비동기 처리: @Async + 이벤트

@Configuration
@EnableAsync
public class AsyncConfig {

    @Bean(name = "taskExecutor")
    public Executor taskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(10);
        executor.setMaxPoolSize(50);
        executor.setQueueCapacity(100);
        executor.setThreadNamePrefix("async-");
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
        return executor;
    }
}


@Service
@RequiredArgsConstructor
public class EmailService {

    @Async("taskExecutor")
    public CompletableFuture<Void> sendWelcomeEmail(String to) {
        log.info("이메일 발송 시작: {}", to);
        // ... SMTP 호출 (느림)
        Thread.sleep(2000);
        log.info("이메일 발송 완료");
        return CompletableFuture.completedFuture(null);
    }
}


@Service
@RequiredArgsConstructor
public class UserService {
    private final EmailService emailService;
    private final UserRepository userRepo;

    public User signup(SignupDto dto) {
        User user = userRepo.save(new User(dto));
        // 이메일 발송이 회원가입 응답을 막지 않음
        emailService.sendWelcomeEmail(user.getEmail());
        return user;
    }
}

// 이벤트 정의
public record OrderPlacedEvent(Long orderId, String userEmail, BigDecimal total) {}


// 발행
@Service
@RequiredArgsConstructor
public class OrderService {
    private final ApplicationEventPublisher publisher;
    private final OrderRepository orderRepo;

    @Transactional
    public Order placeOrder(OrderRequest req) {
        Order order = orderRepo.save(new Order(req));
        // 이벤트 발행 — 트랜잭션 커밋 후 처리됨
        publisher.publishEvent(new OrderPlacedEvent(
            order.getId(), order.getUserEmail(), order.getTotal()
        ));
        return order;
    }
}


// 리스너
@Component
@Slf4j
@RequiredArgsConstructor
public class OrderEventListener {
    private final EmailService emailService;
    private final NotificationService notificationService;
    private final InventoryService inventoryService;

    @Async
    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    public void handleOrderPlaced(OrderPlacedEvent event) {
        log.info("주문 후처리: {}", event.orderId());
        emailService.sendOrderConfirmation(event.userEmail(), event.orderId());
        notificationService.send(event.userEmail(), "주문이 접수되었습니다");
        inventoryService.decrementForOrder(event.orderId());
    }
}

public CompletableFuture<UserDashboard> getDashboard(Long userId) {
    CompletableFuture<User> userFuture = userService.findByIdAsync(userId);
    CompletableFuture<List<Order>> ordersFuture = orderService.findByUserAsync(userId);
    CompletableFuture<List<Notification>> notifsFuture = notifService.findByUserAsync(userId);

    return CompletableFuture.allOf(userFuture, ordersFuture, notifsFuture)
        .thenApply(v -> new UserDashboard(
            userFuture.join(),
            ordersFuture.join(),
            notifsFuture.join()
        ));
}