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Use Apache Flink® with Aiven for Apache Kafka®

Apache Flink® is an open source platform for processing distributed streaming and batch data. Where Apache Kafka® excels at receiving and sending event streams, Flink consumes, transforms, aggregates, and enriches your data.


If you want to experience the power of streaming SQL transformations with Flink, Aiven provides a managed Aiven for Apache Flink® with built-in data flow integration with Aiven for Apache Kafka®.

The example in this article shows you how to create a simple Java Flink job that reads data from a Kafka topic, processes it, and then pushes it to a different Kafka topic. It uses the Java API on a local installation of Apache Flink 1.15.1, but it can be applied to use Aiven for Apache Kafka with any self-hosted cluster.

You need an Aiven for Apache Kafka service up and running with two topics, named test-flink-input and test-flink-output, already created. Furthermore, for the example, you need to collect the following information about the Aiven for Apache Kafka service:

  • APACHE_KAFKA_HOST: The hostname of the Apache Kafka service
  • APACHE_KAFKA_PORT: The port of the Apache Kafka service

You need to have Apache Maven™ installed to build the example.

Setup the truststore and keystore

Create a Java keystore and truststore for the Aiven for Apache Kafka service. For the following example we assume:

  • The keystore is available at KEYSTORE_PATH/client.keystore.p12
  • The truststore is available at TRUSTSTORE_PATH/client.truststore.jks
  • For simplicity, the same secret (password) is used for both the keystore and the truststore, and is shown here as KEY_TRUST_SECRET

The following example shows how to customise the DataStreamJob generated from the Quickstart to work with Aiven for Apache Kafka.


The full code to run this example can be found in the Aiven examples GitHub repository.

  1. Generate a Flink job skeleton named flink-capitalizer using the Maven archetype:

    mvn archetype:generate -DinteractiveMode=false  \
    -DarchetypeGroupId=org.apache.flink \
    -DarchetypeArtifactId=flink-quickstart-java \
    -DarchetypeVersion=1.15.1 \
    -DgroupId=io.aiven.example \
    -DartifactId=flink-capitalizer \
    -Dpackage=io.aiven.example.flinkcapitalizer \
  2. Uncomment the Kafka connector in `pom.xml`:


Customize the DataStreamJob application

In the generated code, DataStreamJob is the main entry point, and has already been configured with all of the context necessary to interact with the cluster for your processing.

  1. Create a new class called io.aiven.example.flinkcapitalizer.StringCapitalizer which performs a simple MapFunction transformation on incoming records with every incoming string will be emitted as uppercase.

    package io.aiven.example.flinkcapitalizer;

    import org.apache.flink.api.common.functions.MapFunction;

    public class StringCapitalizer implements MapFunction<String, String> {
    public String map(String s) {
    return s.toUpperCase();
  2. Import the following classes in the DataStreamJob

    import java.util.Properties;
    import org.apache.flink.api.common.eventtime.WatermarkStrategy;
    import org.apache.flink.api.common.serialization.SimpleStringSchema;
    import org.apache.flink.connector.base.DeliveryGuarantee;
    import org.apache.flink.connector.kafka.sink.KafkaRecordSerializationSchema;
    import org.apache.flink.connector.kafka.sink.KafkaSink;
    import org.apache.flink.connector.kafka.source.KafkaSource;
    import org.apache.flink.connector.kafka.source.enumerator.initializer.OffsetsInitializer;
  3. Modify the main method in DataStreamJob to read and write from the Kafka topics, replacing the APACHE_KAFKA_HOST, APACHE_KAFKA_PORT, KEYSTORE_PATH, TRUSTSTORE_PATH and KEY_TRUST_SECRET placeholders with the values from the prerequisites.

    public static void main(String[] args) throws Exception {
    final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

    Properties props = new Properties();
    props.put("security.protocol", "SSL");
    props.put("ssl.keystore.type", "PKCS12");
    props.put("ssl.keystore.location", "KEYSTORE_PATH/client.keystore.p12");
    props.put("ssl.keystore.password", "KEY_TRUST_SECRET");
    props.put("ssl.key.password", "KEY_TRUST_SECRET");
    props.put("ssl.truststore.type", "JKS");
    props.put("ssl.truststore.location", "TRUSTSTORE_PATH/client.truststore.jks");
    props.put("ssl.truststore.password", "KEY_TRUST_SECRET");

    KafkaSource<String> source = KafkaSource.<String>builder()
    .setValueOnlyDeserializer(new SimpleStringSchema())

    KafkaSink<String> sink = KafkaSink.<String>builder()
    .setValueSerializationSchema(new SimpleStringSchema())

    // ... processing continues here
  4. Tie the Kafka sources and sinks together with the StringCapitalizer in a single processing pipeline.

    // ... processing continues here
    .fromSource(source, WatermarkStrategy.noWatermarks(), "Kafka Source")
    .map(new StringCapitalizer())
    env.execute("Flink Java capitalizer");

Build the application

From the main flink-capitalizer folder, execute the following Maven command to build the application:

mvn -DskipTests=true clean package

The above command should create a jar file named target/flink-capitalizer-0.0.1-SNAPSHOT.jar.

Run the applications

If you have installed a local cluster installation of Apache Flink 1.15.1, you can launch the job on your local machine. $FLINK_HOME is the Flink installation directory.

$FLINK_HOME/bin/flink run target/flink-capitalizer-0.0.1-SNAPSHOT.jar

You can see that the job is running in the Flink web UI at http://localhost:8081.

By following the article Aiven for Apache Flink®, you can send string events to the input topic and verify that the messages are forwarded to the output topic in upper case.