Spring Kafka: architettura event-driven con consumer resilienti

public record OrderEvent(
    // Identificatore univoco dell'evento per idempotenza
    String eventId,
    // Tipo di evento per il routing
    String eventType,
    // Timestamp di creazione
    Instant createdAt,
    // Payload di business
    OrderPayload payload
) {
    // Factory method che garantisce l'unicità di eventId
    public static OrderEvent created(OrderPayload payload) {
        return new OrderEvent(
            UUID.randomUUID().toString(),
            "ORDER_CREATED",
            Instant.now(),
            payload
        );
    }
}

public record OrderPayload(
    Long orderId,
    Long customerId,
    List<OrderItem> items,
    BigDecimal totalAmount
) {}

# application.yml
spring:
  kafka:
    # Indirizzi dei broker Kafka (cluster)
    bootstrap-servers: localhost:9092

    # Configurazione del producer
    producer:
      # Serializzazione delle chiavi String
      key-serializer: org.apache.kafka.common.serialization.StringSerializer
      # Serializzazione dei valori JSON
      value-serializer: org.springframework.kafka.support.serializer.JsonSerializer
      # Attesa della conferma di tutte le repliche
      acks: all
      # Numero di tentativi in caso di errore di rete
      retries: 3

    # Configurazione del consumer
    consumer:
      # Identificatore del consumer group
      group-id: order-service
      # Deserializzazione delle chiavi
      key-deserializer: org.apache.kafka.common.serialization.StringDeserializer
      # Deserializzazione JSON con tipo di destinazione
      value-deserializer: org.springframework.kafka.support.serializer.JsonDeserializer
      # Posizione iniziale se non c'è offset registrato
      auto-offset-reset: earliest
      # Disabilitare auto-commit per controllo manuale
      enable-auto-commit: false
      properties:
        # Pacchetto fidato per la deserializzazione
        spring.json.trusted.packages: com.example.events

@Service
@Slf4j
public class OrderEventPublisher {

    private final KafkaTemplate<String, OrderEvent> kafkaTemplate;

    // Topic di destinazione (esternalizzato in configurazione)
    @Value("${app.kafka.topics.orders}")
    private String ordersTopic;

    public OrderEventPublisher(KafkaTemplate<String, OrderEvent> kafkaTemplate) {
        this.kafkaTemplate = kafkaTemplate;
    }

    public CompletableFuture<SendResult<String, OrderEvent>> publishOrderCreated(
            OrderPayload payload) {

        // Creazione dell'evento con i metadati
        OrderEvent event = OrderEvent.created(payload);

        // Uso di orderId come chiave di partizione
        // Garantisce l'ordine degli eventi per lo stesso ordine
        String partitionKey = payload.orderId().toString();

        log.info("Publishing ORDER_CREATED event: {} to topic: {}",
            event.eventId(), ordersTopic);

        // Invio asincrono con callback
        return kafkaTemplate.send(ordersTopic, partitionKey, event)
            .whenComplete((result, ex) -> {
                if (ex == null) {
                    // Successo: log dei metadati di invio
                    RecordMetadata metadata = result.getRecordMetadata();
                    log.info("Event sent successfully: topic={}, partition={}, offset={}",
                        metadata.topic(), metadata.partition(), metadata.offset());
                } else {
                    // Errore: log per investigazione
                    log.error("Failed to send event: {}", event.eventId(), ex);
                }
            });
    }
}

@Service
@Slf4j
public class OrderEventConsumer {

    private final OrderProcessingService processingService;

    public OrderEventConsumer(OrderProcessingService processingService) {
        this.processingService = processingService;
    }

