Develop Reference

Kafka Consumer returns null value

I am trying to make a kafka consumer in Java but the consumer.poll(5000) method call return null value no matter what. here is the code:
package com.apache.kafka.consumer;
import java.util.Map;
import java.util.Properties;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.log4j.Logger;
import org.apache.kafka.clients.consumer.ConsumerRecords;
public class Consumer {
public static void main(String[] args) throws Exception {
final Logger logger = Logger.getLogger(Consumer.class);
//Kafka consumer configuration settings
String topicName = "mytopic";
Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "test");
props.put("enable.auto.commit", "true");
props.put("auto.offset.reset","earliest");
props.put("auto.commit.interval.ms", "1000");
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");
KafkaConsumer<String, String> consumer = new
KafkaConsumer<String, String>(props);
//Kafka Consumer subscribes list of topics here.
consumer.subscribe("sampletopic");
while (true) {
Map<String,ConsumerRecords<String, String>> records = consumer.poll(0);
for (ConsumerRecords<String, String> record : records.values()) {
System.out.println(records);
}
}
}
}
Please help!!!
I have created the topic already and also have added some data in it plus the zookeeper and kafka are running perfectly. I don;t know why the poll() method is returning null.

The call to poll needs to be in a loop, that's why the literature calls it the poll loop.
If its returning null its either polling too early and exiting the main or no data is in the topic
See the usage examples here https://kafka.apache.org/22/javadoc/org/apache/kafka/clients/consumer/KafkaConsumer.html
Properties props = new Properties();
props.setProperty("bootstrap.servers", "localhost:9092");
props.setProperty("group.id", "test");
props.setProperty("enable.auto.commit", "true");
props.setProperty("auto.commit.interval.ms", "1000");
props.setProperty("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
props.setProperty("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Arrays.asList("foo", "bar"));
while (true) {
ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
for (ConsumerRecord<String, String> record : records)
System.out.printf("offset = %d, key = %s, value = %s%n", record.offset(), record.key(), record.value());
}
Notice the loop ^

Not getting avro messages while reading data from topic in java

I am writing code first time in java to consume AVRO data from kafka topic. I am using kafka-avro-console-producer to produce record.I am using leneseio/fast-data-dev image on Docker to UP kafka stack.
Producing record :
root#fast-data-dev / $ kafka-avro-console-producer --broker-list localhost:9092 --topic payengine --property schema.registry.url=http://localhost:8081 --property value.schema='{"type":"record", "name":"payengine", "fields":[{"name":"tin", "type":"string"},{"name":"ach","type":"string"}] }'
{"tin":"61582","ach":"I"}
{"tin":"97820","ach":"I"}
Now, to read this record, I have written below code. Also, it seems like I don't have to refer the schema while consuming records (as mentioned in the below reference link). I had also gone through one example, where SpecificAvroRecord was being used in place of GenericRecord, but that requires building class based on schema. I am not sure how GenericRecord points to correct schema, but don't see any schema reference in the example.
package com.github.psingh.Kafka;
import io.confluent.kafka.serializers.KafkaAvroDeserializer;
import io.confluent.kafka.serializers.KafkaAvroDeserializerConfig;
import org.apache.avro.generic.GenericRecord;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.serialization.StringSerializer;
import java.time.Duration;
import java.util.Collections;
import java.util.Properties;
public class SimpleConsumer_AvroSchema {
public static void main(String[] args) {
// System.out.println("Hello Kafka ");
// setting properties
Properties props = new Properties();
props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringSerializer.class.getName());
props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, KafkaAvroDeserializer.class.getName());
props.put(ConsumerConfig.GROUP_ID_CONFIG, "group1");
props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
props.put(KafkaAvroDeserializerConfig.SCHEMA_REGISTRY_URL_CONFIG, "http://localhost:8081");
//name topic
String topic = "payengine";
// create the consumer
KafkaConsumer<String, GenericRecord> consumer = new KafkaConsumer<String, GenericRecord>(props);
//subscribe to topic
consumer.subscribe(Collections.singleton(topic));
System.out.println("Waiting for the data...");
while (true) {
ConsumerRecords<String, GenericRecord> records = consumer.poll(Duration.ofMillis(5000));
for(ConsumerRecord<String,GenericRecord> record: records) {
System.out.print(record.value());
}
// consumer.commitSync();
}
}
}
The code built was successful.I was hoping to get console produced record here, but I am not getting anything :
Please suggest.
I have taken reference from here :
https://docs.confluent.io/current/schema-registry/serdes-develop/serdes-avro.html

