Kafka Consumer Advance (Java example)

Updated: Sep 23, 2019

Prerequisite


Commits and Offset in Kafka Consumer


Once client commits the message, Kafka marks the message "deleted" for the consumer and hence the read message would be available in next poll by the client.


Properties used in the below example


bootstrap.servers=localhost:9092

ProducerConfig.RETRIES_CONFIG=0

value.deserializer=org.apache.kafka.common.serialization.StringDeserializer

key.deserializer=org.apache.kafka.common.serialization.StringDeserializer

retries=0

group.id=group1

HQ_TOPIC_NAME=EK.TWEETS.TOPIC

CONSUMER_TIMEOUT=1000

worker.thread.count=5

counsumer.count=3

auto.offset.reset=earliest

enable.auto.commit=false



Configuration Level Setting


This can be done at configuration level in the properties files.

  • auto.commit.offset=false - This is the default setting. Means the consumer API can take the decision to retail the message of the offset or commit it.

  • auto.commit.offset=true - Once the message is consumed by the consumer, the offset is committed if consumer API is not taking any decision in client code.


Consumer API Level Setting


Synchronous Commit

  • Offset is committed as soon consumer API confirms.

  • The latest Offset of the message is committed.

  • Below example is committing the message after processing all messages of the current polling.

  • Synchronous commit blocks until the broker responds to the commit request.


Sample Code


public synchronized void subscribeMessage(String configPropsFile)throws Exception{

try{

if(consumer==null){

consumer =(KafkaConsumer<byte[], byte[]>) getKafkaConnection(configPropsFile);

System.out.println("Kafka Connection created...on TOPIC : "+getTopicName());

}

consumer.subscribe(Collections.singletonList(getTopicName()));

while (true) {

ConsumerRecords<byte[], byte[]> records = consumer.poll(10000L);

for (ConsumerRecord<byte[], byte[]> record : records) {

System.out.printf("Received Message topic =%s, partition =%s, offset = %d, key = %s, value = %s\n", record.topic(), record.partition(), record.offset(), record.key(), record.value());

}

consumer.commitSync();

}

}catch(Exception e){

e.printStackTrace();

consumer.close();

}

}



Asynchronous Commit

  • The consumer does not wait for the the response from the broker

  • This commits just confirms the broker and continue its processing.

  • Throughput is more in compare to Synchronous commit.

  • There could be chances of duplicate read, that application need to handle its own.


Sample code


while (true) {

ConsumerRecords<byte[], byte[]> records = consumer.poll(10000L);

System.out.println("Number of messaged polled by consumer "+records.count());

for (ConsumerRecord<byte[], byte[]> record : records) {

System.out.printf("Received Message topic =%s, partition =%s, offset = %d, key = %s, value = %s\n", record.topic(), record.partition(), record.offset(), record.key(), record.value());

}

consumer.commitAsync(new OffsetCommitCallback() {

public void onComplete(Map<TopicPa