    @KafkaListener(
        // Topic da ascoltare
        topics = "${app.kafka.topics.orders}",
        // Consumer group
        groupId = "${spring.kafka.consumer.group-id}",
        // Factory personalizzata per configurazione avanzata
        containerFactory = "kafkaListenerContainerFactory"
    )
    public void handleOrderEvent(
            // Payload deserializzato automaticamente
            OrderEvent event,
            // Metadati Kafka iniettati
            @Header(KafkaHeaders.RECEIVED_PARTITION) int partition,
            @Header(KafkaHeaders.OFFSET) long offset,
            @Header(KafkaHeaders.RECEIVED_TIMESTAMP) long timestamp,
            // Acknowledgment per commit manuale
            Acknowledgment acknowledgment) {

        log.info("Received event: type={}, partition={}, offset={}",
            event.eventType(), partition, offset);

        try {
            // Elaborazione di business
            processingService.process(event);

            // Commit dell'offset solo dopo elaborazione riuscita
            acknowledgment.acknowledge();

            log.info("Event processed successfully: {}", event.eventId());

        } catch (Exception ex) {
            // L'assenza di acknowledge() causa il riprocessing
            log.error("Failed to process event: {}", event.eventId(), ex);
            throw ex;
        }
    }
}

@Configuration
@EnableKafka
public class KafkaConsumerConfig {

    @Value("${spring.kafka.bootstrap-servers}")
    private String bootstrapServers;

    @Value("${spring.kafka.consumer.group-id}")
    private String groupId;

    @Bean
    public ConsumerFactory<String, OrderEvent> consumerFactory() {
        Map<String, Object> props = new HashMap<>();

        // Configurazione di connessione
        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
        props.put(ConsumerConfig.GROUP_ID_CONFIG, groupId);

        // Deserializzazione
        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG,
            StringDeserializer.class);
        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG,
            JsonDeserializer.class);

        // Gestione degli offset
        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
        props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);

        // Performance: record per poll
        props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 100);

        // Timeout di sessione per rilevamento dei guasti
        props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, 30000);

        // Intervallo di heartbeat (1/3 del session timeout)
        props.put(ConsumerConfig.HEARTBEAT_INTERVAL_MS_CONFIG, 10000);

        // Tempo massimo di elaborazione prima del rebalance
        props.put(ConsumerConfig.MAX_POLL_INTERVAL_MS_CONFIG, 300000);

        // Configurazione del deserializer JSON
        JsonDeserializer<OrderEvent> deserializer = new JsonDeserializer<>(OrderEvent.class);
        deserializer.addTrustedPackages("com.example.events");
        deserializer.setUseTypeMapperForKey(false);

        return new DefaultKafkaConsumerFactory<>(
            props,
            new StringDeserializer(),
            deserializer
        );
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, OrderEvent>
            kafkaListenerContainerFactory() {

        ConcurrentKafkaListenerContainerFactory<String, OrderEvent> factory =
            new ConcurrentKafkaListenerContainerFactory<>();

        factory.setConsumerFactory(consumerFactory());

        // Modalità di acknowledgment manuale
        factory.getContainerProperties()
            .setAckMode(ContainerProperties.AckMode.MANUAL);

        // Numero di thread consumer
        factory.setConcurrency(3);

        // Elaborazione batch disabilitata (un messaggio alla volta)
        factory.setBatchListener(false);

        return factory;
    }
}

@Configuration
public class KafkaRetryConfig {

    @Bean
    public RetryTemplate kafkaRetryTemplate() {
        RetryTemplate template = new RetryTemplate();

        // Politica di retry: 3 tentativi massimi
        SimpleRetryPolicy retryPolicy = new SimpleRetryPolicy();
        retryPolicy.setMaxAttempts(3);
        template.setRetryPolicy(retryPolicy);

        // Backoff esponenziale: 1s, 2s, 4s tra i tentativi
        ExponentialBackOffPolicy backOffPolicy = new ExponentialBackOffPolicy();
        backOffPolicy.setInitialInterval(1000);
        backOffPolicy.setMultiplier(2.0);
        backOffPolicy.setMaxInterval(10000);
        template.setBackOffPolicy(backOffPolicy);

        return template;
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, OrderEvent>
            retryableKafkaListenerContainerFactory(
                ConsumerFactory<String, OrderEvent> consumerFactory,
                RetryTemplate kafkaRetryTemplate) {

