@KafkaListener

Introduction

What is @KafkaListener? @KafkaListener creates message listener containers that poll Kafka and invoke your method.

Understanding the topic

What happens — @KafkaListener:

• Add spring-kafka dependency.
• Configure bootstrap servers in application.yml.
• Use KafkaTemplate to send.
• Use @KafkaListener to consume.

Visual explanation

Pipeline view:

KafkaTemplate
  ↓
topic
  ↓
@KafkaListener container
  ↓
service logic
  ↓
ack/retry/DLT

Step-by-step explanation

1. Configure — Producer/consumer factories.
2. Produce — kafkaTemplate.send().
3. Consume — @KafkaListener method.
4. Handle errors — Retry topics + DLT.

Informative example

Example:

@KafkaListener(topics = "orders", groupId = "inventory-service")
public void handle(OrderCreated event, Acknowledgment ack) {
    inventory.reserve(event);
    ack.acknowledge();
}

Execution workflow

Best practices

• Use typed event classes and schema validation.
• Use manual or record ack modes for critical processing.
• Add retry topics and DLT with alerts.
• Test with embedded Kafka or Testcontainers.

Common mistakes

• Doing slow blocking work inside listener threads without concurrency planning.
• Catching exceptions and still acknowledging bad records.
• No DLT monitoring.

Summary

@KafkaListener — Configure listener containers intentionally: ack mode, concurrency, error handling, retries, DLT, observability, and idempotent processing.