KafkaTemplate is the Spring Kafka producer-facing dependency used to publish application records. Declare it with key and value types, inject it into a Spring component, and use it where a listener has finished processing an event and must emit the result to another topic. section 12.2.1
Its core call is send(topic, key, value). The topic selects the destination; the key can be a customer ID; and the value is the processed event. A loan processor sends an approved or rejected LoanApplication to a selected output topic, then may also send the same result to a QA topic. section 12.2.1
Publish with completion handling
Keep the result returned by send, then attach a callback. On success, read RecordMetadata to log the produced topic, offset, and timestamp. On failure, log the producing error; no produced offset or timestamp is available in that case. section 12.2.2
A single KafkaTemplate can be shared by concurrent listener threads because its underlying KafkaProducer is thread safe. Listener methods should remain stateless when concurrency is increased; shared mutable state requires synchronization for deterministic results. section 12.2.2
Listing 12.16 Adding a callback for notification of completed produce requests
ListenableFuture<SendResult<String, LoanApplication>>
produceResult = #1
kafkaTemplate.send(topicToSend,
processedLoan.getCustomerId(),
processedLoan);
produceResult.addCallback(produceCallback); #2
The listing captures the future returned by send and attaches a callback for produced-record metadata or errors. section 12.2.2
