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)

```python
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)
```

ID generation is a house rule, not a taste call: `Read("${CLAUDE_PLUGIN_ROOT}/skills/domain-driven-design/references/ork-delta.md")`.

### Value Object (Immutable)

```python
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)
```

Canonical Address / DateRange boilerplate is not restated here. See the upstream coverage table below.

## 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 (load `${CLAUDE_PLUGIN_ROOT}/skills/domain-driven-design/rules/aggregate-boundaries.md`) | HIGH | Aggregate root design, reference by ID, one-per-transaction |
| aggregate-invariants (load `${CLAUDE_PLUGIN_ROOT}/skills/domain-driven-design/rules/aggregate-invariants.md`) | HIGH | Business rule enforcement, specification pattern |
| aggregate-sizing (load `${CLAUDE_PLUGIN_ROOT}/skills/domain-driven-design/rules/aggregate-sizing.md`) | 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)

```python
# 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!
```

## Upstream coverage (do not restate)

These topics are documented first-party. Read the source instead of re-deriving them here; only the
house consequences are kept, in `references/ork-delta.md`.

| Topic | Source |
|-------|--------|
| Entity / value-object dataclass mechanics: `frozen`, `__post_init__`, inherited field ordering, `kw_only` | https://docs.python.org/3/library/dataclasses.html |
| Domain event definition, deferred dispatch, handler wiring, dispatch before vs after commit | https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/domain-events-design-implementation |
| Bounded contexts, context map, ubiquitous language, integration patterns (shared kernel, customer-supplier, conformist, open host service, published language) | https://learn.microsoft