layer…**

DomainEvents.raise_event(OrderEvent.HIGH_VOLUME_PRICE, order=self)

How it works

Bellow is a simplified example that should help you understand how and when you might choose to use Domain Events.

Step 1: Define a Domain Event Type in your Domain Layer

class OrderEvent(Enum):
    """Domain Event raised for special order use cases"""
    HIGH_QUANTITY = 'HIGH_QUANTITY'
    HIGH_VOLUME_PRICE = 'HIGH_VOLUME_PRICE'

Step 2: Define a Domain Entity that raises Domain Events

class OrderItem:
    """OrderItem value object that contains order details for a single item"""
    def __init__(self, product_id: str, price: float, quantity: int):
        self.product_id = product_id
        self.price = price
        self.quantity = quantity

class Order:
    """Order entity that contains order items"""
    HIGH_VOLUME_PRICE = 1_000_000
    HIGH_QUANTITY = 10_000

    def __init__(self):
        self._order_items = []

    @property
    def order_items(self):
        for order_item in self._order_items:
            yield order_item

    def add_order_items(self, order_items: List[OrderItem]) -> None:
        total_price = 0
        total_quantity = 0

        for order_item in order_items:
            total_price += (order_item.price * order_item.quantity)
            total_quantity += order_item.quantity
            # Process the actual business logic related to this method,
            # which is add OrderItem value objects to this Order Entity
            self._order_items.append(order_item)

        # Notify whoever might be interested about high price volume orders
        if total_price >= self.HIGH_VOLUME_PRICE:
            DomainEvents.raise_event(OrderEvent.HIGH_VOLUME_PRICE, order=self)

        # Notify whoever might be interested about high quantity volume orders
        if total_quantity >= self.HIGH_QUANTITY:
            DomainEvents.raise_event(OrderEvent.HIGH_QUANTITY, order=self)

Step 3: Define an Application Service that registers to Domain Events

class OrderService:
    """Application Service for handling Order related operations"""
    @classmethod
    def create_order(cls, order_items: List[OrderItem]) -> Order:
        with DomainEvents() as domain_events:
            # Create callbacks for 'side effects' that are related to domain logic,
            # and which should be handled by the Application Layer
            callbacks = [
                DomainEventCallable(OrderEvent.HIGH_VOLUME_PRICE, cls.notify_top_management),
                DomainEventCallable(OrderEvent.HIGH_VOLUME_PRICE, cls.notify_sales_team),
                DomainEventCallable(OrderEvent.HIGH_QUANTITY, cls.notify_inventory_team)
            ]

            # Register for these domain events
            for callback in callbacks:
                domain_events.register_event(callback)

            order = Order()

            order.add_order_items(order_items)

            return order

    @staticmethod
    def notify_sales_team(order: Order) -> None:
        """A callback for notifying the sales team about the important order"""

    @staticmethod
    def notify_top_management(order: Order) -> None:
        """A callback for notifying the top management about the important order"""

    @staticmethod
    def notify_inventory_team(order: Order) -> None:
        """A callback for notifying the inventory team required quantities"""