Reply Kafka Template - Exception handling - apache-kafka

Question

I am using ReplyingKafkaTemplate to establish a synchronous call between two microservices.
The receiver of the event is annotated with SendTo as below:
#KafkaListener(topics = "${kafka.topic.prefix}"
+ "${kafka.topic.name}", containerFactory = "customEventKafkaListenerFactory")
#SendTo
public CustomResponseEvent onMessage(
#Payload #Valid CustomRequestEvent event, #Header(KafkaHeaders.CORRELATION_ID) String correlationId,
#Header(KafkaHeaders.REPLY_TOPIC) String replyTopic) {
//Making some REST API calls to another external system here using RestTemplate
}
The REST API call can throw a 4xx or 5xx. There are multiple such calls, some to internal systems, and some to external systems. It may be a bad design, but let's not get into that.
I would like to have a global exception handler for the RestTemplate where I can catch all the exceptions, and then return a response to the original sender of the event.
I am using the same replyTopic and correlationId as received in the consumer to publish the event.
But still the receiver of the response throws No pending reply exception.
Whatever approach I have above, is it possible to achieve such a central error response event publisher?
Is there any other alternative that is best suited for this exception handling?

Answer 1

The #KafkaListener comes with the:
/**
* Set an {#link org.springframework.kafka.listener.KafkaListenerErrorHandler} bean
* name to invoke if the listener method throws an exception.
* #return the error handler.
* #since 1.3
*/
String errorHandler() default "";
That one is used to catch and process all the downstream exceptions and if it returns a result, it is sent back to the replyTopic:
public void onMessage(ConsumerRecord<K, V> record, Acknowledgment acknowledgment, Consumer<?, ?> consumer) {
Message<?> message = toMessagingMessage(record, acknowledgment, consumer);
logger.debug(() -> "Processing [" + message + "]");
try {
Object result = invokeHandler(record, acknowledgment, message, consumer);
if (result != null) {
handleResult(result, record, message);
}
}
catch (ListenerExecutionFailedException e) { // NOSONAR ex flow control
if (this.errorHandler != null) {
try {
Object result = this.errorHandler.handleError(message, e, consumer);
if (result != null) {
handleResult(result, record, message);
}
}
catch (Exception ex) {
throw new ListenerExecutionFailedException(createMessagingErrorMessage(// NOSONAR stack trace loss
"Listener error handler threw an exception for the incoming message",
message.getPayload()), ex);
}
}
else {
throw e;
}
}
See RecordMessagingMessageListenerAdapter source code for more info.