Testing

To simplify testing of streaming integrations with Alpakka Kafka, it provides the Alpakka Kafka testkit. It provides help for

• Using an embedded Kafka broker
• Using Docker to launch a local Kafka cluster with testcontainers
• Mocking the Alpakka Kafka Consumers and Producers

Maven

<properties>
  <akka.version>2.5.31</akka.version>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-stream-kafka-testkit_${scala.binary.version}</artifactId>
  <version>2.0.5</version>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-stream-testkit_${scala.binary.version}</artifactId>
  <version>${akka.version}</version>
  <scope>test</scope>
</dependency>

sbt

val AkkaVersion = "2.5.31"
libraryDependencies ++= Seq(
  "com.typesafe.akka" %% "akka-stream-kafka-testkit" % "2.0.5" % Test,
  "com.typesafe.akka" %% "akka-stream-testkit" % AkkaVersion % Test
)

Gradle

versions += [
  AkkaVersion: "2.5.31",
  ScalaBinary: "2.13"
]
dependencies {
  test group: 'com.typesafe.akka', name: "akka-stream-kafka-testkit_${versions.ScalaBinary}", version: '2.0.5',
  test group: 'com.typesafe.akka', name: "akka-stream-testkit_${versions.ScalaBinary}", version: versions.AkkaVersion
}

Note that Akka testkits do not promise binary compatibility. The API might be changed even between patch releases.

The table below shows Alpakka Kafka testkit’s direct dependencies and the second tab shows all libraries it depends on transitively. We’ve overriden the commons-compress library to use a version with fewer known security vulnerabilities.

Direct dependencies

Organization	Artifact	Version
com.typesafe.akka	akka-stream-kafka_2.13	2.0.5
com.typesafe.akka	akka-stream-testkit_2.13	2.5.31
org.scala-lang	scala-library	2.13.1

Dependency tree

com.typesafe.akka    akka-stream-kafka_2.13    2.0.5
    com.typesafe.akka    akka-stream_2.13    2.5.31
        com.typesafe.akka    akka-actor_2.13    2.5.31
            com.typesafe    config    1.3.3
            org.scala-lang.modules    scala-java8-compat_2.13    0.9.0
                org.scala-lang    scala-library    2.13.1
            org.scala-lang    scala-library    2.13.1
        com.typesafe.akka    akka-protobuf_2.13    2.5.31
            org.scala-lang    scala-library    2.13.1
        com.typesafe    ssl-config-core_2.13    0.3.8
            com.typesafe    config    1.3.3
            org.scala-lang.modules    scala-parser-combinators_2.13    1.1.2
                org.scala-lang    scala-library    2.13.1
            org.scala-lang    scala-library    2.13.1
        org.reactivestreams    reactive-streams    1.0.2
        org.scala-lang    scala-library    2.13.1
    org.apache.kafka    kafka-clients    2.4.1
        com.github.luben    zstd-jni    1.4.3-1
        org.lz4    lz4-java    1.6.0
        org.slf4j    slf4j-api    1.7.30
        org.xerial.snappy    snappy-java    1.1.7.3
    org.scala-lang.modules    scala-collection-compat_2.13    2.1.6
        org.scala-lang    scala-library    2.13.1
    org.scala-lang    scala-library    2.13.1
com.typesafe.akka    akka-stream-testkit_2.13    2.5.31
    com.typesafe.akka    akka-stream_2.13    2.5.31
        com.typesafe.akka    akka-actor_2.13    2.5.31
            com.typesafe    config    1.3.3
            org.scala-lang.modules    scala-java8-compat_2.13    0.9.0
                org.scala-lang    scala-library    2.13.1
            org.scala-lang    scala-library    2.13.1
        com.typesafe.akka    akka-protobuf_2.13    2.5.31
            org.scala-lang    scala-library    2.13.1
        com.typesafe    ssl-config-core_2.13    0.3.8
            com.typesafe    config    1.3.3
            org.scala-lang.modules    scala-parser-combinators_2.13    1.1.2
                org.scala-lang    scala-library    2.13.1
            org.scala-lang    scala-library    2.13.1
        org.reactivestreams    reactive-streams    1.0.2
        org.scala-lang    scala-library    2.13.1
    com.typesafe.akka    akka-testkit_2.13    2.5.31
        com.typesafe.akka    akka-actor_2.13    2.5.31
            com.typesafe    config    1.3.3
            org.scala-lang.modules    scala-java8-compat_2.13    0.9.0
                org.scala-lang    scala-library    2.13.1
            org.scala-lang    scala-library    2.13.1
        org.scala-lang    scala-library    2.13.1
    org.scala-lang    scala-library    2.13.1
org.scala-lang    scala-library    2.13.1

