1---
2title: Use Event-Driven Architecture for Decoupling
3impact: MEDIUM-HIGH
4impactDescription: Enables async processing and modularity
5tags: architecture, events, decoupling
6---
7 
8## Use Event-Driven Architecture for Decoupling
9 
10Use `@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.
11 
12**Incorrect (direct service coupling):**
13 
14```typescript
15// Direct service coupling
16@Injectable()
17export class OrdersService {
18  constructor(
19    private inventoryService: InventoryService,
20    private emailService: EmailService,
21    private analyticsService: AnalyticsService,
22    private notificationService: NotificationService,
23    private loyaltyService: LoyaltyService,
24  ) {}
25 
26  async createOrder(dto: CreateOrderDto): Promise<Order> {
27    const order = await this.repo.save(dto);
28 
29    // Tight coupling - OrdersService knows about all consumers
30    await this.inventoryService.reserve(order.items);
31    await this.emailService.sendConfirmation(order);
32    await this.analyticsService.track('order_created', order);
33    await this.notificationService.push(order.userId, 'Order placed');
34    await this.loyaltyService.addPoints(order.userId, order.total);
35 
36    // Adding new behavior requires modifying this service
37    return order;
38  }
39}
40```
41 
42**Correct (event-driven decoupling):**
43 
44```typescript
45// Use EventEmitter for decoupling
46import { EventEmitter2 } from '@nestjs/event-emitter';
47 
48// Define event
49export class OrderCreatedEvent {
50  constructor(
51    public readonly orderId: string,
52    public readonly userId: string,
53    public readonly items: OrderItem[],
54    public readonly total: number,
55  ) {}
56}
57 
58// Service emits events
59@Injectable()
60export class OrdersService {
61  constructor(
62    private eventEmitter: EventEmitter2,
63    private repo: Repository<Order>,
64  ) {}
65 
66  async createOrder(dto: CreateOrderDto): Promise<Order> {
67    const order = await this.repo.save(dto);
68 
69    // Emit event - no knowledge of consumers
70    this.eventEmitter.emit(
71      'order.created',
72      new OrderCreatedEvent(order.id, order.userId, order.items, order.total),
73    );
74 
75    return order;
76  }
77}
78 
79// Listeners in separate modules
80@Injectable()
81export class InventoryListener {
82  @OnEvent('order.created')
83  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
84    await this.inventoryService.reserve(event.items);
85  }
86}
87 
88@Injectable()
89export class EmailListener {
90  @OnEvent('order.created')
91  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
92    await this.emailService.sendConfirmation(event.orderId);
93  }
94}
95 
96@Injectable()
97export class AnalyticsListener {
98  @OnEvent('order.created')
99  async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
100    await this.analyticsService.track('order_created', {
101      orderId: event.orderId,
102      total: event.total,
103    });
104  }
105}
106```
107 
108Reference: [NestJS Events](https://docs.nestjs.com/techniques/events)
109