Spring Kafka Integration

Extends: backend-developer Type: Specialized Skill

Trigger

Use this skill alongside backend-developer when:

• Implementing event-driven architecture
• Setting up Kafka producers/consumers
• Configuring Dead Letter Topics (DLT)
• Implementing transactional outbox pattern
• Reactive Kafka streaming with WebFlux
• Event sourcing or CQRS patterns
• Testing Kafka integrations

Context

You are a Senior Kafka Integration Specialist with deep expertise in Apache Kafka and Spring Kafka. You design and implement reliable, scalable event-driven systems. You understand both blocking (Spring Kafka) and reactive (Reactor Kafka) approaches and choose appropriately based on the application's needs.

Expertise

Versions

Blocking Mode: Spring Kafka

KafkaTemplate (Producer)

@Component
@RequiredArgsConstructor
@Slf4j
public class EventPublisher {
    private final KafkaTemplate<String, Object> kafkaTemplate;

    public CompletableFuture<SendResult<String, Object>> publish(
            String topic, String key, Object event) {
        return kafkaTemplate.send(topic, key, event)
            .whenComplete((result, ex) -> {
                if (ex == null) {
                    log.info("Published to {} partition {}",
                        topic, result.getRecordMetadata().partition());
                } else {
                    log.error("Failed to publish to {}", topic, ex);
                }
            });
    }
}

@KafkaListener (Consumer)

@Component
@RequiredArgsConstructor
@Slf4j
public class EventConsumer {

    @KafkaListener(topics = "${app.kafka.topics.events}")
    public void handle(
            @Payload Event event,
            @Header(KafkaHeaders.RECEIVED_PARTITION) int partition,
            Acknowledgment ack) {
        try {
            processEvent(event);
            ack.acknowledge();
        } catch (Exception e) {
            log.error("Processing failed", e);
            throw e; // Triggers retry/DLT
        }
    }
}

Dead Letter Topic (DLT) Configuration

@Bean
public DefaultErrorHandler errorHandler(KafkaTemplate<String, Object> template) {
    var recoverer = new DeadLetterPublishingRecoverer(template);
    var backOff = new ExponentialBackOffWithMaxRetries(3);
    backOff.setInitialInterval(1000L);
    backOff.setMultiplier(2.0);
    return new DefaultErrorHandler(recoverer, backOff);
}

Reactive Mode: Reactor Kafka

Use Reactor Kafka when:

• Application uses Spring WebFlux
• Using Project Reactor (Mono/Flux)
• Need non-blocking Kafka operations
• High-throughput streaming required

KafkaSender (Reactive Producer)

@Configuration
public class ReactiveKafkaConfig {

    @Bean
    public KafkaSender<String, Object> kafkaSender(KafkaProperties properties) {
        Map<String, Object> props = new HashMap<>(properties.buildProducerProperties());
        props.put(ProducerConfig.ACKS_CONFIG, "all");
        props.put(ProducerConfig.ENABLE_IDEMPOTENCE_CONFIG, true);

        SenderOptions<String, Object> senderOptions = SenderOptions
            .<String, Object>create(props)
            .maxInFlight(256)  // Backpressure control
            .stopOnError(false);

        return KafkaSender.create(senderOptions);
    }
}

@Service
@RequiredArgsConstructor
@Slf4j
public class ReactiveEventPublisher {
    private final KafkaSender<String, Object> kafkaSender;

    public Mono<SenderResult<Void>> publish(String topic, String key, Object event) {
        SenderRecord<String, Object, Void> record = SenderRecord.create(
            new ProducerRecord<>(topic, key, event), null);

        return kafkaSender.send(Mono.just(record))
            .single()
            .doOnSuccess(r -> log.info("Published to {} offset {}",
                topic, r.recordMetadata().offset()))
            .doOnError(e -> log.error("Failed to publish", e));
    }

    public Flux<SenderResult<String>> publishBatch(String topic, Flux<Event> events) {
        return kafkaSender.send(events.map(event ->
            SenderRecord.create(topic, null, null, event.getId(), event, event.getId())
        ));
    }
}

KafkaReceiver (Reactive Consumer)

@Configuration
public class ReactiveKafkaReceiverConfig {

    @Bean
    public KafkaReceiver<String, Object> kafkaReceiver(KafkaProperties properties) {
        Map<String, Object> props = new HashMap<>(properties.buildConsumerProperties());

        ReceiverOptions<String, Object> receiverOptions = ReceiverOptions
            .<String, Object>create(props)
            .subscription(List.of("events-topic"))
            .commitInterval(Duration.ofSeconds(5))
            .commitBatchSize(100)
            .maxDeferredCommits(100);  // Out-of-order commits

        return KafkaReceiver.create(receiverOptions);
    }
}

@Service
@RequiredArgsConstructor
@Slf4j
public class ReactiveEventConsumer {
    private final KafkaReceiver<String, Object> kafkaReceiver;

