KafkaTemplate

Introduction

What is KafkaTemplate? KafkaTemplate is Spring's high-level producer abstraction for sending records and handling send results.

Understanding the topic

What happens — KafkaTemplate:

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

Visual explanation

Pipeline view:

App creates message (e.g. "User signed up")
  ↓
Producer sends to topic (e.g. user_signups)
  ↓
Kafka stores in partitioned log (replicated)
  ↓
Consumers / analytics / alerts read in real time

Step-by-step explanation

1. Initialization — Producer connects to bootstrap servers and receives metadata (topics, partitions, leaders).
2. Message creation — Application builds a ProducerRecord with topic, key, value, and optional timestamp.
3. Serialization — Key and value serializers convert objects to bytes (e.g. StringSerializer).
4. Partitioning — Hash of key picks partition; no key → round-robin / sticky partitioner.
5. Batching — Records accumulate in the RecordAccumulator per partition for efficient network use.
6. Sending — Background sender thread ships batches to the partition leader broker.
7. Acknowledgement — Broker responds; producer retries on failure based on acks and retry settings.

Informative example

Example:

kafkaTemplate.send("orders", order.id(), order)
    .whenComplete((result, ex) -> {
        if (ex != null) log.error("send failed", ex);
    });

Execution workflow

Best practices

• Use acks=all with min.insync.replicas for critical events.
• Enable idempotence for retry safety.
• Choose keys intentionally; random keys destroy ordering.
• Monitor record-error-rate, request-latency, batch-size, and retries.

Common mistakes

• Ignoring asynchronous send failures.
• Using null keys for events that require aggregate ordering.
• Making producer timeouts too low and failing during normal broker leader movement.

Summary

KafkaTemplate — Use acks=all, enable idempotence, choose meaningful keys, and handle send callbacks for errors.