Implement Kafka Streams applications for real-time data processing with Java and Scala

Update system packages

Start by updating your package manager to ensure you get the latest versions of all dependencies.

sudo apt update && sudo apt upgrade -y

sudo dnf update -y

Install Java Development Kit

Kafka Streams requires Java 11 or later for optimal performance and compatibility.

sudo apt install -y openjdk-21-jdk openjdk-21-jre
export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
echo 'export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64' >> ~/.bashrc

sudo dnf install -y java-21-openjdk java-21-openjdk-devel
export JAVA_HOME=/usr/lib/jvm/java-21-openjdk
echo 'export JAVA_HOME=/usr/lib/jvm/java-21-openjdk' >> ~/.bashrc

Install Scala build tools

Install SBT (Scala Build Tool) for compiling and managing Scala-based Kafka Streams applications.

echo "deb https://repo.scala-sbt.org/scalasbt/debian all main" | sudo tee /etc/apt/sources.list.d/sbt.list
curl -sL "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x2EE0EA64E40A89B84B2DF73499E82A75642AC823" | sudo apt-key add
sudo apt update
sudo apt install -y sbt scala

sudo dnf install -y dnf-plugins-core
sudo dnf copr enable -y bintray/rpm
curl -L https://github.com/sbt/sbt/releases/download/v1.9.7/sbt-1.9.7.tgz | sudo tar -xz -C /opt/
sudo ln -s /opt/sbt/bin/sbt /usr/local/bin/sbt

Install Apache Maven

Maven handles Java project dependencies and builds for Kafka Streams applications.

sudo apt install -y maven

sudo dnf install -y maven

Create application directory structure

Set up organized directories for both Java and Scala Kafka Streams applications.

mkdir -p ~/kafka-streams-apps/{java-app,scala-app}
cd ~/kafka-streams-apps

Initialize Maven project structure

Create a complete Maven project with the proper directory layout for Java development.

cd java-app
mvn archetype:generate -DgroupId=com.example.streams \
  -DartifactId=kafka-streams-java \
  -DarchetypeArtifactId=maven-archetype-quickstart \
  -DinteractiveMode=false
cd kafka-streams-java

Configure Maven dependencies

Add Kafka Streams dependencies and configure Java version in the POM file.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example.streams</groupId>
  <artifactId>kafka-streams-java</artifactId>
  <version>1.0-SNAPSHOT</version>
  <properties>
    <maven.compiler.source>21</maven.compiler.source>
    <maven.compiler.target>21</maven.compiler.target>
    <kafka.version>3.6.0</kafka.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.apache.kafka</groupId>
      <artifactId>kafka-streams</artifactId>
      <version>${kafka.version}</version>
    </dependency>
    <dependency>
      <groupId>org.apache.kafka</groupId>
      <artifactId>kafka-clients</artifactId>
      <version>${kafka.version}</version>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <version>2.0.9</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.16.0</version>
    </dependency>
  </dependencies>
</project>

Implement stream processing topology

Create a Java class that demonstrates stream transformations, aggregations, and windowing operations.

package com.example.streams;

import org.apache.kafka.common.serialization.Serdes;
import org.apache.kafka.streams.KafkaStreams;
import org.apache.kafka.streams.StreamsBuilder;
import org.apache.kafka.streams.StreamsConfig;
import org.apache.kafka.streams.Topology;
import org.apache.kafka.streams.kstream.*;

import java.time.Duration;
import java.util.Properties;
import java.util.concurrent.CountDownLatch;

public class StreamProcessor {
    public static void main(String[] args) {
        Properties props = new Properties();
        props.put(StreamsConfig.APPLICATION_ID_CONFIG, "streams-processor-java");
        props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
        props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass());
        props.put(StreamsConfig.CACHE_MAX_BYTES_BUFFERING_CONFIG, 10 * 1024 * 1024L);
        props.put(StreamsConfig.COMMIT_INTERVAL_MS_CONFIG, 1000);
        
        StreamsBuilder builder = new StreamsBuilder();
        
        // Read from input topic
        KStream<String, String> source = builder.stream("input-events");
        
        // Transform and filter data
        KStream<String, String> processed = source
            .filter((key, value) -> value != null && value.length() > 0)
            .mapValues(value -> value.toUpperCase())
            .selectKey((key, value) -> extractUserId(value));
        
        // Create windowed aggregations
        KTable<Windowed<String>, Long> windowedCounts = processed
            .groupByKey()
            .windowedBy(TimeWindows.of(Duration.ofMinutes(5)).advanceBy(Duration.ofMinutes(1)))
            .count();
        
        // Output aggregated results
        windowedCounts.toStream()
            .map((windowedKey, count) -> {
                String key = windowedKey.key() + "@" + windowedKey.window().start();
                return KeyValue.pair(key, count.toString());
            })
            .to("aggregated-counts");
        