Running Kafka with your tests

The Testkit provides a variety of ways to test your application against a real Kafka broker or cluster. There are two main options:

1. Embedded Kafka
2. Testcontainers (Docker)

The table below helps guide you to the right Testkit implementation depending on your programming language, testing framework, and use (or not) of Docker containers. You must mix in or implement these types into your test classes to use them. See the documentation for each for more details.

Testing with an embedded Kafka server

To test the Alpakka Kafka connector the Embedded Kafka library is an important tool as it helps to easily start and stop Kafka brokers from test cases.

Add the Embedded Kafka to your test dependencies:

Maven

<dependency>
  <groupId>io.github.embeddedkafka</groupId>
  <artifactId>embedded-kafka_2.12</artifactId>
  <version>2.4.1.1</version>
  <scope>test</scope>
</dependency>

sbt

libraryDependencies += "io.github.embeddedkafka" % "embedded-kafka_2.12" % "2.4.1.1" % Test

Gradle

dependencies {
  test group: 'io.github.embeddedkafka', name: 'embedded-kafka_2.12', version: '2.4.1.1'
}

The testkit contains helper classes used by the tests in the Alpakka Kafka connector and may be used for other testing, as well.

Testing with Avro and Schema Registry

If you need to run tests using Confluent’s Schema Registry, you might include embedded-kafka-schema-registry instead.

Testing with Avro and Schema Registry from Java code

Test classes may extend EmbeddedKafkaTest (JUnit 5) or EmbeddedKafkaJunit4Test (JUnit 4) to automatically start and stop an embedded Kafka broker.

Furthermore it provides

• preconfigured consumer settings (ConsumerSettings<String, String> consumerDefaults),
• preconfigured producer settings (ProducerSettings<String, String> producerDefaults),
• unique topic creation (createTopic(int number, int partitions, int replication)), and
• CompletionStage value extraction helper (<T> T resultOf(CompletionStage<T> stage, java.time.Duration timeout)).

The example below shows skeleton test classes for JUnit 4 and JUnit 5.

Java JUnit 4

import akka.kafka.testkit.javadsl.EmbeddedKafkaJunit4Test;
import akka.testkit.javadsl.TestKit;
import org.junit.AfterClass;
import org.junit.Rule;
import org.junit.Test;
public class AssignmentTest extends EmbeddedKafkaJunit4Test {

  @Rule public final LogCapturingJunit4 logCapturing = new LogCapturingJunit4();

  private static final ActorSystem sys = ActorSystem.create("AssignmentTest");
  private static final Materializer mat = ActorMaterializer.create(sys);

  public AssignmentTest() {
    super(sys, mat, KafkaPorts.AssignmentTest());
  }

  @AfterClass
  public static void afterClass() {
    TestKit.shutdownActorSystem(sys);
  }
}

Java JUnit 5

import akka.kafka.testkit.javadsl.EmbeddedKafkaTest;
import akka.testkit.javadsl.TestKit;
import org.junit.jupiter.api.*;
import org.junit.jupiter.api.TestInstance.Lifecycle;
import org.junit.jupiter.api.extension.ExtendWith;

@TestInstance(Lifecycle.PER_CLASS)
class ProducerExampleTest extends EmbeddedKafkaTest {

  private static final ActorSystem system = ActorSystem.create("ProducerExampleTest");
  private static final Materializer materializer = ActorMaterializer.create(system);

  ProducerExampleTest() {
    super(system, materializer, KafkaPorts.ProducerExamplesTest());
  }

  @AfterAll
  void shutdownActorSystem() {
    TestKit.shutdownActorSystem(system);
    executor.shutdown();
  }

}

The JUnit test base classes run the assertAllStagesStopped check from Akka Stream testkit to ensure all stages are shut down properly within each test. This may interfere with the stop-timeout which delays shutdown for Alpakka Kafka consumers. You might need to configure a shorter timeout in your application.conf for tests.