        ConcurrentKafkaListenerContainerFactory<String, OrderEvent> factory =
            new ConcurrentKafkaListenerContainerFactory<>();

        factory.setConsumerFactory(consumerFactory);
        factory.getContainerProperties()
            .setAckMode(ContainerProperties.AckMode.MANUAL);

        // Configurazione retry con callback di recovery
        factory.setCommonErrorHandler(
            new DefaultErrorHandler(
                // Recovery: azione dopo l'esaurimento dei tentativi
                (record, exception) -> {
                    log.error("All retries exhausted for record: key={}, value={}",
                        record.key(), record.value(), exception);
                },
                // Backoff esponenziale: 1s iniziale, max 30s, 3 tentativi
                new ExponentialBackOff(1000L, 2.0)
            )
        );

        return factory;
    }
}

@Configuration
public class DeadLetterConfig {

    @Value("${app.kafka.topics.orders}")
    private String ordersTopic;

    @Bean
    public DeadLetterPublishingRecoverer deadLetterPublishingRecoverer(
            KafkaTemplate<String, Object> kafkaTemplate) {

        // Strategia di naming del topic DLT: original-topic.DLT
        return new DeadLetterPublishingRecoverer(
            kafkaTemplate,
            (record, exception) -> {
                // Topic DLT basato sul topic di origine
                String dltTopic = record.topic() + ".DLT";

                log.warn("Sending failed record to DLT: topic={}, key={}, error={}",
                    dltTopic, record.key(), exception.getMessage());

                return new TopicPartition(dltTopic, record.partition());
            }
        );
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, OrderEvent>
            dltKafkaListenerContainerFactory(
                ConsumerFactory<String, OrderEvent> consumerFactory,
                DeadLetterPublishingRecoverer deadLetterPublishingRecoverer) {

        ConcurrentKafkaListenerContainerFactory<String, OrderEvent> factory =
            new ConcurrentKafkaListenerContainerFactory<>();

        factory.setConsumerFactory(consumerFactory);
        factory.getContainerProperties()
            .setAckMode(ContainerProperties.AckMode.MANUAL);

        // ErrorHandler con recovery DLT
        DefaultErrorHandler errorHandler = new DefaultErrorHandler(
            deadLetterPublishingRecoverer,
            new FixedBackOff(1000L, 3)
        );

        // Eccezioni non riprovabili (invio diretto al DLT)
        errorHandler.addNotRetryableExceptions(
            ValidationException.class,
            JsonParseException.class,
            NullPointerException.class
        );

        factory.setCommonErrorHandler(errorHandler);

        return factory;
    }
}

@Service
@Slf4j
public class DeadLetterConsumer {

    private final AlertingService alertingService;
    private final FailedEventRepository failedEventRepository;

    public DeadLetterConsumer(AlertingService alertingService,
                              FailedEventRepository failedEventRepository) {
        this.alertingService = alertingService;
        this.failedEventRepository = failedEventRepository;
    }

    @KafkaListener(
        topics = "${app.kafka.topics.orders}.DLT",
        groupId = "order-service-dlt"
    )
    public void handleDeadLetter(
            ConsumerRecord<String, OrderEvent> record,
            @Header(KafkaHeaders.DLT_EXCEPTION_MESSAGE) String exceptionMessage,
            @Header(KafkaHeaders.DLT_EXCEPTION_STACKTRACE) String stacktrace,
            @Header(KafkaHeaders.DLT_ORIGINAL_TOPIC) String originalTopic,
            @Header(KafkaHeaders.DLT_ORIGINAL_PARTITION) int originalPartition,
            @Header(KafkaHeaders.DLT_ORIGINAL_OFFSET) long originalOffset,
            Acknowledgment acknowledgment) {

        log.error("Dead letter received: topic={}, partition={}, offset={}, error={}",
            originalTopic, originalPartition, originalOffset, exceptionMessage);

