Domain-Driven Design Tactical Patterns Skill

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)

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)

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)

# 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("${CLAUDE_PLUGIN_ROOT}/skills/domain-driven-design/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.

Agent Skills: Domain-Driven Design Tactical Patterns

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