Kafka producer and consumer written with Kotlin


I wanted to write a short and sweet blog post on writing a Kafka producer and consumer in Kotlin before delving deeper into the topic of Kafka. Yes, that was a “topic” pun… I’ll see myself out.

This post will contain example producer and consumer code written in Kotlin.


The code below creates a Producer that continually pumps messages to its chosen topic.

fun createProducer(): Producer<String, String> {
  val props = Properties()
  props["bootstrap.servers"] = "localhost:9092"
  props["acks"] = "all"
  props["retries"] = 0
  props["linger.ms"] = 1
  props["key.serializer"] = "org.apache.kafka.common.serialization.StringSerializer"
  props["value.serializer"] = "org.apache.kafka.common.serialization.StringSerializer"

  return KafkaProducer(props)

fun Producer<String, String>.produceMessages(topic: String) {
  fixedRateTimer(daemon = true, period = Duration.ofSeconds(2).toMillis()) {
    val time = LocalDateTime.now()
    val message = ProducerRecord(
      topic, // topic
      time.toString(), // key
      "Message sent at ${LocalDateTime.now()}" // value
    println("Producer sending message: $message")

This is an example, so the code is not particularly useful.

When creating a KafkaProducer, you must provide its configuration via a Properties object or a Map<String, Object>. KafkaProducer’s constructor also accepts values for the key.serializer and value.serializer properties.

In this example, a StringSerializer is used for both keys and values, as the producer is pushing messages comprised purely of Strings. At a minimum, you have to include a single serializer, or an error will be coming your way:

Exception in thread "main" org.apache.kafka.common.config.ConfigException: Missing required configuration "key.serializer" which has no default value.
	at org.apache.kafka.common.config.ConfigDef.parseValue(ConfigDef.java:478)
	at org.apache.kafka.common.config.ConfigDef.parse(ConfigDef.java:468)
	at org.apache.kafka.common.config.AbstractConfig.<init>(AbstractConfig.java:108)
	at org.apache.kafka.common.config.AbstractConfig.<init>(AbstractConfig.java:129)
	at org.apache.kafka.clients.producer.ProducerConfig.<init>(ProducerConfig.java:536)
	at org.apache.kafka.clients.producer.KafkaProducer.<init>(KafkaProducer.java:330)
	at org.apache.kafka.clients.producer.KafkaProducer.<init>(KafkaProducer.java:302)
	at dev.lankydan.kafka.producer.ProducerKt.createProducer(Producer.kt:30)
	at dev.lankydan.kafka.MainKt.main(Main.kt:13)
	at dev.lankydan.kafka.MainKt.main(Main.kt)

Using the wrong serializer will cause the following error:

Exception in thread "Timer-0" org.apache.kafka.common.errors.SerializationException: Can't convert key of class java.lang.String to class org.apache.kafka.common.serialization.UUIDSerializer specified in key.serializer
Caused by: java.lang.ClassCastException: class java.lang.String cannot be cast to class java.util.UUID (java.lang.String and java.util.UUID are in module java.base of loader 'bootstrap')
	at org.apache.kafka.common.serialization.UUIDSerializer.serialize(UUIDSerializer.java:29)
	at org.apache.kafka.common.serialization.Serializer.serialize(Serializer.java:62)
	at org.apache.kafka.clients.producer.KafkaProducer.doSend(KafkaProducer.java:918)
	at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:886)
	at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:774)
	at dev.lankydan.kafka.producer.ProducerKt$produceMessages$$inlined$fixedRateTimer$1.run(Timer.kt:156)
	at java.base/java.util.TimerThread.mainLoop(Timer.java:556)
	at java.base/java.util.TimerThread.run(Timer.java:506)

You can find the available configuration options in Kafka’s documentation.

After creating the producer, it is ready to start sending events/messages/records, or whatever you wish to call them. You can send a record by building a ProducerRecord and executing Producer.send.

Producer is the interface that KafkaProducer implements, which it also shares with MockProducer.

A ProducerRecord consists of 6 components:

  • Topic - The topic to send the record to.
  • Partition - The partition to send the record to. This property is optional. When it is not provided, the partition is calculated by hashing the record’s key and modding it against the number of partitions.
  • Timestamp - The timestamp of the record. When it is not explicitly provided, it will default to System.currentTimeMillis.
  • Key - The key of the record.
  • Value - The value of the record.
  • Headers - Extra metadata to go alongside the record’s value.

Producer.send completes the process as it hands off the record to the Kafka broker/cluster.


The following consumer code compliments the previously shown producer:

fun createConsumer(): Consumer<String, String> {
  val props = 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")
  return KafkaConsumer(props)

fun Consumer<String, String>.consumeMessages(topic: String) {
  while (true) {
    val messages: ConsumerRecords<String, String> = poll(Duration.ofMillis(5000))
    if (!messages.isEmpty) {
      for (message: ConsumerRecord<String, String> in messages) {
        println("Consumer reading message: ${message.value()}")
      commitAsync { offsets, exception ->
        for ((partition, metadata) in offsets) {
          println("Committed offset for topic: ${partition.topic()}, partition: ${partition.partition()}, offset: ${metadata.offset()}")
    } else {
      println("No messages to read and poll timeout reached")

Similar to initialising a KafkaProducer, you must provide configuration when creating a KafkaConsumer.

You can find the available configuration options in Kafka’s documentation.

The consumer is in charge of requesting records from the Kafka broker. To do this, it subscribes (by calling subscribe) to the topic(s) it wants records from. Calling Consumer.poll requests records from the subscribed topics and blocking the executing thread until the consumer receives some records or reaching the timeout duration. Surpassing the timeout causes the code to move on with an empty collection of records. No exceptions are thrown, which makes sense; the lack of messages isn’t unusual behaviour; it is only an indication that no new records are available for processing.

When messages are successfully received, poll will return ConsumerRecords (a collection of ConsumerRecords). These records consist of the following information (most of them match the data contained in ProducerRecord):

  • Topic - The topic the record is from.
  • Partition - The partition the record was placed in.
  • Timestamp - The timestamp that the record was recorded with.
  • Key - The key of the record.
  • Value - The value of the record.
  • Headers - Extra metadata to stored alongside the record’s value.
  • Offset - The offset of the record (the position in the partition).

ConsumerRecords implements Iterable so it can be looped over directly, as shown in the example above.

After processing the batch of received records, you should update the offset. Calling commitAsync will set the offset, for each subscribed topic, to the highest offset of the received records. You can call the commitAsync overload below if you want to specify the offsets to update to yourself:

void commitAsync(Map<TopicPartition, OffsetAndMetadata> offsets, OffsetCommitCallback callback);

You can use the OffsetCommitCallback to trigger a callback when committing the offset, which can be beneficial as it is happening asynchronously.

commitSync is the synchronous version of commitAsync which will block the current thread until the consumer propagates the update to the Kafka broker.

How you decide to update your offset is up to you and the behaviour your application requires.


You have now seen an example of how to write a Kafka producer and consumer in Kotlin. There isn’t anything special about it being written in Kotlin, but now you know what it could like like if you were wondering.

In short, to create a producer, you should instantiate a KafkaProducer, make ProducerRecords and send them to the Kafka broker. To consume these messages, create a KafkaConsumer, poll new records, and remember to call commitAsync/commitSync to update your topics’ offsets.

Dan Newton
Written by Dan Newton