Simple Kafka Consumer Example not working

Question

I have a simple class to consume messages from a kafka server. The majority of codes are copied from the comments of org.apache.kafka.clients.consumer.KafkaConsumer.java.

public class Demo {

    public static void main(String[] args) {
        Properties props = new Properties();
        props.put("metadata.broker.list", "192.168.144.10:29092");
        props.put("group.id", "test");
        props.put("session.timeout.ms", "1000");
        props.put("enable.auto.commit", "true");
        props.put("auto.commit.interval.ms", "10000");
        KafkaConsumer<byte[], byte[]> consumer = new KafkaConsumer<byte[], byte[]>(props);
        consumer.subscribe("voltdbexportAUDIT", "voltdbexportTEST");
        boolean isRunning = true;
        while (isRunning) {
            Map<String, ConsumerRecords<byte[], byte[]>> records = consumer.poll(100);
            process(records);
        }
        consumer.close();
    }

    private static Map<TopicPartition, Long> process(Map<String, ConsumerRecords<byte[], byte[]>> records) {
        Map<TopicPartition, Long> processedOffsets = new HashMap<>();
        for (Map.Entry<String, ConsumerRecords<byte[], byte[]>> recordMetadata : records.entrySet()) {
            List<ConsumerRecord<byte[], byte[]>> recordsPerTopic = recordMetadata.getValue().records();
            for (int i = 0; i < recordsPerTopic.size(); i++) {
                ConsumerRecord<byte[], byte[]> record = recordsPerTopic.get(i);
                // process record
                try {
                    processedOffsets.put(record.topicAndPartition(), record.offset());
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
        return processedOffsets;
    }
}

I am using 'org.apache.kafka:kafka-clients:0.8.2.0'. it throws exception

Exception in thread "main" org.apache.kafka.common.config.ConfigException: Missing required configuration "key.deserializer" which has no default value.
    at org.apache.kafka.common.config.ConfigDef.parse(ConfigDef.java:124)
    at org.apache.kafka.common.config.AbstractConfig.<init>(AbstractConfig.java:48)
    at org.apache.kafka.clients.consumer.ConsumerConfig.<init>(ConsumerConfig.java:194)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:430)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:413)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:400)
    at kafka.integration.Demo.main(Demo.java:26)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

How should I config the key.deserializer?

Answer 1

This works out of the box without implementing your own serializers

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "test");
props.put("enable.auto.commit", "true");
props.put("auto.commit.interval.ms", "1000");
props.put("session.timeout.ms", "30000");
props.put("key.deserializer","org.apache.kafka.common.serialization.StringDeserializer");  
props.put("value.deserializer","org.apache.kafka.common.serialization.StringDeserializer");
props.put("partition.assignment.strategy", "range");

Answer 2

For keys use one of the following

String Key

properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);

JSON Key

properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, JsonDeserializer.class);

Avro Key

properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, io.confluent.kafka.serializers.KafkaAvroDeserializer.class);

ByteArray Key

properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);

---

Likewise, use one of the following for your value deserializer:

String Value

properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);

JSON Value

properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, JsonDeserializer.class);

Avro Value

properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, io.confluent.kafka.serializers.KafkaAvroDeserializer.class);

ByteArray Value

properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);

---

Note that for Avro deserialisers, you will need the following dependencies:

<dependency> 
    <groupId>io.confluent</groupId> 
    <artifactId>kafka-avro-serializer</artifactId> 
    <version>${confluent.version}</version> 
</dependency> 

<dependency> 
    <groupId>org.apache.avro</groupId> 
    <artifactId>avro</artifactId> 
    <version>${avro.version}</version> 
</dependency>

Answer 3

You need to set the properties:

props.put("serializer.class","my.own.serializer.StringSupport");
props.put("key.serializer.class","my.own.serializer.LongSupport");

in your main method so that you pass them to the producer's constructor. Of course, you'd have to specify the right encoders. The serializer class converts the message into a byte array and the key.serializer class turn the key object into a byte array. Generally you'd also have them be able to reverse the process.