Let us create an application for publishing and consuming messages using a Java client. Kafka producer client consists of the following API’s.

KafkaProducer API

Let us understand the most important set of Kafka producer API in this section. The central part of the KafkaProducer API is KafkaProducer class. The KafkaProducer class provides an option to connect a Kafka broker in its constructor with the following methods.

• KafkaProducer class provides send method to send messages asynchronously to a topic. The signature of send() is as follows

producer.send(new ProducerRecord<byte[],byte[]>(topic, 
partition, key1, value1) , callback);

• ProducerRecord − The producer manages a buffer of records waiting to be sent.

• Callback − A user-supplied callback to execute when the record has been acknowl-edged by the server (null indicates no callback).

• KafkaProducer class provides a flush method to ensure all previously sent messages have been actually completed. Syntax of the flush method is as follows −

public void flush()

• KafkaProducer class provides partitionFor method, which helps in getting the partition metadata for a given topic. This can be used for custom partitioning. The signature of this method is as follows −

public Map metrics()

It returns the map of internal metrics maintained by the producer.

• public void close() − KafkaProducer class provides close method blocks until all previously sent requests are completed.

Producer API

The central part of the Producer API is Producer class. Producer class provides an option to connect Kafka broker in its constructor by the following methods.

The Producer Class

The producer class provides send method to send messages to either single or multiple topics using the following signatures.

---

public void send(KeyedMessaget<k,v> message) 
- sends the data to a single topic,par-titioned by key using either sync or async producer.
public void send(List<KeyedMessage<k,v>>messages)
- sends data to multiple topics.
Properties prop = new Properties();
prop.put(producer.type,”async”)
ProducerConfig config = new ProducerConfig(prop);

There are two types of producers – Sync and Async.

The same API configuration applies to Sync producer as well. The difference between them is a sync producer sends messages directly, but sends messages in background. Async producer is preferred when you want a higher throughput. In the previous releases like 0.8, an async producer does not have a callback for send() to register error handlers. This is available only in the current release of 0.9.

public void close()

Producer class provides close method to close the producer pool connections to all Kafka bro-kers.

Configuration Settings

The Producer API’s main configuration settings are listed in the following table for better under-standing −

ProducerRecord API

ProducerRecord is a key/value pair that is sent to Kafka cluster.ProducerRecord class constructor for creating a record with partition, key and value pairs using the following signature.

public ProducerRecord (string topic, int partition, k key, v value)

• Topic − user defined topic name that will appended to record.

• Partition − partition count

• Key − The key that will be included in the record.

• Value − Record contents

public ProducerRecord (string topic, k key, v value)

ProducerRecord class constructor is used to create a record with key, value pairs and without partition.

• Topic − Create a topic to assign record.

• Key − key for the record.

• Value − record contents.

public ProducerRecord (string topic, v value)

ProducerRecord class creates a record without partition and key.

• Topic − create a topic.

• Value − record contents.

The ProducerRecord class methods are listed in the following table −

SimpleProducer application

Before creating the application, first start ZooKeeper and Kafka broker then create your own topic in Kafka broker using create topic command. After that create a java class named Sim-pleProducer.java and type in the following coding.

//import util.properties packages
import java.util.Properties;

//import simple producer packages
import org.apache.kafka.clients.producer.Producer;

//import KafkaProducer packages
import org.apache.kafka.clients.producer.KafkaProducer;

//import ProducerRecord packages
import org.apache.kafka.clients.producer.ProducerRecord;

//Create java class named “SimpleProducer”
public class SimpleProducer {
   
   public static void main(String[] args) throws Exception{
      
      // Check arguments length value
      if(args.length == 0){
         System.out.println("Enter topic name");
         return;
      }
      
      //Assign topicName to string variable
      String topicName = args[0].toString();
      
      // create instance for properties to access producer configs   
      Properties props = new Properties();
      
      //Assign localhost id
      props.put("bootstrap.servers", “localhost:9092");
      
      //Set acknowledgements for producer requests.      
      props.put("acks", “all");
      
      //If the request fails, the producer can automatically retry,
      props.put("retries", 0);
      
      //Specify buffer size in config
      props.put("batch.size", 16384);
      
