@Service
public class NotificationService {
    @Async
    public void sendEmail(String to, String subject, String body) {
        // 이메일 전송 로직 (blocking I/O)
    }
}
 
@Service
public class OrderService {
    private final NotificationService notificationService;
 
    @Transactional
    public void placeOrder(OrderDto dto) {
        // 1) 재고 확인
        // 2) 결제 처리
        // 3) 주문 저장
 
        // 비동기 이메일 알림
        notificationService.sendEmail(dto.getEmail(), "주문 완료",
            "주문이 접수되었습니다.");
    }
}

public CompletableFuture<OrderResult> processOrderAsync(OrderDto dto) {
    return CompletableFuture
        .supplyAsync(() -> validate(dto))
        .thenCompose(validDto -> CompletableFuture.supplyAsync(() -> charge(validDto)))
        .thenCombine(
            CompletableFuture.supplyAsync(() -> reserveStock(dto)),
            (payment, stock) -> new OrderResult(payment, stock)
        )
        .exceptionally(ex -> {
            // 예외 처리 로직
            return OrderResult.failure(ex);
        });
}

@RestController
@RequestMapping("/orders")
public class OrderController {
    private final OrderService orderService;
 
    @PostMapping
    public Mono<OrderResult> createOrder(@RequestBody OrderDto dto) {
        return orderService.processOrderReactive(dto);
    }
}
 
@Service
public class OrderService {
    public Mono<OrderResult> processOrderReactive(OrderDto dto) {
        return Mono.fromCallable(() -> validate(dto))
            .publishOn(Schedulers.boundedElastic())
            .flatMap(validDto -> Mono.fromCallable(() -> charge(validDto)))
            .zipWith(Mono.fromCallable(() -> reserveStock(dto)))
            .map(tuple -> new OrderResult(tuple.getT1(), tuple.getT2()));
    }
}