ffsshhttiikk Domain-Driven Design
What I do
I provide a comprehensive approach to software design that emphasizes collaboration between technical and domain experts. DDD focuses on modeling the core domain—the area of expertise that provides the business with competitive advantage. It involves creating a shared model that accurately represents domain knowledge, separating complex domain logic from infrastructure concerns, and building a flexible architecture that can evolve with changing business requirements.
When to use me
Use DDD for complex domains where business rules are critical and evolving. It's ideal when you have domain experts available for collaboration, when technical staff struggle to understand domain concepts, or when projects require long-term maintainability. DDD is overkill for simple CRUD applications, utility software, or projects with trivial domain logic that can be adequately expressed in data models.
Core Concepts
- Ubiquitous Language: Shared vocabulary used consistently across all team communication
- Bounded Context: Explicit boundary where a particular model applies
- Aggregate: Cluster of related objects treated as a single unit
- Entity: Objects with distinct identity that persists over time
- Value Object: Objects defined by their attributes, not identity
- Domain Event: Something significant that happened in the domain
- Repository: Abstraction for accessing aggregates
- Service: Operation that doesn't naturally belong to an entity
- Factory: Responsible for creating complex objects and aggregates
- Anti-Corruption Layer: Translation layer between bounded contexts
- Shared Kernel: Small model shared between contexts
Code Examples
Entities and Value Objects
from abc import ABC
from dataclasses import dataclass
from datetime import datetime
from uuid import UUID, uuid4
class CustomerId:
def __init__(self, value: UUID):
self._value = value
@classmethod
def create(cls) -> 'CustomerId':
return cls(uuid4())
@property
def value(self) -> UUID:
return self._value
class Email:
def __init__(self, address: str):
if "@" not in address:
raise ValueError(f"Invalid email: {address}")
self._address = address.lower()
@property
def address(self) -> str:
return self._address
class CustomerName:
def __init__(self, first: str, last: str):
if not first or not last:
raise ValueError("First and last name required")
self._first = first
self._last = last
@property
def full_name(self) -> str:
return f"{self._first} {self._last}"
@property
def first(self) -> str:
return self._first
@property
def last(self) -> str:
return self._last
class Address(ValueObject):
def __init__(self, street: str, city: str, state: str, zip_code: str):
self._street = street
self._city = city
self._state = state
self._zip_code = zip_code
def __eq__(self, other: object) -> bool:
if not isinstance(other, Address):
return False
return (
self._street == other._street and
self._city == other._city and
self._state == other._state and
self._zip_code == other._zip_code
)
class Customer(Entity):
def __init__(self, customer_id: CustomerId, name: CustomerName, email: Email):
self._id = customer_id
self._name = name
self._email = email
self._addresses: list[Address] = []
self._created_at = datetime.now()
self._is_active = True
@property
def id(self) -> CustomerId:
return self._id
def add_address(self, address: Address) -> None:
self._addresses.append(address)
def change_email(self, new_email: Email) -> None:
self._email = new_email
def deactivate(self) -> None:
self._is_active = False
Aggregate Root
from abc import ABC
from dataclasses import field
from datetime import datetime
from uuid import UUID, uuid4
class OrderId:
def __init__(self, value: UUID):
self._value = value
@classmethod
def create(cls) -> 'OrderId':
return cls(uuid4())
class OrderLine:
def __init__(self, product_id: str, product_name: str, quantity: int, unit_price: float):
self._product_id = product_id
self._product_name = product_name
self._quantity = quantity
self._unit_price = unit_price
@property
def line_total(self) -> float:
return self._quantity * self._unit_price
class OrderStatus:
PENDING = "pending"
CONFIRMED = "confirmed"
SHIPPED = "shipped"
DELIVERED = "delivered"
CANCELLED = "cancelled"
class Order(ABC):
def __init__(self, order_id: OrderId, customer_id: UUID):
self._id = order_id
self._customer_id = customer_id
self._order_lines: list[OrderLine] = []
self._status = OrderStatus.PENDING
self._created_at = datetime.now()
self._shipped_at: datetime | None = None
@property
def id(self) -> OrderId:
return self._id
@property
def total(self) -> float:
return sum(line.line_total for line in self._order_lines)
def add_line(self, product_id: str, product_name: str, quantity: int, unit_price: float) -> None:
line = OrderLine(product_id, product_name, quantity, unit_price)
self._order_lines.append(line)
def confirm(self) -> None:
if self._status == OrderStatus.PENDING:
self._status = OrderStatus.CONFIRMED
def ship(self) -> None:
if self._status == OrderStatus.CONFIRMED:
self._status = OrderStatus.SHIPPED
self._shipped_at = datetime.now()
def cancel(self) -> None:
