DDD Conventions (NestJS)

See also: php-ddd-conventions for the same principles in PHP/Symfony context.

Domain Layer Purity

The domain layer MUST NOT depend on infrastructure concerns.

Forbidden in Domain Layer

• NestJS decorators: @Injectable(), @Controller(), @Module(), @Inject()
• HTTP-related: Request, Response, @Body(), @Param(), @Query()
• ORM/Database: Drizzle schemas (pgTable), Drizzle query builders, PrismaClient
• Framework services: ConfigService, HttpService, Logger (NestJS)
• External libraries: anything tied to infrastructure

Allowed in Domain Layer

• Pure TypeScript types (string, number, Date, Record, etc.)
• Domain value objects (classes with no framework dependencies)
• Domain interfaces (defined in domain, implemented in infrastructure)
• DTOs from packages/shared (Zod schemas + inferred types)
• Enums and as const objects
• Branded types

Architecture

src/
├── domain/                    # PURE — no framework imports
│   ├── model/                 # Entities, value objects, aggregates
│   ├── port/                  # Interfaces (repository contracts)
│   ├── service/               # Domain services (pure logic)
│   └── error/                 # Domain exceptions
│
├── application/               # Use cases — orchestration
│   └── use-case/              # May use @Injectable() for DI
│
└── infrastructure/            # Framework & external — implements ports
    ├── persistence/           # Drizzle repositories (implements port/)
    ├── http/                  # Controllers, guards, pipes
    └── config/                # NestJS module, providers

Bounded Contexts = NestJS Modules

Each bounded context maps to a NestJS module:

// ✅ CORRECT - One module per bounded context
@Module({
  controllers: [ReceiptController],
  providers: [
    GenerateReceiptUseCase,
    { provide: RECEIPT_REPOSITORY, useClass: DrizzleReceiptRepository },
  ],
})
export class ReceiptModule {}

Dependency Rule

Controllers → Use Cases → Domain Services → Domain Ports
                                                ↑
Infrastructure Adapters ────────────────────────┘
  (implement domain ports)

Domain NEVER imports from infrastructure. Infrastructure imports from domain.

Example: Domain Port + Infrastructure Adapter

// ✅ domain/port/receipt-repository.ts — pure interface
export interface ReceiptRepository {
  findByTenantId(tenantId: TenantId): Promise<Receipt[]>;
  save(receipt: Receipt): Promise<void>;
}

export const RECEIPT_REPOSITORY = Symbol('ReceiptRepository');

// ✅ infrastructure/persistence/drizzle-receipt-repository.ts
import { Injectable, Inject } from '@nestjs/common';
import { ReceiptRepository } from '../../domain/port/receipt-repository';

@Injectable()
export class DrizzleReceiptRepository implements ReceiptRepository {
  constructor(@Inject('DRIZZLE') private readonly db: DrizzleDatabase) {}

  async findByTenantId(tenantId: TenantId): Promise<Receipt[]> {
    // Drizzle query here — infrastructure concern
  }

  async save(receipt: Receipt): Promise<void> {
    // Drizzle insert here
  }
}

Quick Reference

Layer	Can import from	Cannot import from
Domain	Pure TS, shared DTOs	NestJS, Drizzle, HTTP, any framework
Application	Domain, NestJS DI	Infrastructure directly
Infrastructure	Domain, NestJS, Drizzle	—

Rule	Principle
Domain purity	No decorators, no ORM, no HTTP in domain
Ports & adapters	Interfaces in domain, implementations in infrastructure
Bounded context	1 NestJS module = 1 bounded context
Dependency direction	Always inward: infra → application → domain