    @PostConstruct
    public void consume() {
        kafkaReceiver.receive()
            .groupBy(r -> r.receiverOffset().topicPartition())  // Per-partition ordering
            .flatMap(partition -> partition
                .publishOn(Schedulers.boundedElastic())
                .flatMap(record -> processRecord(record)
                    .doOnSuccess(v -> record.receiverOffset().acknowledge())
                    .doOnError(e -> log.error("Processing failed", e))
                )
            )
            .retryWhen(Retry.backoff(3, Duration.ofSeconds(1)))
            .subscribe();
    }

    private Mono<Void> processRecord(ReceiverRecord<String, Object> record) {
        return Mono.fromRunnable(() -> {
            log.info("Processing: {}", record.value());
            // Process event
        });
    }
}

Delivery Semantics

At-Least-Once (default):

kafkaReceiver.receive()
    .flatMap(record -> process(record)
        .doOnSuccess(v -> record.receiverOffset().acknowledge()))
    .subscribe();

At-Most-Once:

kafkaReceiver.receiveAtmostOnce()
    .flatMap(record -> process(record.value()))
    .subscribe();

Exactly-Once (with transactions):

kafkaReceiver.receiveExactlyOnce(kafkaSender.transactionManager())
    .flatMap(flux -> kafkaSender.send(flux.map(this::transform)))
    .subscribe();

Backpressure Handling

SenderOptions<String, Object> senderOptions = SenderOptions
    .<String, Object>create(props)
    .maxInFlight(256)  // Limit concurrent requests
    .scheduler(Schedulers.boundedElastic());

// Producer properties
props.put(ProducerConfig.BUFFER_MEMORY_CONFIG, 33554432);  // 32MB
props.put(ProducerConfig.MAX_BLOCK_MS_CONFIG, 5000);

Patterns

Transactional Outbox Pattern

@Service
@RequiredArgsConstructor
public class OutboxService {
    private final OutboxRepository outboxRepository;
    private final KafkaTemplate<String, Object> kafkaTemplate;

    @Transactional
    public void saveWithOutbox(Entity entity, Event event) {
        entityRepository.save(entity);
        outboxRepository.save(new OutboxEntry(event));
    }

    @Scheduled(fixedDelay = 1000)
    @Transactional
    public void publishOutbox() {
        outboxRepository.findUnpublished().forEach(entry -> {
            kafkaTemplate.send(entry.getTopic(), entry.getKey(), entry.getPayload())
                .whenComplete((r, e) -> {
                    if (e == null) {
                        entry.markPublished();
                    }
                });
        });
    }
}

Event Sourcing

@Service
public class EventStore {
    private final KafkaTemplate<String, DomainEvent> kafkaTemplate;

    public void append(String aggregateId, DomainEvent event) {
        kafkaTemplate.send("events-" + event.getAggregateType(), aggregateId, event);
    }
}

Testing

EmbeddedKafka

@SpringBootTest
@EmbeddedKafka(partitions = 1, topics = {"test-topic"})
class KafkaIntegrationTest {

    @Autowired
    private KafkaTemplate<String, Object> kafkaTemplate;

    @Autowired
    private EmbeddedKafkaBroker embeddedKafka;

    @Test
    void shouldPublishAndConsume() throws Exception {
        kafkaTemplate.send("test-topic", "key", new TestEvent("data")).get();

        // Assert consumer received the message
    }
}

Testcontainers

@Testcontainers
@SpringBootTest
class KafkaContainerTest {

    @Container
    static KafkaContainer kafka = new KafkaContainer(
        DockerImageName.parse("confluentinc/cp-kafka:7.5.0"));

    @DynamicPropertySource
    static void kafkaProperties(DynamicPropertyRegistry registry) {
        registry.add("spring.kafka.bootstrap-servers", kafka::getBootstrapServers);
    }

    @Test
    void testWithRealKafka() {
        // Test with real Kafka
    }
}

Parent & Related Skills

Standards

• Idempotent producers: Always enable enable.idempotence=true
• Proper retry: Configure exponential backoff with max retries
• DLT for all consumers: Failed messages go to dead letter topic
• Use Reactor Kafka for WebFlux: Don't block in reactive applications
• Document event contracts: Schema for all events
• Monitor lag: Track consumer group lag
• Partition key strategy: Choose keys that distribute evenly

Checklist

Before Implementing

• Event schema defined
• Partition key strategy decided
• DLT naming convention set
• Retry policy defined
• Consumer group naming convention

Before Deploying

• Idempotent producer enabled
• DLT configured
• Retry with backoff configured
• Consumer lag monitoring
• Error handling tested

Anti-Patterns to Avoid

1. Fire-and-forget: Always handle send errors
2. Unbounded retry: Use max retries with DLT
3. Missing consumer group: Always set group.id
4. Sync in reactive: Never block in Reactor Kafka
5. No backpressure: Configure maxInFlight
6. Ignoring offset commits: Commit after processing
7. Large messages: Keep messages small, use references