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)
```

Load `Read("${CLAUDE_SKILL_DIR}/references/entities-value-objects.md")` for complete patterns.

### 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)
```

Load `Read("${CLAUDE_SKILL_DIR}/references/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 (load `${CLAUDE_SKILL_DIR}/rules/aggregate-boundaries.md`) | HIGH | Aggregate root design, reference by ID, one-per-transaction |
| aggregate-invariants (load `${CLAUDE_SKILL_DIR}/rules/aggregate-invariants.md`) | HIGH | Business rule enforcement, specification pattern |
| aggregate-sizing (load `${CLAUDE_SKILL_DIR}/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!
```

## Related Skills

- `aggregate-patterns` - Deep dive on aggregate design
- `ork:distributed-systems` - Cross-aggregate coordination
- `ork:database-patterns` - Schema design for DDD

## References

Load on demand with `Read("${CLAUDE_SKILL_DIR}/references/<file>")`:
| File | Content |
|------|---------|
| `entities-value-objects.md` | Full entity and value object patterns |
| `repositories.md` | Repository pattern implementation |
| `domain-events.md` | Event collection and publishing |
| `bounded-contexts.md` | 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