        // Persistenza per analisi e riprocessing successivo
        FailedEvent failedEvent = FailedEvent.builder()
            .eventId(record.value().eventId())
            .eventType(record.value().eventType())
            .payload(serializePayload(record.value()))
            .originalTopic(originalTopic)
            .originalPartition(originalPartition)
            .originalOffset(originalOffset)
            .exceptionMessage(exceptionMessage)
            .stacktrace(stacktrace)
            .status(FailedEventStatus.PENDING)
            .createdAt(Instant.now())
            .build();

        failedEventRepository.save(failedEvent);

        // Alert per intervento umano
        alertingService.notifyDeadLetter(failedEvent);

        acknowledgment.acknowledge();
    }
}

@Service
@Slf4j
public class IdempotentOrderProcessor {

    private final ProcessedEventRepository processedEventRepository;
    private final OrderService orderService;

    public IdempotentOrderProcessor(ProcessedEventRepository processedEventRepository,
                                    OrderService orderService) {
        this.processedEventRepository = processedEventRepository;
        this.orderService = orderService;
    }

    @Transactional
    public void processIdempotent(OrderEvent event) {
        String eventId = event.eventId();

        // Verifica: evento già elaborato?
        if (processedEventRepository.existsByEventId(eventId)) {
            log.info("Event already processed, skipping: {}", eventId);
            return;
        }

        // Elaborazione di business
        switch (event.eventType()) {
            case "ORDER_CREATED" -> orderService.createOrder(event.payload());
            case "ORDER_UPDATED" -> orderService.updateOrder(event.payload());
            case "ORDER_CANCELLED" -> orderService.cancelOrder(event.payload());
            default -> log.warn("Unknown event type: {}", event.eventType());
        }

        // Registrare l'elaborazione nella stessa transazione
        ProcessedEvent processed = ProcessedEvent.builder()
            .eventId(eventId)
            .eventType(event.eventType())
            .processedAt(Instant.now())
            .build();

        processedEventRepository.save(processed);

        log.info("Event processed and recorded: {}", eventId);
    }
}

@Repository
public interface ProcessedEventRepository extends JpaRepository<ProcessedEvent, Long> {

    // Indice su eventId per performance ottimali
    boolean existsByEventId(String eventId);

    // Pulizia dei record obsoleti
    @Modifying
    @Query("DELETE FROM ProcessedEvent e WHERE e.processedAt < :before")
    int deleteOlderThan(@Param("before") Instant before);
}

@Entity
@Table(name = "outbox_events")
public class OutboxEvent {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, unique = true)
    private String eventId;

    @Column(nullable = false)
    private String aggregateType;

    @Column(nullable = false)
    private String aggregateId;

    @Column(nullable = false)
    private String eventType;

    @Column(columnDefinition = "TEXT", nullable = false)
    private String payload;

    @Enumerated(EnumType.STRING)
    @Column(nullable = false)
    private OutboxStatus status;

    @Column(nullable = false)
    private Instant createdAt;

    private Instant publishedAt;
}

public enum OutboxStatus {
    PENDING, PUBLISHED, FAILED
}

@Service
@Transactional
public class OrderService {

    private final OrderRepository orderRepository;
    private final OutboxRepository outboxRepository;
    private final ObjectMapper objectMapper;

    public OrderService(OrderRepository orderRepository,
                       OutboxRepository outboxRepository,
                       ObjectMapper objectMapper) {
        this.orderRepository = orderRepository;
        this.outboxRepository = outboxRepository;
        this.objectMapper = objectMapper;
    }

    public Order createOrder(CreateOrderRequest request) {
        // Creare l'ordine
        Order order = Order.builder()
            .customerId(request.customerId())
            .items(request.items())
            .totalAmount(calculateTotal(request.items()))
            .status(OrderStatus.CREATED)
            .createdAt(Instant.now())
            .build();

        Order savedOrder = orderRepository.save(order);

        // Creare l'evento outbox nella stessa transazione
        OrderEvent event = OrderEvent.created(toPayload(savedOrder));