Java Consumer for Kafka in Cloudera Quickstart not working

I have a cloudera Quickstart VM. i have installed Kafka parcels using Cloudera Manager and its working fine inside the VM using console based consumer and producer.
But when i try to use java based consumer it does not produce or consume messages. I can list the topics.
But i cannot consume messages.
following is my code.
package kafka_consumer;
import java.util.Arrays;
import java.util.Properties;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.serialization.StringDeserializer;
public class mclass {
public static void main(String[] args) {
Properties props = new Properties();
props.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "10.0.75.1:9092");
// Just a user-defined string to identify the consumer group
props.put(ConsumerConfig.GROUP_ID_CONFIG, "test");
// Enable auto offset commit
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "true");
props.put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, "1000");
props.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
props.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
try (KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props)) {
// List of topics to subscribe to
consumer.subscribe(Arrays.asList("second_topic"));
for (String k_topic : consumer.listTopics().keySet()) {
System.out.println(k_topic);
}
while (true) {
try {
ConsumerRecords<String, String> records = consumer.poll(100);
for (ConsumerRecord<String, String> record : records) {
System.out.printf("Offset = %d\n", record.offset());
System.out.printf("Key = %s\n", record.key());
System.out.printf("Value = %s\n", record.value());
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
And following is the output of the code. while the console producer is producing messages but consumer is not able to receive it.
PS: i can telnet the port and ip of the Kafka broker. I can even list the topics. Consumer is constantly running without crashing but no messages is being consumed.

How to acknowledge current offset in spring kafka for manual commit

I am using Spring Kafka first time and I am not able to use Acknowledgement.acknowledge() method for manual commit in my consumer code. please let me know if anything missing in my consumer configuration or listener code. or else is there other way to handle acknowledge offset based on condition.
Here i'm looking solution like if the offset is not committed/ acknowledge manually, it should pick same message/offset by consumer.
Configuration
import java.util.HashMap;
import java.util.Map;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.common.serialization.StringDeserializer;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.kafka.annotation.EnableKafka;
import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
import org.springframework.kafka.listener.AbstractMessageListenerContainer.AckMode;
#EnableKafka
#Configuration
public class ConsumerConfig {
#Value(value = "${kafka.bootstrapAddress}")
private String bootstrapAddress;
#Value(value = "${kafka.groupId}")
private String groupId;
#Bean
public ConcurrentKafkaListenerContainerFactory<String, String> containerFactory() {
Map<String, Object> props = new HashMap<String, Object>();
props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
props.put(ConsumerConfig.GROUP_ID_CONFIG, groupId);
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);
props.put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, 100);
props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG,
StringDeserializer.class);
props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG,
StringDeserializer.class);
props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");
ConcurrentKafkaListenerContainerFactory<String, String> factory = new ConcurrentKafkaListenerContainerFactory<String, String>();
factory.setConsumerFactory(new DefaultKafkaConsumerFactory<String, String>(
props));
factory.getContainerProperties().setAckMode(AckMode.MANUAL_IMMEDIATE);
factory.getContainerProperties().setSyncCommits(true);
return factory;
}
}
Listener
private static int value = 1;
#KafkaListener(id = "baz", topics = "${message.topic.name}", containerFactory = "containerFactory")
public void listenPEN_RE(#Payload String message,
#Header(KafkaHeaders.RECEIVED_PARTITION_ID) int partition,
#Header(KafkaHeaders.OFFSET) int offsets,
Acknowledgment acknowledgment) {
if (value%2==0){
acknowledgment.acknowledge();
}
value++;
}

