If my producer producing, then why the consumer couldn't consume? it stuck @ poll()

Question

Im publishing to the remote kafka server and try to consume messages from that remote server. (Kafka v 0.90.1) Publishing works fine but nor the consuming.

Publisher

package org.test;

import java.io.IOException;
import java.util.Properties;

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.ProducerRecord;


public class Producer {

    private void generateMessgaes() throws IOException {
        String topic = "MY_TOPIC";

        Properties props = new Properties();

        props.put("bootstrap.servers", "kafka.xx.com:9092");
        props.put("acks", "all");
        props.put("retries", 0);
        props.put("batch.size", 16384);
        props.put("linger.ms", 1);
        props.put("buffer.memory", 33554432);
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("serializer.class", "org.apache.kafka.common.serialization.StringSerializer");


        KafkaProducer<String, String> producer = null;
        try {
             producer = new KafkaProducer<>(props);
            for (int i = 0; i < 10; i++) {
                producer.send(new ProducerRecord<String, String>(topic, "test msg"));
                System.out.println("producing---");
            }

        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println("Error in publishing messages to the topic : " + topic);

        } finally {
            producer.close();
        }
    }

    public static void main(String[] args) throws IOException {
        Producer producer = new Producer();
        producer.generateMessgaes();
        System.out.println("$$$$$");
    }
}

I can see "producing--- and $$$$ prints. But when i try to consume, i do not see "polling " print messages.. It got stuck at poll(timeout).

Any clue?

Consumer

package org.test;

import java.util.Arrays;
import java.util.List;
import java.util.Properties;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;


public class Listener {

    public void start() throws CoreException {

        String topic = "MY_TOPIC";

        List<String> topics = Arrays.asList(topic);

        Properties props = new Properties();
        props.put("bootstrap.servers", "kafka.xx.com:9092");
        props.put("enable.auto.commit", true);
        props.put("receive.buffer.bytes", 262144);
        props.put("consumer.timeout.ms", 10000);
        props.put("session.timeout.ms", 7000);
        props.put("heartbeat.interval.ms", 1000);
        props.put("auto.offset.reset", "earliest");
        props.put("group.id", "test");
        props.put("fetch.min.bytes", 1);
        props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");

        props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        props.put("serializer.class", "org.apache.kafka.common.serialization.StringDeserializer");
        KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(props);
        consumer.subscribe(topics);

        try {
            while (true) {

                ConsumerRecords<String, String> records = consumer.poll(100);
                System.out.println("polling msges : " + records.count());
                for (ConsumerRecord<String, String> record : records) {
    System.out.println("kafka record : " + record.value());
                }
            }
        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println("eror in polling");
        } finally {
            consumer.close();
        }
    }

    public static void main(String args[]) throws CoreException {

        Listener listener = new Listener();
        listener.start();

    }
}

Answer 1

Disclaimer: I don't know Kafka but I do know messaging.

First thing about topics: by default, subscriptions are not persistent, so if a producers sends messages to the topic and there's no one listening, the messages are dropped.

Second thing is the polling: you're polling for 100ms and then if there's nothing there an exception is thrown, dropping you out of the loop.

If there's no messages when the consumer starts - because, as I described, the producer's messages went to the bit bucket - and then the consumer failed because there was nothing to consume. So I would say everything is working as you should expect.

Two options: -add a much larger initial polling to give the consumer a chance to see a message (assuming you know the producer is going to produce in the time frame) -change your logic such that an exception causes you to stay in the while loop and continue to consume, and figure out a different way to stop the consumer.