        OutboxEvent outboxEvent = OutboxEvent.builder()
            .eventId(event.eventId())
            .aggregateType("Order")
            .aggregateId(savedOrder.getId().toString())
            .eventType(event.eventType())
            .payload(serialize(event))
            .status(OutboxStatus.PENDING)
            .createdAt(Instant.now())
            .build();

        outboxRepository.save(outboxEvent);

        return savedOrder;
    }

    private String serialize(OrderEvent event) {
        try {
            return objectMapper.writeValueAsString(event);
        } catch (JsonProcessingException e) {
            throw new SerializationException("Failed to serialize event", e);
        }
    }
}

@Component
@Slf4j
public class OutboxRelayScheduler {

    private final OutboxRepository outboxRepository;
    private final KafkaTemplate<String, String> kafkaTemplate;

    @Value("${app.kafka.topics.orders}")
    private String ordersTopic;

    public OutboxRelayScheduler(OutboxRepository outboxRepository,
                               KafkaTemplate<String, String> kafkaTemplate) {
        this.outboxRepository = outboxRepository;
        this.kafkaTemplate = kafkaTemplate;
    }

    @Scheduled(fixedDelay = 1000)
    @Transactional
    public void relayPendingEvents() {
        // Recuperare gli eventi pendenti con lock
        List<OutboxEvent> pendingEvents = outboxRepository
            .findByStatusOrderByCreatedAtAsc(OutboxStatus.PENDING);

        for (OutboxEvent event : pendingEvents) {
            try {
                // Pubblicare su Kafka
                kafkaTemplate.send(
                    ordersTopic,
                    event.getAggregateId(),
                    event.getPayload()
                ).get(5, TimeUnit.SECONDS);

                // Aggiornare lo stato
                event.setStatus(OutboxStatus.PUBLISHED);
                event.setPublishedAt(Instant.now());
                outboxRepository.save(event);

                log.debug("Outbox event published: {}", event.getEventId());

            } catch (Exception ex) {
                log.error("Failed to publish outbox event: {}",
                    event.getEventId(), ex);
                event.setStatus(OutboxStatus.FAILED);
                outboxRepository.save(event);
            }
        }
    }
}

@Configuration
public class KafkaMetricsConfig {

    @Bean
    public MeterBinder kafkaConsumerMetrics(
            ConcurrentKafkaListenerContainerFactory<?, ?> factory) {

        return registry -> {
            // Metrica personalizzata: eventi elaborati
            Counter.builder("kafka.consumer.events.processed")
                .description("Number of events successfully processed")
                .tag("topic", "orders")
                .register(registry);

            // Metrica personalizzata: eventi falliti
            Counter.builder("kafka.consumer.events.failed")
                .description("Number of events that failed processing")
                .tag("topic", "orders")
                .register(registry);
        };
    }
}

@Service
@Slf4j
public class InstrumentedOrderConsumer {

    private final OrderProcessingService processingService;
    private final MeterRegistry meterRegistry;
    private final Counter processedCounter;
    private final Counter failedCounter;
    private final Timer processingTimer;

    public InstrumentedOrderConsumer(OrderProcessingService processingService,
                                     MeterRegistry meterRegistry) {
        this.processingService = processingService;
        this.meterRegistry = meterRegistry;

        // Inizializzare i contatori
        this.processedCounter = Counter.builder("kafka.consumer.events.processed")
            .tag("topic", "orders")
            .register(meterRegistry);

        this.failedCounter = Counter.builder("kafka.consumer.events.failed")
            .tag("topic", "orders")
            .register(meterRegistry);

        // Timer per misurare la latenza di elaborazione
        this.processingTimer = Timer.builder("kafka.consumer.processing.duration")
            .tag("topic", "orders")
            .publishPercentiles(0.5, 0.95, 0.99)
            .register(meterRegistry);
    }

    @KafkaListener(topics = "${app.kafka.topics.orders}")
    public void handleOrderEvent(OrderEvent event, Acknowledgment acknowledgment) {

        Timer.Sample sample = Timer.start(meterRegistry);

        try {
            processingService.process(event);
            acknowledgment.acknowledge();

            processedCounter.increment();
            sample.stop(processingTimer);

        } catch (Exception ex) {
            failedCounter.increment();
            sample.stop(processingTimer);
            throw ex;
        }
    }
}