ghostfolio/.agents/skills/nestjs-best-practices/rules/arch-avoid-circular-deps.md

title	impact	impactDescription	tags
Avoid Circular Dependencies	CRITICAL	#1 cause of runtime crashes	architecture, modules, dependencies

Avoid Circular Dependencies

Circular dependencies occur when Module A imports Module B, and Module B imports Module A (directly or transitively). NestJS can sometimes resolve these through forward references, but they indicate architectural problems and should be avoided. This is the #1 cause of runtime crashes in NestJS applications.

Incorrect (circular module imports):

// users.module.ts
@Module({
  imports: [OrdersModule], // Orders needs Users, Users needs Orders = circular
  providers: [UsersService],
  exports: [UsersService],
})
export class UsersModule {}

// orders.module.ts
@Module({
  imports: [UsersModule], // Circular dependency!
  providers: [OrdersService],
  exports: [OrdersService],
})
export class OrdersModule {}

Correct (extract shared logic or use events):

// Option 1: Extract shared logic to a third module
// shared.module.ts
@Module({
  providers: [SharedService],
  exports: [SharedService],
})
export class SharedModule {}

// users.module.ts
@Module({
  imports: [SharedModule],
  providers: [UsersService],
})
export class UsersModule {}

// orders.module.ts
@Module({
  imports: [SharedModule],
  providers: [OrdersService],
})
export class OrdersModule {}

// Option 2: Use events for decoupled communication
// users.service.ts
@Injectable()
export class UsersService {
  constructor(private eventEmitter: EventEmitter2) {}

  async createUser(data: CreateUserDto) {
    const user = await this.userRepo.save(data);
    this.eventEmitter.emit('user.created', user);
    return user;
  }
}

// orders.service.ts
@Injectable()
export class OrdersService {
  @OnEvent('user.created')
  handleUserCreated(user: User) {
    // React to user creation without direct dependency
  }
}

Reference: NestJS Circular Dependency