Async fire and forget sending message doesn't works correctly, it's block on waiting for metadata request, i guess. P.S. It's not actually bug, maybe this feature will be introduce in the future releases, look at KIP-286.

kafka-metadata-bug.java

package org.stilya.kafka;

import java.util.Properties;
import org.apache.kafka.clients.producer.Callback;
import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.apache.kafka.clients.producer.RecordMetadata;

public class Main {

  public static void main(String[] args) {

    String test = "test";
    String test1 = "test1";

    Properties props = new Properties();
    props.put("bootstrap.servers", "localhost:9092");
    props.put("acks", "all");
    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");

    Producer<String, String> producer = new KafkaProducer<>(props);

    producer.send(new ProducerRecord<>(test,
        "test", "test"));

    System.out.println("First message in topic test");

    System.out.println("STOP BROKER"); // MAKE BREAKPOINT HERE AND STOP BROKER

    producer.send(new ProducerRecord<>(test,
        "test", "test"));

    System.out.println("Second message in topic test");

    producer.send(new ProducerRecord<>(test1,
        "test", "test"));

    System.out.println("First message in topic test1");
    producer.close();
  }
}
//
//class MyProducerCallback implements Callback {
//
//  @Override
//  public void onCompletion(RecordMetadata recordMetadata, Exception e) {
//    if (e != null)
//      System.out.println("AsynchronousProducer failed with an exception " + e.getMessage());
//    else
//      System.out.println("AsynchronousProducer call Success:");
//  }
//}