Extensions

Druid implements an extension system that allows for adding functionality at runtime. Extensions are commonly used to add support for deep storages (like HDFS and S3), metadata stores (like MySQL and PostgreSQL), new aggregators, new input formats, and so on.

Production clusters will generally use at least two extensions; one for deep storage and one for a metadata store. Many clusters will also use additional extensions.

Core extensions

Core extensions are maintained by Druid committers.

Community extensions

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Community extensions are not maintained by Druid committers, although we accept patches from community members using these extensions. They may not have been as extensively tested as the core extensions.

A number of community members have contributed their own extensions to Druid that are not packaged with the default Druid tarball. If you’d like to take on maintenance for a community extension, please post on dev@druid.apache.org to let us know!

All of these community extensions can be downloaded using pull-deps while specifying a -c coordinate option to pull org.apache.druid.extensions.contrib:{EXTENSION_NAME}:{DRUID_VERSION}.

Promoting community extensions to core extensions

Please post on dev@druid.apache.org if you’d like an extension to be promoted to core. If we see a community extension actively supported by the community, we can promote it to core based on community feedback.

For information how to create your own extension, please see here.

Loading extensions

Loading core extensions

Apache Druid bundles all core extensions out of the box. See the list of extensions for your options. You can load bundled extensions by adding their names to your common.runtime.properties druid.extensions.loadList property. For example, to load the postgresql-metadata-storage and druid-hdfs-storage extensions, use the configuration:

druid.extensions.loadList=["postgresql-metadata-storage", "druid-hdfs-storage"]

These extensions are located in the extensions directory of the distribution.

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Druid bundles two sets of configurations: one for the quickstart and one for a clustered configuration. Make sure you are updating the correct common.runtime.properties for your setup.

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Because of licensing, the mysql-metadata-storage extension does not include the required MySQL JDBC driver. For instructions on how to install this library, see the MySQL extension page.

Loading community extensions

You can also load community and third-party extensions not already bundled with Druid. To do this, first download the extension and then install it into your extensions directory. You can download extensions from their distributors directly, or if they are available from Maven, the included pull-deps can download them for you. To use pull-deps, specify the full Maven coordinate of the extension in the form groupId:artifactId:version. For example, for the (hypothetical) extension com.example:druid-example-extension:1.0.0, run:

java \
  -cp "lib/*" \
  -Ddruid.extensions.directory="extensions" \
  -Ddruid.extensions.hadoopDependenciesDir="hadoop-dependencies" \
  org.apache.druid.cli.Main tools pull-deps \
  --no-default-hadoop \
  -c "com.example:druid-example-extension:1.0.0"

You only have to install the extension once. Then, add "druid-example-extension" to druid.extensions.loadList in common.runtime.properties to instruct Druid to load the extension.

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Please make sure all the Extensions related configuration properties listed here are set correctly.

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The Maven groupId for almost every community extension is org.apache.druid.extensions.contrib. The artifactId is the name of the extension, and the version is the latest Druid stable version.

Loading extensions from the classpath

If you add your extension jar to the classpath at runtime, Druid will also load it into the system. This mechanism is relatively easy to reason about, but it also means that you have to ensure that all dependency jars on the classpath are compatible. That is, Druid makes no provisions while using this method to maintain class loader isolation so you must make sure that the jars on your classpath are mutually compatible.