Set the enable-auto-commit property to false:
propsMap.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);
Set the ack-mode to MANUAL_IMMEDIATE:
factory.getContainerProperties().setAckMode(AbstractMessageListenerContainer.AckMode.MANUAL_IMMEDIATE);
Then, in your consumer/listener code, you can commit the offset manually, like this:
#KafkaListener(topics = "testKafka")
public void receive(ConsumerRecord<?, ?> consumerRecord,
Acknowledgment acknowledgment) {
System.out.println("Received message: ");
System.out.println(consumerRecord.value().toString());
acknowledgment.acknowledge();
}
Update: I created a small POC for this. Check it out here, might help you.

You need to do the following
1) Set enable-auto-commit property to false
consumerConfigProperties.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
2) Set the ACK Mode to MANUL_IMMEDIATE
factory.getContainerProperties().setAckMode(AckMode.MANUAL_IMMEDIATE);
3) For processed records you need to call acknowledgment.acknowledge();
4) for failed records call acknowledgment.nack(10);
Note: the nack method takes a long parameter which is the sleep time and it should be less than max.poll.interval.ms
Below is a sample code
#KafkaListener(id = "baz", topics = "${message.topic.name}", containerFactory = "containerFactory")
public void listenPEN_RE(#Payload String message,
#Header(KafkaHeaders.RECEIVED_PARTITION_ID) int partition,
#Header(KafkaHeaders.OFFSET) int offsets,
Acknowledgment acknowledgment) {
if (value%2==0){
acknowledgment.acknowledge();
} else {
acknowledgment.nack(10); //sleep time should be less than max.poll.interval.ms
}
value++;
}

You can do following:
1. store the current record offset to file or DB.
2. Implement your kafka listener class with ConsumerAware.
3. Call registerSeekCallback as given below:
(registerSeekCallback(ConsumerSeekCallback callback)
{
callback.seek(topic, partition, offset)
}
So when the consumer goes down or new consumer is assigned , it start reading fomr the offset stored in your DB.

That doesn't work that way in Apache Kafka.
For the currently running consumer we may never worry about committing offsets. We need them persisted only for new consumers in the same consumer group. The current one track its offset in the memory. I guess somewhere on Broker.
If you need to refetch the same message in the same consumer maybe the next poll round, you should consider to use seek() functionality: https://docs.spring.io/spring-kafka/docs/2.0.1.RELEASE/reference/html/_reference.html#seek

I have a small nitpick with OP's configuration. If using #KafkaListener with ConcurrentKafkaListenerContainerFactory, dont forget to make the state thread-safe. Using private static int value = 1; isnt going to help. Use AtomicInteger

How to detect connection errors in Kafka

I'm using KafkaProducer with java. This is an example of code:
package com.mypackage.kafka.producer;
import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.ProducerRecord;
import java.util.Properties;
public class ProducerToTest
{
public static void main(String[] args)
{
Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
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");
KafkaProducer<String, String> producer = new KafkaProducer<String, String>(props);
for (int i = 0; i < 100; i++)
{
System.out.println(i);
producer.send(new ProducerRecord<String, String>("my-topic", Integer.toString(i), Integer.toString(i)));
}
producer.close();
}
}
I have a problem. If my Kafka is down and I run my code it stops at the first send and waits until Kafka is up again.
How can I detect connection errors or use send in a way that don't stop all execution?

Producer will try to re-connect till request.timeout.ms. It will not wait for broker up indefinitely. please refer to below detail information for request.timeout.ms.
The configuration controls the maximum amount of time the client will wait for the response of a request. If the response is not received before the timeout elapses the client will resend the request if necessary or fail the request if retries are exhausted.