Kafka Java consumer never receives any messages

Question

I'm trying to setup a basic Java consumer to receive messages from a Kafka topic. I've followed the sample at - https://cwiki.apache.org/confluence/display/KAFKA/Consumer+Group+Example - and have this code:

package org.example.kafka.client;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import kafka.consumer.ConsumerConfig;
import kafka.consumer.KafkaStream;
import kafka.javaapi.consumer.ConsumerConnector;

public class KafkaClientMain 
{

    private final ConsumerConnector consumer;
    private final String topic;
    private  ExecutorService executor;  


    public KafkaClientMain(String a_zookeeper, String a_groupId, String a_topic) 
    {
        this.consumer = kafka.consumer.Consumer.createJavaConsumerConnector(
                createConsumerConfig(a_zookeeper, a_groupId));

        this.topic = a_topic;
    }    


    private static ConsumerConfig createConsumerConfig(String a_zookeeper, String a_groupId) {
        Properties props = new Properties();
        props.put("zookeeper.connect", a_zookeeper);
        props.put("group.id", a_groupId);
        props.put("zookeeper.session.timeout.ms", "1000");
        props.put("zookeeper.sync.time.ms", "1000");
        props.put("auto.commit.interval.ms", "1000");
        props.put("auto.offset.reset", "smallest");

        return new ConsumerConfig(props);
    }    

    public void shutdown() {
        if (consumer != null) consumer.shutdown();
        if (executor != null) executor.shutdown();
    }    


    public void run(int a_numThreads) {
        Map<String, Integer> topicCountMap = new HashMap<String, Integer>();
        topicCountMap.put(topic, new Integer(a_numThreads));
        Map<String, List<KafkaStream<byte[], byte[]>>> consumerMap = consumer.createMessageStreams(topicCountMap);
        List<KafkaStream<byte[], byte[]>> streams = consumerMap.get(topic);

        System.out.println( "streams.size = " + streams.size() );

        // now launch all the threads
        //
        executor = Executors.newFixedThreadPool(a_numThreads);

        // now create an object to consume the messages
        //
        int threadNumber = 0;
        for (final KafkaStream stream : streams) {
            executor.submit(new ConsumerTest(stream, threadNumber));
            threadNumber++;
        }
    }    


    public static void main(String[] args) 
    {


        String zooKeeper = "ec2-whatever.compute-1.amazonaws.com:2181";
        String groupId = "group1";
        String topic = "test";

        int threads = 1;

        KafkaClientMain example = new KafkaClientMain(zooKeeper, groupId, topic);

        example.run(threads);

    }

}

and

package org.example.kafka.client;

import kafka.consumer.ConsumerIterator;
import kafka.consumer.KafkaStream;

public class ConsumerTest implements Runnable 
{

    private KafkaStream m_stream;
    private int m_threadNumber;

    public ConsumerTest(KafkaStream a_stream, int a_threadNumber) 
    {
        m_threadNumber = a_threadNumber;
        m_stream = a_stream;
    }

    public void run() 
    {
        System.out.println( "calling ConsumerTest.run()" );
        ConsumerIterator<byte[], byte[]> it = m_stream.iterator();

        while (it.hasNext())
        {    
            System.out.println("Thread " + m_threadNumber + ": " + new String(it.next().message()));
        }


        System.out.println("Shutting down Thread: " + m_threadNumber);
    }
}

Kafka is running on the EC2 host in question, and I can send and receive messages on the topic "test" using the kafka-console-producer.sh and kafka-console-consumer.sh tools. Port 2181 is open and available from the machine where the consumer is running (and so is 9092 for good measure, but that didn't seem to help either).

Unfortunately, I never receive any messages in my consumer when I run this. Neither existing messages on the topic, nor newly sent messages that I send using kafka-console-producer.sh, while the consumer is running.

This is using Kafka 0.8.1.1 running on CentOS 6.4 x64, using OpenJDK 1.7.0_65.

Edit: FWIW, when the consumer program starts, I see this Zookeeper output:

[2014-08-01 15:56:38,045] INFO Accepted socket connection from /98.101.159.194:24218 (org.apache.zookeeper.server.NIOServerCnxn)
[2014-08-01 15:56:38,049] INFO Client attempting to establish new session at /98.101.159.194:24218 (org.apache.zookeeper.server.NIOServerCnxn)
[2014-08-01 15:56:38,053] INFO Established session 0x1478e963fb30008 with negotiated timeout 6000 for client /98.101.159.194:24218 (org.apache.zookeeper.server.NIOServerCnxn)

Any idea what might be going on with this? Any and all help is much appreciated.

Answer 1

Answering this myself for posterity, in case anybody else runs across a similar problem.

The issue was this: The Kafka broker and Zookeeper were on an EC2 node, and the consumer was on my laptop running locally. When connecting to Zookeeper, the client was getting handed a reference to "ip-10-0-x-x.ec2.internal", which does not resolve (by default) from outside of EC2. This became clear once I properly configured log4j on the client so I was getting all of the log messages.

The workaround was to just put an entry in my /etc/hosts file, mapping the ec2 internal hostname to the publicly routable IP address.

Answer 2

You can solve this problem using setting following property in server.properties file located under kafka config folder

advertised.host.name=public dns of Ec2 server