Domain Driven Design

Domain-Driven Design Tactical Patterns

Model complex business domains with entities, value objects, and bounded contexts.

Overview

• Modeling complex business logic
• Separating domain from infrastructure
• Establishing clear boundaries between subdomains
• Building rich domain models with behavior
• Implementing ubiquitous language in code

Building Blocks Overview

┌─────────────────────────────────────────────────────────────┐
│                    DDD Building Blocks                       │
├─────────────────────────────────────────────────────────────┤
│  ENTITIES           VALUE OBJECTS        AGGREGATES         │
│  Order (has ID)     Money (no ID)        [Order]→Items      │
│                                                              │
│  DOMAIN SERVICES    REPOSITORIES         DOMAIN EVENTS      │
│  PricingService     IOrderRepository     OrderSubmitted     │
│                                                              │
│  FACTORIES          SPECIFICATIONS       MODULES            │
│  OrderFactory       OverdueOrderSpec     orders/, payments/ │
└─────────────────────────────────────────────────────────────┘

Quick Reference

Entity (Has Identity)

from dataclasses import dataclass, field
from uuid import UUID
from uuid_utils import uuid7

@dataclass
class Order:
    """Entity: Has identity, mutable state, lifecycle."""
    id: UUID = field(default_factory=uuid7)
    customer_id: UUID = field(default=None)
    status: str = "draft"

    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Order):
            return NotImplemented
        return self.id == other.id  # Identity equality

    def __hash__(self) -> int:
        return hash(self.id)

See entities-value-objects.md for complete patterns.

Value Object (Immutable)

from dataclasses import dataclass
from decimal import Decimal

@dataclass(frozen=True)  # MUST be frozen!
class Money:
    """Value Object: Defined by attributes, not identity."""
    amount: Decimal
    currency: str

    def __add__(self, other: "Money") -> "Money":
        if self.currency != other.currency:
            raise ValueError("Cannot add different currencies")
        return Money(self.amount + other.amount, self.currency)

See entities-value-objects.md for Address, DateRange examples.

Key Decisions

Anti-Patterns (FORBIDDEN)

# NEVER have anemic domain models (data-only classes)
@dataclass
class Order:
    id: UUID
    items: list  # WRONG - no behavior!

# NEVER leak infrastructure into domain
class Order:
    def save(self, session: Session):  # WRONG - knows about DB!

# NEVER use mutable value objects
@dataclass  # WRONG - missing frozen=True
class Money:
    amount: Decimal

# NEVER have repositories return ORM models
async def get(self, id: UUID) -> OrderModel:  # WRONG - return domain!

Related Skills

• aggregate-patterns - Deep dive on aggregate design
• distributed-locks - Cross-aggregate coordination
• database-schema-designer - Schema design for DDD

References

• Entities & Value Objects - Full patterns
• Repositories - Repository pattern implementation
• Domain Events - Event collection and publishing
• Bounded Contexts - Context mapping and ACL

Capability Details

entities

Keywords: entity, identity, lifecycle, mutable, domain object Solves: Model entities in Python, identity equality, adding behavior

value-objects

Keywords: value object, immutable, frozen, dataclass, structural equality Solves: Create immutable value objects, when to use VO vs entity

domain-services

Keywords: domain service, business logic, cross-aggregate, stateless Solves: When to use domain service, logic spanning aggregates

repositories

Keywords: repository, persistence, collection, IRepository, protocol Solves: Implement repository pattern, abstract DB access, ORM mapping

bounded-contexts

Keywords: bounded context, context map, ACL, subdomain, ubiquitous language Solves: Define bounded contexts, integrate with ACL, context relationships