Order → Payment → Inventory → Shipment
  ↓        ↓        ↓          ↓
Cancel  Refund   Release    Cancel

@Service
@RequiredArgsConstructor
public class OrderService {
    private final OrderRepository orderRepository;
    private final KafkaTemplate<String, Object> kafka;

    @Transactional
    public Order createOrder(CreateOrderCommand cmd) {
        Order order = orderRepository.save(new Order(cmd.orderId(), cmd.items()));
        kafka.send("order.created", new OrderCreatedEvent(order.getId(), order.getItems()));
        return order;
    }
}

@Service
@RequiredArgsConstructor
public class PaymentService {
    private final PaymentRepository paymentRepository;
    private final KafkaTemplate<String, Object> kafka;

    public void processPayment(PaymentRequest request) {
        Payment payment = paymentRepository.save(new Payment(request.orderId(), request.amount()));
        kafka.send("payment.processed", new PaymentProcessedEvent(payment.getId(), request.orderId()));
    }

    @Transactional
    public void refundPayment(String paymentId) {
        paymentRepository.findById(paymentId)
            .ifPresent(p -> {
                p.setStatus(REFUNDED);
                paymentRepository.save(p);
                kafka.send("payment.refunded", new PaymentRefundedEvent(paymentId));
            });
    }
}

@Configuration
@EnableKafka
public class KafkaConfig {
    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, Object> kafkaListenerContainerFactory(
            ConsumerFactory<String, Object> consumerFactory) {
        ConcurrentKafkaListenerContainerFactory<String, Object> factory =
            new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(consumerFactory);
        factory.setCommonErrorHandler(new DefaultErrorHandler());
        return factory;
    }
}

@Service
@RequiredArgsConstructor
public class OrderSagaOrchestrator {
    private final KafkaTemplate<String, Object> kafka;
    private final SagaStateRepository sagaStateRepo;

    public void startSaga(OrderRequest request) {
        String sagaId = UUID.randomUUID().toString();
        sagaStateRepo.save(new SagaState(sagaId, STARTED, LocalDateTime.now()));
        kafka.send("saga.order.start", new StartOrderSagaCommand(sagaId, request));
    }

    @KafkaListener(topics = "payment.failed")
    public void handlePaymentFailed(PaymentFailedEvent event) {
        kafka.send("order.compensate", new CompensateOrderCommand(event.getSagaId()));
        kafka.send("inventory.compensate", new ReleaseInventoryCommand(event.getSagaId()));
        sagaStateRepo.updateStatus(event.getSagaId(), FAILED);
    }
}

@Service
public class OrderEventHandler {
    private final OrderService orderService;
    private final KafkaTemplate<String, Object> kafka;

    @KafkaListener(topics = "payment.processed", groupId = "order-service")
    public void onPaymentProcessed(PaymentProcessedEvent event) {
        try {
            InventoryReservedEvent result = orderService.reserveInventory(event.toInventoryRequest());
            kafka.send("inventory.reserved", result);
        } catch (InsufficientInventoryException e) {
            kafka.send("inventory.insufficient", new InsufficientInventoryEvent(event.getOrderId(), event.getPaymentId()));
        }
    }
}

@Configuration
public class SagaMetricsConfig {
    @Bean
    public MeterRegistry meterRegistry() {
        return new PrometheusMeterRegistry(PrometheusConfig.DEFAULT);
    }
}

// Application.java
@SpringBootApplication
@EnableKafka
@EnableKafkaListeners
public class OrderApplication {
    public static void main(String[] args) {
        SpringApplication.run(OrderApplication.class, args);
    }
}

// Event Classes (immutable)
public record OrderCreatedEvent(String orderId, List<OrderItem> items) {}
public record PaymentProcessedEvent(String paymentId, String orderId) {}
public record InventoryReservedEvent(String reservationId, String orderId) {}
public record PaymentFailedEvent(String orderId, String reason) {}
public record InsufficientInventoryEvent(String orderId, String paymentId) {}

// OrderService with compensation
@Service
@RequiredArgsConstructor
public class OrderService {
    private final OrderRepository orderRepository;
    private final KafkaTemplate<String, Object> kafka;

    @KafkaListener(topics = "payment.failed", groupId = "order-service")
    public void handleCompensation(PaymentFailedEvent event) {
        orderRepository.findByOrderId(event.orderId())
            .ifPresent(order -> {
                order.setStatus(CANCELLED);
                orderRepository.save(order);
            });
    }
}

// Command
@Aggregate
public class OrderAggregate {
    @AggregateIdentifier
    private String orderId;

    @CommandHandler
    public OrderAggregate(CreateOrderCommand cmd) {
        apply(new OrderCreatedEvent(cmd.orderId(), cmd.items()));
    }

    @EventSourcingHandler
    public void on(OrderCreatedEvent event) {
        this.orderId = event.orderId();
    }

    @CommandHandler
    public void handle(CancelOrderCommand cmd) {
        apply(new OrderCancelledEvent(cmd.orderId(), cmd.reason()));
    }
}