DDD Tactical Design Patterns in Go

Purpose

To guide implementation of Domain-Driven Design tactical patterns in idiomatic Go. This skill covers Entities, Value Objects, Aggregates, Repositories, Domain Services, Domain Events, Factories, and Specifications using Go-native idioms (composition over inheritance, interfaces, unexported fields, functional options).

When to Use This Skill

• Implementing domain models with rich business logic in Go
• Designing aggregate boundaries and consistency rules
• Creating repository interfaces and infrastructure implementations
• Building event-driven domain models
• Structuring a Go project following DDD layered architecture
• Reviewing domain code for DDD pattern adherence

Core Principles

1. Go idioms first - No Java-style OOP. Use composition, interfaces, and package boundaries
2. Unexported fields - All entity/aggregate fields lowercase; expose through getters and behavior methods
3. Pointer receivers for entities - Mutable domain objects use *T receivers
4. Value receivers for value objects - Immutable types use T receivers
5. Factory functions - Use NewX() constructors to enforce invariants at creation
6. Interface in domain, implementation in infrastructure - Repository interfaces live with aggregates
7. One package per aggregate - Each aggregate root gets its own package under internal/domain/
8. Context propagation - Pass context.Context as first parameter in repository and service methods

Project Structure

internal/
├── domain/                     # Domain layer (no external deps)
│   ├── customer/               # One package per aggregate
│   │   ├── customer.go         # Aggregate root entity
│   │   ├── repository.go       # Repository interface
│   │   ├── email.go            # Value objects
│   │   ├── events.go           # Domain events
│   │   └── errors.go           # Domain errors
│   ├── order/
│   │   ├── order.go
│   │   ├── repository.go
│   │   ├── item.go             # Child entity
│   │   └── money.go            # Value object
│   └── shared/                 # Shared kernel
│       ├── events.go           # Event interface
│       └── specification.go    # Generic specification
│
├── application/                # Application services (orchestration)
│   ├── command/
│   │   └── place_order.go
│   └── query/
│       └── get_customer.go
│
└── infrastructure/             # Technical implementations
    ├── postgres/
    │   ├── customer_repo.go
    │   └── order_repo.go
    └── eventbus/
        └── in_memory.go

Dependency rule: Domain has zero imports from application or infrastructure. Dependencies point inward.

Pattern Quick Reference

Implementation Workflow

When implementing a new aggregate or domain concept:

1. Define value objects - Create self-validating types for domain primitives (Email, Money, Address)
2. Define entities - Create types with identity, unexported fields, and behavior methods
3. Define aggregate root - Designate one entity as root; enforce all invariants through its methods
4. Define repository interface - Place interface in same package as aggregate root
5. Define domain events - Create immutable event structs for significant state changes
6. Implement infrastructure - Create repository implementations in infrastructure/ package
7. Wire application layer - Create command/query handlers that orchestrate domain operations

Pattern Details

For detailed implementation guides with full code examples, see:

• references/entities-and-value-objects.md - Entities, Value Objects, and Factories
• references/aggregates-and-repositories.md - Aggregates, Repositories, and Domain Services
• references/events-and-specifications.md - Domain Events and Specifications
• references/anti-patterns.md - Common mistakes and how to avoid them

Entities

Entities have unique identity and mutable state. Use unexported fields, pointer receivers, and factory functions.

type Order struct {
    id        uuid.UUID
    status    Status
    items     []Item
    createdAt time.Time
}

func NewOrder(customerID uuid.UUID) (*Order, error) {
    return &Order{
        id:        uuid.New(),
        status:    StatusDraft,
        items:     make([]Item, 0),
        createdAt: time.Now(),
    }, nil
}

func (o *Order) AddItem(product ProductID, qty int, price Money) error {
    if o.status != StatusDraft {
        return ErrOrderNotDraft
    }
    o.items = append(o.items, NewItem(product, qty, price))
    return nil
}

Value Objects

Immutable types validated at creation. Use value receivers. Return new instances for operations.

type Money struct {
    amount   int64
    currency string
}

func NewMoney(amount int64, currency string) (Money, error) {
    if currency == "" {
        return Money{}, ErrInvalidCurrency
    }
    return Money{amount: amount, currency: currency}, nil
}

func (m Money) Add(other Money) (Money, error) {
    if m.currency != other.currency {
        return Money{}, ErrCurrencyMismatch
    }
    return Money{amount: m.amount + other.amount, currency: m.currency}, nil
}

Aggregates

Aggregate roots enforce invariants across child entities. All mutations go through the root.

func (o *Order) Place() error {
    if len(o.items) == 0 {
        return ErrEmptyOrder
    }
    if o.status != StatusDraft {
        return ErrOrderNotDraft
    }
    o.status = StatusPlaced
    o.events = append(o.events, NewOrderPlacedEvent(o.id, o.Total()))
    return nil
}

Repositories

Interface in domain, implementation in infrastructure. One repository per aggregate root.

// domain/order/repository.go
type Repository interface {
    Find(ctx context.Context, id uuid.UUID) (*Order, error)
    Save(ctx context.Context, order *Order) error
    Update(ctx context.Context, id uuid.UUID, fn func(*Order) error) error
}

Domain Events

Immutable structs collected by aggregates, published by application layer.

type OrderPlaced struct {
    orderID    uuid.UUID
    total      Money
    occurredAt time.Time
}

func (o *Order) PullEvents() []Event {
    events := o.events
    o.events = nil
    return events
}

Domain Services

Stateless operations spanning multiple aggregates. Domain logic only, no orchestration.

type TransferService struct {
    accountRepo account.Repository
}

func (s *TransferService) Transfer(ctx context.Context, from, to uuid.UUID, amount Money) error {
    // Load aggregates, validate domain rules, coordinate changes
}

Specifications

Composable business rules using Go generics.

type Specification[T any] interface {
    IsSatisfiedBy(T) bool
}

func And[T any](specs ...Specification[T]) Specification[T] { /* ... */ }
func Or[T any](specs ...Specification[T]) Specification[T]  { /* ... */ }
func Not[T any](spec Specification[T]) Specification[T]     { /* ... */ }

Key Rules

• Never expose aggregate internals - No public fields, no getters that return mutable child collections
• No setters - Replace SetStatus() with domain methods like Place(), Cancel(), Ship()
• Reference other aggregates by ID - Never hold direct pointers to other aggregate roots
• Reconstitution factories - Create separate Reconstruct() functions for loading from DB (bypass validation)
• Domain errors - Define sentinel errors (var ErrNotFound = errors.New(...)) per aggregate package
• Accept interfaces, return structs - Repository parameters use interfaces; factories return concrete types

References

Detailed guides with full code examples are in the references/ directory.