zio / zio-kafka

A Kafka client for ZIO and ZIO Streams

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Welcome to ZIO Kafka

ZIO Kafka provides a purely functional, streams-based interface to the Kafka client. It integrates effortlessly with ZIO and ZIO Streams.



Add the following dependencies to your build.sbt file:

libraryDependencies ++= Seq(
  "dev.zio" %% "zio-streams" % "1.0.2",
  "dev.zio" %% "zio-kafka"   % "<version>"

Somewhere in your application, configure the zio.kafka.ConsumerSettings data type:

import zio._, zio.duration._
import zio.kafka.consumer._

val settings: ConsumerSettings = 

For a lot of use cases where you just want to do something with all messages on a Kafka topic, ZIO Kafka provides the convenience method Consumer.consumeWith. This method lets you execute a ZIO effect for each message. Topic partitions will be processed in parallel and offsets are committed after running the effect automatically.

import zio._
import zio.console._
import zio.kafka.consumer._
import zio.kafka.serde._

val subscription = Subscription.topics("topic")

Consumer.consumeWith(settings, subscription, Serde.string, Serde.string) { case (key, value) =>
  putStrLn(s"Received message ${key}: ${value}")
  // Perform an effect with the received message

If you require more control over the consumption process, read on!

Consuming Kafka topics using ZIO Streams

First, create a consumer using the ConsumerSettings instance:

import zio.ZLayer, zio.ZManaged, zio.blocking.Blocking, zio.clock.Clock
import zio.kafka.consumer.{ Consumer, ConsumerSettings }

val consumerSettings: ConsumerSettings = ConsumerSettings(List("localhost:9092")).withGroupId("group")
val consumerManaged: ZManaged[Clock with Blocking, Throwable, Consumer.Service] =
val consumer: ZLayer[Clock with Blocking, Throwable, Consumer] =

The consumer returned from Consumer.make is wrapped in a ZLayer to allow for easy composition with other ZIO environment components. You may provide that layer to effects that require a consumer. Here's an example:

import zio._, zio.blocking.Blocking, zio.clock.Clock 
import zio.kafka.consumer._
import zio.kafka.serde._

val data: RIO[Clock with Blocking, 
              List[CommittableRecord[String, String]]] = 
  (Consumer.subscribe(Subscription.topics("topic")) *>
  Consumer.plainStream(Serde.string, Serde.string).take(50).runCollect)

You may stream data from Kafka using the subscribeAnd and plainStream methods:

import zio.blocking.Blocking, zio.clock.Clock, zio.console.putStrLn
import zio.kafka.consumer._

  .plainStream(Serde.string, Serde.string)
  .tap(cr => putStrLn(s"key: ${cr.record.key}, value: ${cr.record.value}"))

If you need to distinguish between the different partitions assigned to the consumer, you may use the Consumer#partitionedStream method, which creates a nested stream of partitions:

import zio.blocking.Blocking, zio.clock.Clock, zio.console.putStrLn
import zio.kafka.consumer._

  .partitionedStream(Serde.string, Serde.string)
  .tap(tpAndStr => putStrLn(s"topic: ${tpAndStr._1.topic}, partition: ${tpAndStr._1.partition}"))
  .tap(cr => putStrLn(s"key: ${cr.record.key}, value: ${cr.record.value}"))

Example: consuming, producing and committing offset

This example shows how to consume messages from topic topic_a and produce transformed messages to topic_b, after which consumer offsets are committed. Processing is done in chunks using ZStreamChunk for more efficiency.

import zio.ZLayer
import zio.kafka.consumer._
import zio.kafka.producer._
import zio.kafka.serde._
import org.apache.kafka.clients.producer.ProducerRecord

val consumerSettings: ConsumerSettings = ConsumerSettings(List("localhost:9092")).withGroupId("group")
val producerSettings: ProducerSettings = ProducerSettings(List("localhost:9092"))

val consumerAndProducer = 
  ZLayer.fromManaged(Consumer.make(consumerSettings)) ++
    ZLayer.fromManaged(Producer.make(producerSettings, Serde.int, Serde.string))

val consumeProduceStream = Consumer
  .plainStream(Serde.int, Serde.long)
  .map { record =>
    val key: Int    = record.record.key()
    val value: Long = record.record.value()
    val newValue: String = value.toString

    val producerRecord: ProducerRecord[Int, String] = new ProducerRecord("my-output-topic", key, newValue)
    (producerRecord, record.offset)
  .mapChunksM { chunk =>
    val records     = chunk.map(_._1)
    val offsetBatch = OffsetBatch(chunk.map(_._2).toSeq)

    Producer.produceChunk[Any, Int, String](records) *> offsetBatch.commit.as(Chunk(()))

Partition assignment and offset retrieval

zio-kafka offers several way to control which Kafka topics and partitions are assigned to your application.

Use case Method
One or more topics, automatic partition assignment Consumer.subscribe(Subscription.topics("my_topic", "other_topic"))
Topics matching a pattern Consumer.subscribe(Subscription.pattern("topic.*"))
Manual partition assignment Consumer.subscribe(Subscription.manual("my_topic" -> 1, "my_topic" -> 2))

By default zio-kafka will start streaming a partition from the last committed offset for the consumer group, or the latest message on the topic if no offset has yet been committed. You can also choose to store offsets outside of kafka. This can be useful in cases where consistency between data stores and consumer offset is required.

Use case Method
Offsets in kafka, start at latest message if no offset committed OffsetRetrieval.Auto()
Offsets in kafka, start at earliest message if no offset committed OffsetRetrieval.Auto(AutoOffsetStrategy.Earliest)
Manual/external offset storage Manual(getOffsets: Set[TopicPartition] => Task[Map[TopicPartition, Long]])

For manual offset retrieval, the getOffsets function will be called for each topic-partition that is assigned to the consumer, either via Kafka's rebalancing or via a manual assignment.

Custom data type serdes

Serializers and deserializers (serdes) for custom data types can be constructed from scratch or by converting existing serdes. For example, to create a serde for an Instant:

import java.time.Instant
import zio.kafka.serde._

val instantSerde: Serde[Any, Instant] = Serde.long.inmap(java.time.Instant.ofEpochMilli)(_.toEpochMilli)

Handling deserialization failures

The default behavior for a consumer stream when encountering a deserialization failure is to fail the stream. In many cases you may want to handle this situation differently, eg by skipping the message that failed to deserialize or by executing an alternative effect. For this purpose, any Deserializer[T] for some type T can be easily converted into a Deserializer[Try[T]] where deserialization failures are converted to a Failure using the asTry method.

Below is an example of skipping messages that fail to deserialize. The offset is passed downstream to be committed.

import zio.blocking.Blocking, zio.clock.Clock, zio.console.putStrLn
import zio.stream._
import zio.kafka.consumer._
import zio.kafka.serde._
import scala.util.{Try, Success, Failure}
import zio._

val consumer = Consumer.make(consumerSettings)

val stream = Consumer
  .plainStream(Serde.string, Serde.string.asTry)

  .mapM { record => 
    val tryValue: Try[String] = record.record.value()
    val offset: Offset = record.offset
    tryValue match {
      case Success(value) =>
        // Action for successful deserialization
      case Failure(exception) =>
        // Possibly log the exception or take alternative action

Getting help

Join us on the ZIO Discord server at the #zio-kafka channel.


This library is heavily inspired and made possible by the research and implementation done in Alpakka Kafka, a library maintained by the Akka team and originally written as Reactive Kafka by SoftwareMill.


Copyright 2020 Itamar Ravid and the zio-kafka contributors. All rights reserved.