Reactor Kafka: Exactly Once Processing Sample

Question

I've read many articles where there are many different configurations to achieve exactly once processing.

Here is my producer config:

final Map<String, Object> props = Maps.newConcurrentMap();

props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, JsonSerializer.class);


props.put(ProducerConfig.ENABLE_IDEMPOTENCE_CONFIG, "true");
props.put(ProducerConfig.ACKS_CONFIG, "all"); 
props.put(ProducerConfig.TRANSACTIONAL_ID_CONFIG, "producer-tx-1");

Here is my consumer config:

final Map<String, Object> props = Maps.newHashMap();

props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
props.put(ConsumerConfig.GROUP_ID_CONFIG, consumerGroupId);
props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, JsonDeserializer.class);

props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
props.put(ConsumerConfig.ISOLATION_LEVEL_CONFIG, "read_committed");

I read this [sample scenarios][1]

And I try to follow but i'm getting some issues:

Here is my producer code:

    @Override
public Mono<SenderResult<Void>> buy(Message msg) {
    final ReactiveKafkaProducerTemplate kafkaProducerTemplate = kafkaConfig.getKafkaProducerTemplate();
    return kafkaProducerTemplate.transactionManager().begin().then(kafkaProducerTemplate.send(mytopic, msg));

}

My Consumer code:

@Override
public void run(ApplicationArguments arg0) throws Exception {
    final ReactiveKafkaProducerTemplate kafkaProducerTemplate = kafkaConfig.getKafkaProducerTemplate();
    final ReactiveKafkaConsumerTemplate kafkaConsumerTemplate = kafkaConfig.getKafkaConsumerTemplate(mytopic, Message.class);

    final Flux<ConsumerRecord<String, Message>> flux = kafkaConsumerTemplate.receiveExactlyOnce(kafkaProducerTemplate.transactionManager())
            .concatMap(receiverRecordFlux -> receiverRecordFlux );

    flux.subscribe(record -> {
        final Message message = record.value();

        System.out.printf("received message: timestamp=%s key=%d value=%s\n",
                dateFormat.format(new Date(record.timestamp())),
                record.key(),
                message);
 transactionService.processAndSendToNextTopic(message)
                .doOnSuccess(aVoid -> kafkaProducerTemplate.transactionManager().commit())
                .subscribe();

    });
}

I'm always getting following error when trying to produce and consume message:

Invalid transition attempted from state IN_TRANSACTION to state IN_TRANSACTION

Answer 1

See the javadocs for receiveExactlyOnce

/**
 * Returns a {@link Flux} of consumer record batches that may be used for exactly once
 * delivery semantics. A new transaction is started for each inner Flux and it is the
 * responsibility of the consuming application to commit or abort the transaction
 * using {@link TransactionManager#commit()} or {@link TransactionManager#abort()}
 * after processing the Flux. 

begin() has already been called so you don't need to call it.

@Override
public Flux<Flux<ConsumerRecord<K, V>>> receiveExactlyOnce(TransactionManager transactionManager) {
    this.ackMode = AckMode.EXACTLY_ONCE;
    Flux<ConsumerRecords<K, V>> flux = withDoOnRequest(createConsumerFlux());
    return  flux.map(consumerRecords -> transactionManager.begin()
                             .then(Mono.fromCallable(() -> awaitingTransaction.getAndSet(true)))
                             .thenMany(transactionalRecords(transactionManager, consumerRecords)))
                             .publishOn(transactionManager.scheduler());
}