Testing with Avro and Schema Registry from Scala code

The KafkaSpec class offers access to

• preconfigured consumer settings (consumerDefaults: ConsumerSettings[String, String]),
• preconfigured producer settings (producerDefaults: ProducerSettings[String, String]),
• unique topic creation (createTopic(number: Int = 0, partitions: Int = 1, replication: Int = 1)),
• an implicit LoggingAdapter for use with the log() operator, and
• other goodies.

EmbeddedKafkaLike extends KafkaSpec to add automatic starting and stopping of the embedded Kafka broker.

Some Alpakka Kafka tests implemented in Scala use Scalatest with the mix-ins shown below. You need to add Scalatest explicitly in your test dependencies (this release of Alpakka Kafka uses Scalatest 3.0.8.)

Maven

<dependency>
  <groupId>org.scalatest</groupId>
  <artifactId>scalatest</artifactId>
  <version>3.0.8</version>
  <scope>test</scope>
</dependency>

sbt

libraryDependencies += "org.scalatest" % "scalatest" % "3.0.8" % Test

Gradle

dependencies {
  test group: 'org.scalatest', name: 'scalatest', version: '3.0.8'
}

Scala

import akka.kafka.testkit.scaladsl.ScalatestKafkaSpec
import org.scalatest.concurrent.{Eventually, IntegrationPatience, ScalaFutures}
import org.scalatest.{Matchers, WordSpecLike}

abstract class SpecBase(kafkaPort: Int)
    extends ScalatestKafkaSpec(kafkaPort)
    with WordSpecLike
    with Matchers
    with ScalaFutures
    with Eventually {

  protected def this() = this(kafkaPort = -1)
}

By mixing in EmbeddedKafkaLike an embedded Kafka instance will be started before the tests in this test class execute shut down after all tests in this test class are finished.

Scala

import akka.kafka.testkit.scaladsl.EmbeddedKafkaLike
import com.github.ghik.silencer.silent
import net.manub.embeddedkafka.EmbeddedKafkaConfig

@silent
class EmbeddedKafkaSampleSpec extends SpecBase(kafkaPort = 1234) with EmbeddedKafkaLike {

  // if a specific Kafka broker configuration is desired
  override def createKafkaConfig: EmbeddedKafkaConfig =
    EmbeddedKafkaConfig(kafkaPort,
                        zooKeeperPort,
                        Map(
                          "offsets.topic.replication.factor" -> "1",
                          "offsets.retention.minutes" -> "1",
                          "offsets.retention.check.interval.ms" -> "100"
                        ))

  // ...
}

With this EmbeddedKafkaSpecBase class test classes can extend it to automatically start and stop a Kafka broker to test with. To configure the Kafka broker non-default, override the createKafkaConfig as shown above.

To ensure proper shutdown of all stages in every test, wrap your test code in assertAllStagesStopped. This may interfere with the stop-timeout which delays shutdown for Alpakka Kafka consumers. You might need to configure a shorter timeout in your application.conf for tests.

Alternative testing libraries

If using Maven and Java, an alternative library that provides running Kafka broker instance during testing is kafka-unit by salesforce. It has support for Junit 4 and 5 and supports many different versions of Kafka.

Mocking the Consumer or Producer

The testkit contains factories to create the messages emitted by Consumer sources in akka.kafka.testkit.ConsumerResultFactory and Producer flows in akka.kafka.testkit.ProducerResultFactory.

To create the materialized value of Consumer sources, ConsumerControlFactoryConsumerControlFactory offers a wrapped KillSwitchKillSwitch.

