agent-skills/skills/nestjs-best-practices/rules/arch-use-events.md

---
title: Use Event-Driven Architecture for Decoupling
impact: MEDIUM-HIGH
impactDescription: Enables async processing and modularity
tags: architecture, events, decoupling
---

## Use Event-Driven Architecture for Decoupling

Use `@nestjs/event-emitter` for intra-service events and message brokers for inter-service communication. Events allow modules to react to changes without direct dependencies, improving modularity and enabling async processing.

**Incorrect (direct service coupling):**

```typescript
// Direct service coupling
@Injectable()
export class OrdersService {
  constructor(
    private inventoryService: InventoryService,
    private emailService: EmailService,
    private analyticsService: AnalyticsService,
    private notificationService: NotificationService,
    private loyaltyService: LoyaltyService,
  ) {}

  async createOrder(dto: CreateOrderDto): Promise<Order> {
    const order = await this.repo.save(dto);

    // Tight coupling - OrdersService knows about all consumers
    await this.inventoryService.reserve(order.items);
    await this.emailService.sendConfirmation(order);
    await this.analyticsService.track('order_created', order);
    await this.notificationService.push(order.userId, 'Order placed');
    await this.loyaltyService.addPoints(order.userId, order.total);

    // Adding new behavior requires modifying this service
    return order;
  }
}
```

**Correct (event-driven decoupling):**

```typescript
// Use EventEmitter for decoupling
import { EventEmitter2 } from '@nestjs/event-emitter';

// Define event
export class OrderCreatedEvent {
  constructor(
    public readonly orderId: string,
    public readonly userId: string,
    public readonly items: OrderItem[],
    public readonly total: number,
  ) {}
}

// Service emits events
@Injectable()
export class OrdersService {
  constructor(
    private eventEmitter: EventEmitter2,
    private repo: Repository<Order>,
  ) {}

  async createOrder(dto: CreateOrderDto): Promise<Order> {
    const order = await this.repo.save(dto);

    // Emit event - no knowledge of consumers
    this.eventEmitter.emit(
      'order.created',
      new OrderCreatedEvent(order.id, order.userId, order.items, order.total),
    );

    return order;
  }
}

// Listeners in separate modules
@Injectable()
export class InventoryListener {
  @OnEvent('order.created')
  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
    await this.inventoryService.reserve(event.items);
  }
}

@Injectable()
export class EmailListener {
  @OnEvent('order.created')
  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
    await this.emailService.sendConfirmation(event.orderId);
  }
}

@Injectable()
export class AnalyticsListener {
  @OnEvent('order.created')
  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
    await this.analyticsService.track('order_created', {
      orderId: event.orderId,
      total: event.total,
    });
  }
}
```

Reference: [NestJS Events](https://docs.nestjs.com/techniques/events)