lcbp3/.agents/skills/nestjs-best-practices/rules/micro-use-patterns.md

---
title: Use Message and Event Patterns Correctly
impact: MEDIUM
impactDescription: Proper patterns ensure reliable microservice communication
tags: microservices, message-pattern, event-pattern, communication
---

## Use Message and Event Patterns Correctly

NestJS microservices support two communication patterns: request-response (MessagePattern) and event-based (EventPattern). Use MessagePattern when you need a response, and EventPattern for fire-and-forget notifications. Understanding the difference prevents communication bugs.

**Incorrect (using wrong pattern for use case):**

```typescript
// Use @MessagePattern for fire-and-forget
@Controller()
export class NotificationsController {
  @MessagePattern('user.created')
  async handleUserCreated(data: UserCreatedEvent) {
    // This WAITS for response, blocking the sender
    await this.emailService.sendWelcome(data.email);
    // If email fails, sender gets an error (coupling!)
  }
}

// Use @EventPattern expecting a response
@Controller()
export class OrdersController {
  @EventPattern('inventory.check')
  async checkInventory(data: CheckInventoryDto) {
    const available = await this.inventory.check(data);
    return available; // This return value is IGNORED with @EventPattern!
  }
}

// Tight coupling in client
@Injectable()
export class UsersService {
  async createUser(dto: CreateUserDto): Promise<User> {
    const user = await this.repo.save(dto);

    // Blocks until notification service responds
    await this.client.send('user.created', user).toPromise();
    // If notification service is down, user creation fails!

    return user;
  }
}
```

**Correct (use MessagePattern for request-response, EventPattern for fire-and-forget):**

```typescript
// MessagePattern: Request-Response (when you NEED a response)
@Controller()
export class InventoryController {
  @MessagePattern({ cmd: 'check_inventory' })
  async checkInventory(data: CheckInventoryDto): Promise<InventoryResult> {
    const result = await this.inventoryService.check(data.productId, data.quantity);
    return result; // Response sent back to caller
  }
}

// Client expects response
@Injectable()
export class OrdersService {
  async createOrder(dto: CreateOrderDto): Promise<Order> {
    // Check inventory - we NEED this response to proceed
    const inventory = await firstValueFrom(
      this.inventoryClient.send<InventoryResult>(
        { cmd: 'check_inventory' },
        { productId: dto.productId, quantity: dto.quantity },
      ),
    );

    if (!inventory.available) {
      throw new BadRequestException('Insufficient inventory');
    }

    return this.repo.save(dto);
  }
}

// EventPattern: Fire-and-Forget (for notifications, side effects)
@Controller()
export class NotificationsController {
  @EventPattern('user.created')
  async handleUserCreated(data: UserCreatedEvent): Promise<void> {
    // No return value needed - just process the event
    await this.emailService.sendWelcome(data.email);
    await this.analyticsService.track('user_signup', data);
    // If this fails, it doesn't affect the sender
  }
}

// Client emits event without waiting
@Injectable()
export class UsersService {
  async createUser(dto: CreateUserDto): Promise<User> {
    const user = await this.repo.save(dto);

    // Fire and forget - doesn't block, doesn't wait
    this.eventClient.emit('user.created', {
      userId: user.id,
      email: user.email,
      timestamp: new Date(),
    });

    return user; // User creation succeeds regardless of event handling
  }
}

// Hybrid pattern for critical events
@Injectable()
export class OrdersService {
  async createOrder(dto: CreateOrderDto): Promise<Order> {
    const order = await this.repo.save(dto);

    // Critical: inventory reservation (use MessagePattern)
    const reserved = await firstValueFrom(
      this.inventoryClient.send({ cmd: 'reserve_inventory' }, {
        orderId: order.id,
        items: dto.items,
      }),
    );

    if (!reserved.success) {
      await this.repo.delete(order.id);
      throw new BadRequestException('Could not reserve inventory');
    }

    // Non-critical: notifications (use EventPattern)
    this.eventClient.emit('order.created', {
      orderId: order.id,
      userId: dto.userId,
      total: dto.total,
    });

    return order;
  }
}

// Error handling patterns
// MessagePattern errors propagate to caller
@MessagePattern({ cmd: 'get_user' })
async getUser(userId: string): Promise<User> {
  const user = await this.repo.findOne({ where: { id: userId } });
  if (!user) {
    throw new RpcException('User not found'); // Received by caller
  }
  return user;
}

// EventPattern errors should be handled locally
@EventPattern('order.created')
async handleOrderCreated(data: OrderCreatedEvent): Promise<void> {
  try {
    await this.processOrder(data);
  } catch (error) {
    // Log and potentially retry - don't throw
    this.logger.error('Failed to process order event', error);
    await this.deadLetterQueue.add(data);
  }
}
```

Reference: [NestJS Microservices](https://docs.nestjs.com/microservices/basics)