      //Reduce the no of requests less than 0   
      props.put("linger.ms", 1);
      
      //The buffer.memory controls the total amount of memory available to the producer for buffering.   
      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");
      
      Producer<String, String> producer = new KafkaProducer
         <String, String>(props);
            
      for(int i = 0; i < 10; i++)
         producer.send(new ProducerRecord<String, String>(topicName, 
            Integer.toString(i), Integer.toString(i)));
               System.out.println(“Message sent successfully”);
               producer.close();
   }
}

Compilation − The application can be compiled using the following command.

javac -cp “/path/to/kafka/kafka_2.11-0.9.0.0/lib/*” *.java

Execution − The application can be executed using the following command.

java -cp “/path/to/kafka/kafka_2.11-0.9.0.0/lib/*”:. SimpleProducer <topic-name>

Output

Message sent successfully
To check the above output open new terminal and type Consumer CLI command to receive messages.
>> bin/kafka-console-consumer.sh --zookeeper localhost:2181 —topic <topic-name> —from-beginning
1
2
3
4
5
6
7
8
9
10

Simple Consumer Example

As of now we have created a producer to send messages to Kafka cluster. Now let us create a consumer to consume messages form the Kafka cluster. KafkaConsumer API is used to consume messages from the Kafka cluster. KafkaConsumer class constructor is defined below.

public KafkaConsumer(java.util.Map<java.lang.String,java.lang.Object> configs)

configs − Return a map of consumer configs.

KafkaConsumer class has the following significant methods that are listed in the table below.

ConsumerRecord API

The ConsumerRecord API is used to receive records from the Kafka cluster. This API consists of a topic name, partition number, from which the record is being received and an offset that points to the record in a Kafka partition. ConsumerRecord class is used to create a consumer record with specific topic name, partition count and <key, value> pairs. It has the following signature.

public ConsumerRecord(string topic,int partition, long offset,K key, V value)

• Topic − The topic name for consumer record received from the Kafka cluster.

• Partition − Partition for the topic.

• Key − The key of the record, if no key exists null will be returned.

• Value − Record contents.

ConsumerRecords API

ConsumerRecords API acts as a container for ConsumerRecord. This API is used to keep the list of ConsumerRecord per partition for a particular topic. Its Constructor is defined below.

public ConsumerRecords(java.util.Map<TopicPartition,java.util.List
<Consumer-Record>K,V>>> records)

• TopicPartition − Return a map of partition for a particular topic.

• Records − Return list of ConsumerRecord.

ConsumerRecords class has the following methods defined.

Configuration Settings

The configuration settings for the Consumer client API main configuration settings are listed below −

SimpleConsumer Application

The producer application steps remain the same here. First, start your ZooKeeper and Kafka broker. Then create a SimpleConsumer application with the java class named SimpleCon-sumer.java and type the following code.

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

public class SimpleConsumer {
   public static void main(String[] args) throws Exception {
      if(args.length == 0){
         System.out.println("Enter topic name");
         return;
      }
      //Kafka consumer configuration settings
      String topicName = args[0].toString();
      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");
      KafkaConsumer<String, String> consumer = new KafkaConsumer
         <String, String>(props);
      
      //Kafka Consumer subscribes list of topics here.
      consumer.subscribe(Arrays.asList(topicName))
      
      //print the topic name
      System.out.println("Subscribed to topic " &plus; topicName);
      int i = 0;
      
      while (true) {
         ConsumerRecords<String, String> records = con-sumer.poll(100);
         for (ConsumerRecord<String, String> record : records)
         
         // print the offset,key and value for the consumer records.
         System.out.printf("offset = %d, key = %s, value = %sn", 
            record.offset(), record.key(), record.value());
      }
   }
}

Compilation − The application can be compiled using the following command.

javac -cp “/path/to/kafka/kafka_2.11-0.9.0.0/lib/*” *.java

Execution − The application can be executed using the following command

java -cp “/path/to/kafka/kafka_2.11-0.9.0.0/lib/*”:. SimpleConsumer <topic-name>

Input − Open the producer CLI and send some messages to the topic. You can put the smple input as ‘Hello Consumer’.

Output − Following will be the output.

Subscribed to topic Hello-Kafka
offset = 3, key = null, value = Hello Consumer