ddd-domain-events 0.1.6

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

Why should you consider using Domain Events?

Domain Events are based on the Observer Design Pattern, which is a convenient way to decouple your Domain Layer Business Logic from the Application Layer code - especially as the Domain code should have no Infrastructure Layer dependencies (such as calling Repositories or Providers that call external services).

In other words, your Domain Entities should have no access to the Application Layer Services - and hence, your Entities might use Domain Events to indirectly communicate with Application Services.

For example, say you have a Gaming Domain with a User entity that has add_points(number_of_points) behavior. Whenever add_points is executed, the obvious expected behavior is to add the provided number of points to the specific user. However, in our sample game, whenever a user reaches 1,000 points then she should receive the “Master” badge, and whenever the user reaches 1,000,000 points then she should receive the “Champion” badge.

Your Application Layer pseudo python code might look like this:

user.add_points(10,000)
if user.has_reached_master_level:
    # TODO: Send the user a congratulation email...
elif user.has_reached_champion_level:
    # TODO: Send the user both a congratulation email and a check...

The above code is perfectly o.k. - yet, as there will be more actions / options that are the consequence of add_points, then the code will become more and more cumbersome.

Using the Domain Events alternative, will allow you to write code that looks more like this pseudo code:

domain_events.register_event(has_reached_master_level_callback)
domain_events.register_event(has_reached_champion_level_callback)

user.add_points(10,000)

This code is cleaner and arguably easier to extend, as shown bellow:

domain_events.register_event(has_reached_master_level_callback)
domain_events.register_event(has_reached_champion_level_callback)
# introduce new level...
domain_events.register_event(has_reached_intermediate_level_callback)

user.add_points(50)

Last but not least, Domain Events allow you to keep your Entity’s code cleaner - with less methods (such as has_reached_master_level and has_reached_champion_level which can be omitted) that might be relatively complex - as they might require you to use additional state in the entity (so that you know if the user has reached the current level following the recent call to add_points).

Using DDD Domain Events

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

For example:

Somewhere in the Application Layer…

with DomainEvents() as domain_events:
    # create a callback to the notify_top_management Application Layer function
    high_price_volume_callback = DomainEventCallable(OrderEvent.HIGH_VOLUME_PRICE, notify_top_management),

    # register callback - so it can be triggered from the Domain Layer
    domain_events.register_event(high_price_volume_callback)

    # create Domain Entity
    order = Order()

    # execute a Domain method that might raise the relevant Domain Event
    order.add_order_items(order_items)

Somewhere in the Domain Layer…

# Domain entity raises a Domain Event - allowing the Application Layer
# to take a relevant action.
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

from ddd_domain_events import DomainEvents, DomainEventCallable

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"""