1// Order service: sync payment (needs result) + async events (don't need to wait)
2class OrderService {
3  constructor(
4    private paymentClient: PaymentServiceClient,  // gRPC
5    private kafka: Producer,
6  ) {}
7 
8  async placeOrder(userId: string, items: CartItem[]): Promise<Order> {
9    // 1. SYNC: charge payment — need result to proceed
Synchronous for payment — you need the result to proceed. Can't confirm order without charge
10    const payment = await this.paymentClient.charge({
11      userId,
12      amountCents: calculateTotal(items),
13    });
14 
15    if (!payment.success) {
16      throw new Error(`Payment failed: ${payment.error}`);
17    }
18 
19    // 2. Save to DB
20    const order = await db.orders.create({
21      userId, items, paymentId: payment.id, status: 'confirmed'
22    });
Async event — downstream services (inventory, email) don't block the order response
23 
24    // 3. ASYNC: publish event — don't wait for downstream services
25    await this.kafka.send({
26      topic: 'order-placed',
27      messages: [{ key: order.id, value: JSON.stringify({
28        orderId: order.id, userId, items,
29      })}]
30    });
31    // inventory-service, email-service, analytics consume this event
32    // Order returns immediately — no blocking on downstream
33 
34    return order;
35  }
36}
37 
38// Circuit Breaker: prevent cascade failures
39import CircuitBreaker from 'opossum';
40 
Circuit breaker: after 50% failure rate, open circuit. Fail fast instead of timing out
41const paymentCircuitBreaker = new CircuitBreaker(
42  (req: ChargeRequest) => paymentClient.charge(req),
43  {
44    timeout: 3000,
45    errorThresholdPercentage: 50,  // open if 50% of calls fail
46    resetTimeout: 30000,           // retry after 30s
47  }
48);
49// When payment service is down, circuit opens
50// Subsequent calls fail immediately — don't pile up on a down service