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

Decision	Recommendation
Entity vs VO	Has unique ID + lifecycle? Entity. Otherwise VO
Entity equality	By ID, not attributes
Value object mutability	Always immutable (frozen=True)
Repository scope	One per aggregate root
Domain events	Collect in entity, publish after persist
Context boundaries	By business capability, not technical

Rules Quick Reference

Rule	Impact	What It Covers
aggregate-boundaries	HIGH	Aggregate root design, reference by ID, one-per-transaction
aggregate-invariants	HIGH	Business rule enforcement, specification pattern
aggregate-sizing	HIGH	Right-sizing, when to split, eventual consistency

When NOT to Use

Under 5 entities? Skip DDD entirely. The ceremony costs more than the benefit.

Pattern	Interview	Hackathon	MVP	Growth	Enterprise	Simpler Alternative
Aggregates	OVERKILL	OVERKILL	OVERKILL	SELECTIVE	APPROPRIATE	Plain dataclasses with validation
Bounded contexts	OVERKILL	OVERKILL	OVERKILL	BORDERLINE	APPROPRIATE	Python packages with clear imports
CQRS	OVERKILL	OVERKILL	OVERKILL	OVERKILL	WHEN JUSTIFIED	Single model for read/write
Value objects	OVERKILL	OVERKILL	BORDERLINE	APPROPRIATE	REQUIRED	Typed fields on the entity
Domain events	OVERKILL	OVERKILL	OVERKILL	SELECTIVE	APPROPRIATE	Direct method calls between services
Repository pattern	OVERKILL	OVERKILL	BORDERLINE	APPROPRIATE	REQUIRED	Direct ORM queries in service layer

Rule of thumb: DDD adds ~40% code overhead. Only worth it when domain complexity genuinely demands it (5+ entities with invariants spanning multiple objects). A CRUD app with DDD is a red flag.

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-systems - Cross-aggregate coordination
• database-patterns - 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