Apache Kafka is an open-source distributed event streaming platform used by thousands of companies for high-performance data pipelines, streaming analytics, data integration, and mission-critical applications.

You need to have Java installed.

We build and test Apache Kafka with Java versions 17 and 25. The release parameter in javac is set to 11 for the clients and streams modules, and 17 for the rest, ensuring compatibility with their respective minimum Java versions. Similarly, the release parameter in scalac is set to 11 for the streams modules and 17 for the rest.

Scala 2.13 is the only supported version in Apache Kafka.

Build a JAR and run it

./gradlew jar

Follow instructions in https://kafka.apache.org/quickstart

Build source JAR

./gradlew srcJar

Build aggregated javadoc

./gradlew aggregatedJavadoc --no-parallel

Build javadoc and scaladoc

./gradlew javadoc
./gradlew javadocJar # builds a javadoc jar for each module
./gradlew scaladoc
./gradlew scaladocJar # builds a scaladoc jar for each module
./gradlew docsJar # builds both (if applicable) javadoc and scaladoc jars for each module

Run unit/integration tests

./gradlew test  # runs both unit and integration tests
./gradlew unitTest
./gradlew integrationTest
./gradlew test -Pkafka.test.run.flaky=true  # runs tests that are marked as flaky

Force re-running tests without code change

./gradlew test --rerun-tasks
./gradlew unitTest --rerun-tasks
./gradlew integrationTest --rerun-tasks

Running a particular unit/integration test

./gradlew clients:test --tests RequestResponseTest
./gradlew streams:integration-tests:test --tests RestoreIntegrationTest

Running a particular unit/integration test N times

N=500; I=0; while [ $I -lt $N ] && ./gradlew clients:test --tests RequestResponseTest --rerun --fail-fast; do (( I=$I+1 )); echo "Completed run: $I"; sleep 1; done

Running a particular test method within a unit/integration test

./gradlew clients:test --tests org.apache.kafka.clients.MetadataTest.testTimeToNextUpdate
./gradlew clients:clients-integration-tests:test --tests org.apache.kafka.clients.producer.ProducerFailureHandlingTest.testCannotSendToInternalTopic
./gradlew streams:integration-tests:test --tests org.apache.kafka.streams.integration.RestoreIntegrationTest.shouldRestoreNullRecord

Running a particular unit/integration test with log4j output

By default, there will be only a small number of logs output while testing. You can adjust it by changing the log4j2.yaml file in the module's src/test/resources directory.

For example, if you want to see more logs for clients project tests, you can modify the line in clients/src/test/resources/log4j2.yaml to level: INFO and then run:

./gradlew cleanTest clients:test --tests NetworkClientTest

And you should see INFO level logs in the file under the clients/build/test-results/test directory.

Specifying test retries

Retries are disabled by default, but you can set maxTestRetryFailures and maxTestRetries to enable retries.

The following example declares -PmaxTestRetries=1 and -PmaxTestRetryFailures=3 to enable a failed test to be retried once, with a total retry limit of 3.

./gradlew test -PmaxTestRetries=1 -PmaxTestRetryFailures=3

See Test Retry Gradle Plugin and build.yml for more details.

Generating test coverage reports

Generate coverage reports for the whole project:

./gradlew reportCoverage -PenableTestCoverage=true -Dorg.gradle.parallel=false

Generate coverage for a single module, i.e.:

./gradlew clients:reportCoverage -PenableTestCoverage=true -Dorg.gradle.parallel=false

Coverage reports are located within the module's build directory, categorized by module type:

Core Module (:core): core/build/reports/scoverageTest/index.html

Other Modules: /build/reports/jacoco/test/html/index.html

Building a binary release gzipped tarball

./gradlew clean releaseTarGz

The release file can be found inside ./core/build/distributions/.

Building auto-generated messages

Sometimes it is only necessary to rebuild the RPC auto-generated message data when switching between branches, as they could fail due to code changes. You can just run:

./gradlew processMessages processTestMessages

See Apache Kafka Message Definitions for details on Apache Kafka message protocol.

Running a Kafka broker

Using compiled files:

KAFKA_CLUSTER_ID="$(./bin/kafka-storage.sh random-uuid)"
./bin/kafka-storage.sh format --standalone -t $KAFKA_CLUSTER_ID -c config/server.properties
./bin/kafka-server-start.sh config/server.properties

Using docker image:

docker run -p 9092:9092 apache/kafka:latest

See docker/README.md for detailed information.

Cleaning the build

./gradlew clean

Running a task for a specific project

This is for core, examples and clients

./gradlew core:jar
./gradlew core:test

Streams has multiple sub-projects, but you can run all the tests:

./gradlew :streams:testAll

Listing all gradle tasks

./gradlew tasks

Building IDE project

Note: Please ensure that JDK 17 is used when developing Kafka.

IntelliJ supports Gradle natively, and it will automatically check Java syntax and compatibility for each module, even if the Java version shown in the Structure > Project Settings > Modules may not be the correct one.

When it comes to Eclipse, run:

./gradlew eclipse

The eclipse task has been configured to use ${project_dir}/build_eclipse as Eclipse's build directory. Eclipse's default build directory (${project_dir}/bin) clashes with Kafka's scripts directory, and we don't use Gradle's build directory to avoid known issues with this configuration.

Publishing the streams quickstart archetype artifact to maven

For the Streams archetype project, one cannot use gradle to upload to maven; instead the mvn deploy command needs to be called at the quickstart folder:

cd streams/quickstart
mvn deploy

Please note for this to work you should create/update user maven settings (typically, ${USER_HOME}/.m2/settings.xml) to assign the following variables

...                           