        // Branch streams for different processing paths
        KStream<String, String>[] branches = processed.branch(
            (key, value) -> value.contains("ERROR"),
            (key, value) -> value.contains("WARN"),
            (key, value) -> true
        );
        
        branches[0].to("error-events");
        branches[1].to("warning-events");
        branches[2].to("info-events");
        
        Topology topology = builder.build();
        KafkaStreams streams = new KafkaStreams(topology, props);
        
        CountDownLatch latch = new CountDownLatch(1);
        Runtime.getRuntime().addShutdownHook(new Thread("streams-shutdown-hook") {
            @Override
            public void run() {
                streams.close();
                latch.countDown();
            }
        });
        
        try {
            streams.start();
            latch.await();
        } catch (Throwable e) {
            System.exit(1);
        }
        System.exit(0);
    }
    
    private static String extractUserId(String value) {
        // Simple extraction - in production, use proper JSON parsing
        int start = value.indexOf("userId:") + 7;
        int end = value.indexOf(",", start);
        if (end == -1) end = value.length();
        return start > 6 ? value.substring(start, end).trim() : "unknown";
    }
}

Initialize SBT project

Set up a Scala project with SBT build configuration and Kafka Streams dependencies.

cd ~/kafka-streams-apps/scala-app
mkdir -p src/main/scala/com/example/streams
mkdir project

Configure SBT build file

Define project dependencies and Scala version for the streams application.

name := "kafka-streams-scala"
version := "1.0"
scalaVersion := "2.13.12"

libraryDependencies ++= Seq(
  "org.apache.kafka" % "kafka-streams" % "3.6.0",
  "org.apache.kafka" %% "kafka-streams-scala" % "3.6.0",
  "org.apache.kafka" % "kafka-clients" % "3.6.0",
  "org.slf4j" % "slf4j-simple" % "2.0.9",
  "com.fasterxml.jackson.core" % "jackson-databind" % "2.16.0",
  "com.fasterxml.jackson.module" %% "jackson-module-scala" % "2.16.0"
)

Configure SBT plugins

Add plugins for better Scala development experience.

addSbtPlugin("com.github.sbt" % "sbt-native-packager" % "1.9.16")

Implement Scala stream processor

Create a Scala implementation with functional programming patterns and type safety.

package com.example.streams

import org.apache.kafka.streams.scala._
import org.apache.kafka.streams.scala.kstream._
import org.apache.kafka.streams.{KafkaStreams, StreamsConfig, Topology}
import org.apache.kafka.streams.kstream.{TimeWindows, Windowed}

import java.time.Duration
import java.util.Properties
import scala.concurrent.duration._
import scala.util.{Failure, Success, Try}

object StreamProcessor extends App {
  import org.apache.kafka.streams.scala.ImplicitConversions._
  import org.apache.kafka.streams.scala.serialization.Serdes._
  
  val props: Properties = {
    val p = new Properties()
    p.put(StreamsConfig.APPLICATION_ID_CONFIG, "streams-processor-scala")
    p.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")
    p.put(StreamsConfig.CACHE_MAX_BYTES_BUFFERING_CONFIG, 10 * 1024 * 1024L: java.lang.Long)
    p.put(StreamsConfig.COMMIT_INTERVAL_MS_CONFIG, 1000: java.lang.Integer)
    p
  }
  
  val builder = new StreamsBuilder()
  
  // Define the topology
  val sourceStream: KStream[String, String] = builder.stream<a>String, String</a>
  
  // Data transformation pipeline
  val processedStream: KStream[String, String] = sourceStream
    .filter((_, value) => Option(value).exists(_.nonEmpty))
    .mapValues(_.toUpperCase)
    .selectKey((_, value) => extractUserId(value))
  
  // Windowed aggregations with functional approach
  val windowedCounts: KTable[Windowed[String], Long] = processedStream
    .groupByKey
    .windowedBy(TimeWindows.of(Duration.ofMinutes(5)).advanceBy(Duration.ofMinutes(1)))
    .count()
  
  // Transform windowed results
  windowedCounts.toStream
    .map { case (windowedKey, count) =>
      val key = s"${windowedKey.key()}@${windowedKey.window().start()}"
      (key, count.toString)
    }
    .to("aggregated-counts")
  
  // Pattern matching for stream branching
  val branches = processedStream.branch(
    (_, value) => value.contains("ERROR"),
    (_, value) => value.contains("WARN"),
    (_, _) => true
  )
  
  branches(0).to("error-events")
  branches(1).to("warning-events")
  branches(2).to("info-events")
  
  // Create and run streams
  val topology: Topology = builder.build()
  val streams = new KafkaStreams(topology, props)
  
  // Graceful shutdown
  sys.addShutdownHook {
    streams.close(Duration.ofSeconds(30))
  }
  
  Try {
    streams.start()
    // Keep the application running
    Thread.currentThread().join()
  } match {
    case Success(_) => println("Stream processing completed successfully")
    case Failure(exception) =>
      println(s"Stream processing failed: ${exception.getMessage}")
      sys.exit(1)
  }
  
  private def extractUserId(value: String): String = {
    val pattern = "userId:(\\w+)".r
    pattern.findFirstMatchIn(value)
      .map(_.group(1))
      .getOrElse("unknown")
  }
}

Create streams configuration file

Define reusable configuration properties for different environments.