Scala

import akka.kafka.testkit.scaladsl.ConsumerControlFactory
import akka.kafka.testkit.{ConsumerResultFactory, ProducerResultFactory}

// create elements emitted by the mocked Consumer
val elements = (0 to 10).map { i =>
  val nextOffset = startOffset + i
  ConsumerResultFactory.committableMessage(
    new ConsumerRecord(topic, partition, nextOffset, "key", s"value $i"),
    ConsumerResultFactory.committableOffset(groupId, topic, partition, nextOffset, s"metadata $i")
  )
}

// create a source imitating the Consumer.committableSource
val mockedKafkaConsumerSource: Source[ConsumerMessage.CommittableMessage[String, String], Consumer.Control] =
  Source(elements).viaMat(ConsumerControlFactory.controlFlow())(Keep.right)

// create a source imitating the Producer.flexiFlow
val mockedKafkaProducerFlow: Flow[ProducerMessage.Envelope[String, String, CommittableOffset],
                                  ProducerMessage.Results[String, String, CommittableOffset],
                                  NotUsed] =
  Flow[ProducerMessage.Envelope[String, String, CommittableOffset]]
    .map {
      case msg: ProducerMessage.Message[String, String, CommittableOffset] =>
        ProducerResultFactory.result(msg)
      case other => throw new Exception(s"excluded: $other")
    }

// run the flow as if it was connected to a Kafka broker
val (control, streamCompletion) = mockedKafkaConsumerSource
  .map(
    msg =>
      ProducerMessage.Message(
        new ProducerRecord[String, String](targetTopic, msg.record.value),
        msg.committableOffset
      )
  )
  .via(mockedKafkaProducerFlow)
  .map(_.passThrough)
  .toMat(Committer.sink(committerSettings))(Keep.both)
  .run()

Java

import akka.kafka.testkit.ConsumerResultFactory;
import akka.kafka.testkit.ProducerResultFactory;
import akka.kafka.testkit.javadsl.ConsumerControlFactory;

// create elements emitted by the mocked Consumer
List<ConsumerMessage.CommittableMessage<String, String>> elements =
    Arrays.asList(
        ConsumerResultFactory.committableMessage(
            new ConsumerRecord<>(topic, partition, startOffset, "key", "value 1"),
            ConsumerResultFactory.committableOffset(
                groupId, topic, partition, startOffset, "metadata")),
        ConsumerResultFactory.committableMessage(
            new ConsumerRecord<>(topic, partition, startOffset + 1, "key", "value 2"),
            ConsumerResultFactory.committableOffset(
                groupId, topic, partition, startOffset + 1, "metadata 2")));

// create a source imitating the Consumer.committableSource
Source<ConsumerMessage.CommittableMessage<String, String>, Consumer.Control>
    mockedKafkaConsumerSource =
        Source.cycle(elements::iterator)
            .viaMat(ConsumerControlFactory.controlFlow(), Keep.right());

// create a source imitating the Producer.flexiFlow
Flow<
        ProducerMessage.Envelope<String, String, ConsumerMessage.CommittableOffset>,
        ProducerMessage.Results<String, String, ConsumerMessage.CommittableOffset>,
        NotUsed>
    mockedKafkaProducerFlow =
        Flow
            .<ProducerMessage.Envelope<String, String, ConsumerMessage.CommittableOffset>>
                create()
            .map(
                msg -> {
                  if (msg instanceof ProducerMessage.Message) {
                    ProducerMessage.Message<String, String, ConsumerMessage.CommittableOffset>
                        msg2 =
                            (ProducerMessage.Message<
                                    String, String, ConsumerMessage.CommittableOffset>)
                                msg;
                    return ProducerResultFactory.result(msg2);
                  } else throw new RuntimeException("unexpected element: " + msg);
                });

// run the flow as if it was connected to a Kafka broker
Pair<Consumer.Control, CompletionStage<Done>> stream =
    mockedKafkaConsumerSource
        .map(
            msg -> {
              ProducerMessage.Envelope<String, String, ConsumerMessage.CommittableOffset>
                  message =
                      new ProducerMessage.Message<>(
                          new ProducerRecord<>(
                              targetTopic, msg.record().key(), msg.record().value()),
                          msg.committableOffset());
              return message;
            })
        .via(mockedKafkaProducerFlow)
        .map(ProducerMessage.Results::passThrough)
        .toMat(Committer.sink(committerSettings), Keep.both())
        .run(mat);