   ...

      apache.snapshots.https
      ${maven_username}
      ${maven_password}


      apache.releases.https
      ${maven_username}
      ${maven_password}

    ...

 ...

Installing all projects to the local Maven repository

./gradlew -PskipSigning=true publishToMavenLocal

Installing specific projects to the local Maven repository

./gradlew -PskipSigning=true :streams:publishToMavenLocal

Building the test JAR

./gradlew testJar

Running code quality checks

There are two code quality analysis tools that we regularly run, SpotBugs and Checkstyle.

Checkstyle

Checkstyle enforces a consistent coding style in Kafka. You can run Checkstyle using:

./gradlew checkstyleMain checkstyleTest spotlessCheck

The Checkstyle warnings will be found in reports/checkstyle/reports/main.html and reports/checkstyle/reports/test.html files in the subproject build directories. They are also printed to the console. The build will fail if Checkstyle fails. For experiments (or regression testing purposes) add -PcheckstyleVersion=X.y.z switch (to override project-defined checkstyle version).

Spotless

The import order is a part of static check. Please call spotlessApply to optimize Java imports before filing a pull request.

./gradlew spotlessApply

SpotBugs

SpotBugs uses static analysis to look for bugs in the code. You can run SpotBugs using:

./gradlew spotbugsMain spotbugsTest -x test

The SpotBugs warnings will be found in reports/spotbugs/main.html and reports/spotbugs/test.html files in the subproject build directories. Use -PxmlSpotBugsReport=true to generate an XML report instead of an HTML one.

JMH microbenchmarks

We use JMH to write microbenchmarks that produce reliable results in the JVM.

See jmh-benchmarks/README.md for details on how to run the microbenchmarks.

Dependency Analysis

The gradle dependency debugging documentation mentions using the dependencies or dependencyInsight tasks to debug dependencies for the root project or individual subprojects.

Alternatively, use the allDeps or allDepInsight tasks for recursively iterating through all subprojects:

./gradlew allDeps

./gradlew allDepInsight --configuration runtimeClasspath --dependency com.fasterxml.jackson.core:jackson-databind

These take the same arguments as the built-in variants.

Determining if any dependencies could be updated

./gradlew dependencyUpdates

Common build options

The following options should be set with a -P switch, for example ./gradlew -PmaxParallelForks=1 test.

• commitId: sets the build commit ID as .git/HEAD might not be correct if there are local commits added for build purposes.
• mavenUrl: sets the URL of the maven deployment repository (file://path/to/repo can be used to point to a local repository).
• maxParallelForks: maximum number of test processes to start in parallel. Defaults to the number of processors available to the JVM.
• maxScalacThreads: maximum number of worker threads for the scalac backend. Defaults to the lowest of 8 and the number of processors available to the JVM. The value must be between 1 and 16 (inclusive).
• ignoreFailures: ignore test failures from junit
• showStandardStreams: shows standard output and standard error of the test JVM(s) on the console.
• skipSigning: skips signing of artifacts.
• testLoggingEvents: unit test events to be logged, separated by comma. For example ./gradlew -PtestLoggingEvents=started,passed,skipped,failed test.
• xmlSpotBugsReport: enable XML reports for SpotBugs. This also disables HTML reports as only one can be enabled at a time.
• maxTestRetries: maximum number of retries for a failing test case.
• maxTestRetryFailures: maximum number of test failures before retrying is disabled for subsequent tests.
• enableTestCoverage: enables test coverage plugins and tasks, including bytecode enhancement of classes required to track said coverage. Note that this introduces some overhead when running tests and hence why it's disabled by default (the overhead varies, but 15-20% is a reasonable estimate).
• keepAliveMode: configures the keep-alive mode for the Gradle compilation daemon - reuse improves start-up time. The values should be one of daemon or session (the default is daemon). daemon keeps the daemon alive until it's explicitly stopped while session keeps it alive until the end of the build session. This currently only affects the Scala compiler, see https://github.com/gradle/gradle/pull/21034 for a PR that attempts to do the same for the Java compiler.
• scalaOptimizerMode: configures the optimizing behavior of the Scala compiler, the value should be one of none, method, inline-kafka or inline-scala (the default is inline-kafka). none is the Scala compiler default, which only eliminates unreachable code. method also includes method-local optimizations. inline-kafka adds inlining of methods within the kafka packages. Finally, inline-scala also includes inlining of methods within the scala library (which avoids lambda allocations for methods like Option.exists). inline-scala is only safe if the Scala library version is the same at compile time and runtime. Since we cannot guarantee this for all cases (for example, users may depend on the kafka jar for integration tests where they may include a scala library with a different version), we don't enable it by default. See https://www.lightbend.com/blog/scala-inliner-optimizer for more details.

Upgrading Gradle version

See gradle/wrapper/README.md for instructions on upgrading the Gradle version.

Running system tests

See tests/README.md.

Using Trogdor for testing

We use Trogdor as a test framework for Apache Kafka. You can use it to run benchmarks and other workloads.

See trogdor/README.md.

Running in Vagrant

See vagrant/README.md.

Kafka client examples

See examples/README.md.

Contribution

Apache Kafka is interested in building the community; we would welcome any thoughts or patches. You can reach us on the Apache mailing lists.

To contribute follow the instructions here:

• https://kafka.apache.org/contributing.html