# Kafka Streams Application Configuration
application.id=production-streams-app
bootstrap.servers=localhost:9092

# Performance tuning
num.stream.threads=4
cache.max.bytes.buffering=16777216
commit.interval.ms=1000
default.deserialization.exception.handler=org.apache.kafka.streams.errors.LogAndContinueExceptionHandler

# Security (uncomment for SSL)
# security.protocol=SSL
# ssl.truststore.location=/path/to/kafka.client.truststore.jks
# ssl.truststore.password=truststore-password
# ssl.keystore.location=/path/to/kafka.client.keystore.jks
# ssl.keystore.password=keystore-password

# Monitoring
metric.reporters=org.apache.kafka.common.metrics.JmxReporter
auto.include.jmx.reporter=true

Create logging configuration

Set up structured logging for monitoring and debugging stream processing applications.

# Root logger
log4j.rootLogger=INFO, stdout, file

# Console appender
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# File appender
log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.File=/var/log/kafka-streams/streams.log
log4j.appender.file.MaxFileSize=100MB
log4j.appender.file.MaxBackupIndex=10
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# Kafka Streams specific logging
log4j.logger.org.apache.kafka.streams=INFO
log4j.logger.org.apache.kafka.clients=WARN

Compile Java application

Build the Java Kafka Streams application with Maven.

cd ~/kafka-streams-apps/java-app/kafka-streams-java
mvn clean compile
mvn package

Compile Scala application

Build the Scala application using SBT with optimization flags.

cd ~/kafka-streams-apps/scala-app
sbt clean compile
sbt assembly

Create systemd service files

Set up systemd services for production deployment with proper resource management.

[Unit]
Description=Kafka Streams Java Application
After=network.target kafka.service
Requires=kafka.service

[Service]
Type=simple
User=kafka
Group=kafka
ExecStart=/usr/bin/java -Xms1g -Xmx2g -XX:+UseG1GC \
  -Djava.awt.headless=true \
  -Dlog4j.configuration=file:/opt/kafka-streams/log4j.properties \
  -cp /opt/kafka-streams/java/target/kafka-streams-java-1.0-SNAPSHOT.jar \
  com.example.streams.StreamProcessor
WorkingDirectory=/opt/kafka-streams
Restart=on-failure
RestartSec=30
TimeoutStartSec=60
TimeoutStopSec=30

# Resource limits
LimitNOFILE=65536
LimitNPROC=32768

[Install]
WantedBy=multi-user.target

Create Scala service configuration

Configure systemd service for the Scala streams application.

[Unit]
Description=Kafka Streams Scala Application
After=network.target kafka.service
Requires=kafka.service

[Service]
Type=simple
User=kafka
Group=kafka
ExecStart=/usr/bin/java -Xms1g -Xmx2g -XX:+UseG1GC \
  -Djava.awt.headless=true \
  -Dlog4j.configuration=file:/opt/kafka-streams/log4j.properties \
  -jar /opt/kafka-streams/scala/target/scala-2.13/kafka-streams-scala-assembly-1.0.jar
WorkingDirectory=/opt/kafka-streams
Restart=on-failure
RestartSec=30
TimeoutStartSec=60
TimeoutStopSec=30

# Resource limits
LimitNOFILE=65536
LimitNPROC=32768

[Install]
WantedBy=multi-user.target

Deploy applications to production directory

Copy built applications and configuration files to the production directory structure.

sudo mkdir -p /opt/kafka-streams/{java,scala}
sudo mkdir -p /var/log/kafka-streams
sudo cp ~/kafka-streams-apps/java-app/kafka-streams-java/target/kafka-streams-java-1.0-SNAPSHOT.jar /opt/kafka-streams/java/
sudo cp ~/kafka-streams-apps/scala-app/target/scala-2.13/*.jar /opt/kafka-streams/scala/
sudo cp streams.properties /opt/kafka-streams/
sudo cp log4j.properties /opt/kafka-streams/
sudo chown -R kafka:kafka /opt/kafka-streams /var/log/kafka-streams
sudo chmod 755 /opt/kafka-streams
sudo chmod 644 /opt/kafka-streams/*.properties

Enable and start services

Start the Kafka Streams applications and enable them for automatic startup.

sudo systemctl daemon-reload
sudo systemctl enable kafka-streams-java kafka-streams-scala
sudo systemctl start kafka-streams-java
sudo systemctl start kafka-streams-scala

Configure JMX monitori