domain-driven-design · yonatangross — Claude Code skill: DDD patterns in code

---
name: domain-driven-design
license: MIT
compatibility: "Claude Code 2.1.277+."
description: "DDD tactical patterns for complex business modeling including entities, value objects, aggregates, domain services, repositories, specifications, and bounded contexts. Python dataclass implementations with TypeScript alternatives. Use when building rich domain models, enforcing invariants, or separating domain logic from infrastructure."
user-invocable: false
disable-model-invocation: true
metadata:
  owner-agent: "backend-system-architect"
  category: document-asset-creation
  version: "1.0.0"
  author: "OrchestKit"
  complexity: "medium"
  tags: "ddd, domain-modeling, entities, value-objects, bounded-contexts, python"
allowed-tools: "Read Glob Grep WebFetch WebSearch"
---

# 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("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 `rules/aggregate-boundaries.md`) | HIGH | Aggregate root design, reference by ID, one-per-transaction |
| aggregate-invariants (load `rules/aggregate-invariants.md`) | HIGH | Business rule enforcement, specification pattern |
| aggregate-sizing (load `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.com/en-us/azure/architecture/microservices/model/domain-analysis |
| Anti-corruption layer: what it translates and what it costs | https://learn.microsoft.com/en-us/azure/architecture/patterns/anti-corruption-layer |
| UUIDv7 generation, server side and in Python | https://www.postgresql.org/docs/18/functions-uuid.html and https://github.com/aminalaee/uuid-utils |
| Payment amounts in minor units, zero-decimal currencies | https://docs.stripe.com/currencies |
| Publishing events to a Redis Stream (`XADD` field maps, pipelining) | https://redis.io/docs/latest/commands/xadd/ |
| Layered architecture enforcement, project-structure validation, test standards | the `architecture-patterns` skill in this plugin |

Two subjects deliberately stay in this skill rather than routing upstream: the repository and
Unit of Work implementation, which lives in full in `references/repositories.md`, and aggregate
boundaries, invariants, and sizing, which live in full in `rules/`.

## Related Skills

- `rules/aggregate-boundaries.md`, `rules/aggregate-invariants.md`, `rules/aggregate-sizing.md` - aggregate design, in this skill
- `ork:architecture-patterns` - Layer boundaries and project structure validation
- `ork:distributed-systems` - Cross-aggregate coordination
- `ork:database-patterns` - Schema design for DDD

## References

Load on demand with `Read("references/<file>")`:
| File | Content |
|------|---------|
| `ork-delta.md` | House rules: UUIDv7, event drain ordering, ACL boundary, source layout |
| `repositories.md` | Repository pattern, Unit of Work, SQLAlchemy mapping |

## Capability Details

### entities
**Keywords:** entity, identity, lifecycle, mutable, domain object
**Solves:** Identity equality by ID, and the house UUIDv7 ID rule in `references/ork-delta.md`. Dataclass mechanics route upstream.

### value-objects
**Keywords:** value object, immutable, frozen, dataclass, structural equality
**Solves:** When to use VO vs entity. `frozen=True` semantics and inherited field ordering route upstream.

### 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:** The house ACL boundary and context-first source layout in `references/ork-delta.md`. Context mapping and integration patterns route upstream.