Spring Kafka in 30 Lines

Key points

• Add spring-kafka. Boot auto-configures a KafkaTemplate, a consumer factory and the listener containers.
• KafkaTemplate.send() returns a CompletableFuture<SendResult> in Spring Kafka 3.x. Do not block on it in a hot path.
• @KafkaListener creates one container per method. concurrency is the thread count, and each thread owns whole partitions.
• Keep concurrency at or below the partition count. Extra threads just sit idle.
• Writing to a database? Use manual ack: spring.kafka.listener.ack-mode=manual plus an Acknowledgment parameter.
• Spring hides the poll loop, not the partition maths. Every rule from the earlier lessons still applies.

Example

@Service
public class OrderEvents {

    private static final Logger log = LoggerFactory.getLogger(OrderEvents.class);

    private final KafkaTemplate<String, OrderEvent> template;
    private final BillingService billing;

    public OrderEvents(KafkaTemplate<String, OrderEvent> template, BillingService billing) {
        this.template = template;
        this.billing = billing;
    }

    public void publish(OrderEvent event) {
        // key = orderId keeps every event for one order in the same partition
        template.send("orders", event.orderId(), event)
                .whenComplete((result, ex) -> {
                    if (ex != null) log.error("publish failed for {}", event.orderId(), ex);
                });
    }

    @KafkaListener(topics = "orders", groupId = "billing-service", concurrency = "3")
    public void onOrder(OrderEvent event, Acknowledgment ack) {
        billing.charge(event);   // must be idempotent -- this can be redelivered
        ack.acknowledge();       // commit only after the work succeeded
    }
}

Spring hides the poll loop. It does not hide the partition maths.