How can i write kafka consumer without using infinite loop during deserialization?

Question

How to write kafka consumer in java without using infinite loop for polling?

I have created kafka consumer by using this link as reference. here in processing incoming records function while(true) loop was written in which it is polling for new events. If i use this in my project i am not able to do anything else except this. is there a way to avoid using this infinite loop to get new events?

 public static void main(String[] str) throws InterruptedException {
    System.out.println("Starting  AtMostOnceConsumer ...");
    execute();
}
private static void execute() throws InterruptedException {
    KafkaConsumer<String, Event> consumer = createConsumer();
    // Subscribe to all partition in that topic. 'assign' could be used here
    // instead of 'subscribe' to subscribe to specific partition.
    consumer.subscribe(Arrays.asList("topic"));
    processRecords(consumer);
}
private static KafkaConsumer<String, Event> createConsumer() {
    Properties props = new Properties();
    String consumeGroup = "group_id";
    props.put("group.id", consumeGroup);
    props.put("org.slf4j.simpleLogger.defaultLogLevel", "INFO");
    props.put("client.id", "clientId");
    props.put("security.protocol", "SASL_SSL");

    props.put(CommonClientConfigs.BOOTSTRAP_SERVERS_CONFIG, "servers");
    props.put(CommonClientConfigs.SECURITY_PROTOCOL_CONFIG, "SASL_SSL");
    props.put(SaslConfigs.SASL_MECHANISM, "PLAIN");
    props.put(SaslConfigs.SASL_JAAS_CONFIG, "org.apache.kafka.common.security.plain.PlainLoginModule required username="" + "username" + " password="" + "password";");
    props.put("enable.auto.commit", "true");
    // Auto commit interval, kafka would commit offset at this interval.
    props.put("auto.commit.interval.ms", "101");
    // This is how to control number of records being read in each poll
    props.put("max.partition.fetch.bytes", "135");
    // Set this if you want to always read from beginning.
    // props.put("auto.offset.reset", "earliest");
    props.put("heartbeat.interval.ms", "3000");
    props.put("session.timeout.ms", "6001");
    props.put("schema.registry.url", "https://avroregistry.octanner.io");
    props.put("key.deserializer",
            "io.confluent.kafka.serializers.KafkaAvroDeserializer");
    props.put("value.deserializer",
            "io.confluent.kafka.serializers.KafkaAvroDeserializer");
    return new KafkaConsumer<String, Event>(props);
}
private static void processRecords(KafkaConsumer<String, Event> consumer) throws InterruptedException {
    while (true) {
        ConsumerRecords<String, Event> records = consumer.poll(TimeUnit.MINUTES.toMillis(1));
        long lastOffset = 0;
        for (ConsumerRecord<String, Event> record : records) {
            System.out.printf("\n\n\n\n\n\n\roffset = %d, key = %s\n\n\n\n\n\n", record.offset(), record.value());
            lastOffset = record.offset();
        }
        System.out.println("lastOffset read: " + lastOffset);
        process();
    }
}
private static void process() throws InterruptedException {
    // create some delay to simulate processing of the message.
    Thread.sleep(TimeUnit.MINUTES.toMillis(1));
}

Can someone help me to modify this so that i can avoid while(true) loop and can just listen to my incoming events?

Answer 1

If you want to be able to do multiple things at the same time in the code, you need background threads.

In order to do this more easily, you could use a higher-level Kafka library like Spring (already answered), Vert.x or Smallrye

Here is a Vert.x example, first create a KafkaConsumer, then assign the handler and subscribe to your topic(s)

consumer.handler(record -> {
  System.out.println("Processing key=" + record.key() + ",value=" + record.value() +
    ",partition=" + record.partition() + ",offset=" + record.offset());
});

// subscribe to a single topic
consumer.subscribe("a-single-topic");

Answer 2

you could try something like this:

public class ConsumerDemoWithThread {
private Logger logger = LoggerFactory.getLogger(ConsumerDemoWithThread.class.getName());
private String bootstrapServers = "127.0.0.1:9092";
private String groupId = "my-first-application";
private String topic = "first-topic";

KafkaConsumer consumer = createConsumer(bootstrapServers, groupId, topic);

private void pollForRecords() {
    ExecutorService executor = Executors.newSingleThreadExecutor();
    executor.submit(() -> processRecords());
}


private KafkaConsumer createConsumer(String bootstrapServers, String groupId, String topic) {
    Properties properties = new Properties();
    properties.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
    properties.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
    properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
    properties.setProperty(ConsumerConfig.GROUP_ID_CONFIG, groupId);
    properties.setProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
    // create consumer
    KafkaConsumer consumer = new KafkaConsumer<String, String>(properties);
    // subscribe consumer to our topic(s)
    consumer.subscribe(Arrays.asList(topic));
    return consumer;
}


private void processRecords() {
    try {
        while (true) {
            ConsumerRecords<String, String> records =
                    consumer.poll(Duration.ofMillis(100));

            for (ConsumerRecord<String, String> record : records) {
                logger.info("Key: " + record.key() + ", Value: " + record.value());
                logger.info("Partition: " + record.partition() + ", Offset:" + record.offset());
            }
        }
    } catch (WakeupException e) {
        logger.info("Received shutdown signal!");
    } finally {
        consumer.close();
    }
}

public static void main(String[] args) {
    ConsumerDemoWithThread consumerDemoWithThread = new ConsumerDemoWithThread();
    consumerDemoWithThread.pollForRecords();
}
}

Basically, as Joachim has mentioned, the entire poll and process logic needs to be delegated to a Thread

Answer 3

You can use @KafkaListener (https://docs.spring.io/spring-kafka/api/org/springframework/kafka/annotation/KafkaListener.html). However, it will also be polling in an infinite loop because that's how Kafka was designed - it's not a queue, but an event bus that stores records for some time. There's no mechanism to notify its consumers.

Poll on a different thread and have a graceful way to exit from the loop.