if self._status not in [OrderStatus.SHIPPED, OrderStatus.DELIVERED]:
self._status = OrderStatus.CANCELLED
def is_shippable(self) -> bool:
return self._status == OrderStatus.CONFIRMED
Domain Events
from abc import ABC
from dataclasses import dataclass
from datetime import datetime
from uuid import UUID
@dataclass
class DomainEvent:
event_id: UUID
occurred_at: datetime
event_type: str
@dataclass
class OrderCreated(DomainEvent):
order_id: UUID
customer_id: UUID
total: float
def __init__(self, order_id: UUID, customer_id: UUID, total: float):
super().__init__(
event_id=uuid4(),
occurred_at=datetime.now(),
event_type="OrderCreated"
)
self.order_id = order_id
self.customer_id = customer_id
self.total = total
@dataclass
class OrderShipped(DomainEvent):
order_id: UUID
tracking_number: str
carrier: str
def __init__(self, order_id: UUID, tracking_number: str, carrier: str):
super().__init__(
event_id=uuid4(),
occurred_at=datetime.now(),
event_type="OrderShipped"
)
self.order_id = order_id
self.tracking_number = tracking_number
self.carrier = carrier
class EventPublisher:
def __init__(self):
self._handlers: dict[str, list] = {}
def subscribe(self, event_type: str, handler: callable) -> None:
if event_type not in self._handlers:
self._handlers[event_type] = []
self._handlers[event_type].append(handler)
def publish(self, event: DomainEvent) -> None:
event_type = event.event_type
if event_type in self._handlers:
for handler in self._handlers[event_type]:
handler(event)
class OrderService:
def __init__(self, event_publisher: EventPublisher):
self._events = event_publisher
def create_order(self, customer_id: UUID, total: float) -> UUID:
order_id = uuid4()
event = OrderCreated(order_id, customer_id, total)
self._events.publish(event)
return order_id
Repository Pattern
from abc import ABC, abstractmethod
from datetime import datetime
from uuid import UUID
class CustomerRepository(ABC):
@abstractmethod
def save(self, customer: 'Customer') -> None:
pass
@abstractmethod
def find_by_id(self, customer_id: UUID) -> 'Customer | None':
pass
@abstractmethod
def find_by_email(self, email: str) -> 'Customer | None':
pass
@abstractmethod
def find_all_active(self) -> list['Customer']:
pass
class InMemoryCustomerRepository(CustomerRepository):
def __init__(self):
self._customers: dict[UUID, 'Customer'] = {}
def save(self, customer: 'Customer') -> None:
self._customers[customer.id.value] = customer
def find_by_id(self, customer_id: UUID) -> 'Customer | None':
return self._customers.get(customer_id)
def find_by_email(self, email: str) -> 'Customer | None':
for customer in self._customers.values():
if customer._email.address == email:
return customer
return None
def find_all_active(self) -> list['Customer']:
return [c for c in self._customers.values() if c._is_active]
Anti-Corruption Layer
from abc import ABC, abstractmethod
from dataclasses import dataclass
from datetime import datetime
from typing import Optional
@dataclass
class ExternalOrderDTO:
external_id: str
customer_name: str
order_date: str
items: list[dict]
total_amount: float
class OrderTranslator:
@staticmethod
def to_internal(dto: ExternalOrderDTO) -> 'InternalOrder':
return InternalOrder(
external_reference=dto.external_id,
customer_name=dto.customer_name,
order_date=datetime.fromisoformat(dto.order_date),
items=[
TranslatedItem(
sku=item["sku"],
description=item["desc"],
qty=item["quantity"],
price=item["unit_price"]
)
for item in dto.items
],
total=dto.total_amount
)
class ExternalOrderServiceAdapter:
def __init__(self, acl: OrderTranslator, external_client):
self._acl = acl
self._external = external_client
def get_order(self, order_id: str) -> Optional[InternalOrder]:
dto = self._external.fetch_order(order_id)
if dto is None:
return None
return self._acl.to_internal(dto)
class InternalOrder:
def __init__(
self,
external_reference: str,
customer_name: str,
order_date: datetime,
items: list['TranslatedItem'],
total: float
):
self.external_reference = external_reference
self.customer_name = customer_name
self.order_date = order_date
self.items = items
self.total = total
class TranslatedItem:
def __init__(self, sku: str, description: str, qty: int, price: float):
self.sku = sku
self.description = description
self.quantity = qty
self.price = price
Best Practices
- Model the Domain: Deep understanding leads to better models
- Use Ubiquitous Language: Speak the domain expert's language everywhere
- Define Bounded Contexts: Clear boundaries prevent model pollution
- Keep Aggregates Small: Only include objects that must change together
- Design Events First: Start with domain events for complex workflows
- Separate Core Domain: Focus effort on what makes the business unique
- Collaborate with Domain Experts: They know the business rules
- Apply Strategic DDD First: Context mapping before tactical patterns
- Refactor Towards Deeper Insight: Models evolve with understanding
- Use Value Objects: Replace primitives with meaningful domain concepts
- Avoid Anemic Domain Models: Domain objects should have behavior
- Preserve Aggregate Invariants: Keep consistency within boundaries