--- layout: doc_page --- # Druid 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. ## Including extensions Please see [here](../operations/including-extensions.html). ## Core extensions Core extensions are maintained by Druid committers. |Name|Description|Docs| |----|-----------|----| |druid-avro-extensions|Support for data in Apache Avro data format.|[link](../development/extensions-core/avro.html)| |druid-basic-security|Support for Basic HTTP authentication and role-based access control.|[link](../development/extensions-core/druid-basic-security.html)| |druid-caffeine-cache|A local cache implementation backed by Caffeine.|[link](../development/extensions-core/caffeine-cache.html)| |druid-datasketches|Support for approximate counts and set operations with [DataSketches](http://datasketches.github.io/).|[link](../development/extensions-core/datasketches-extension.html)| |druid-hdfs-storage|HDFS deep storage.|[link](../development/extensions-core/hdfs.html)| |druid-histogram|Approximate histograms and quantiles aggregator.|[link](../development/extensions-core/approximate-histograms.html)| |druid-kafka-eight|Kafka ingest firehose (high level consumer) for realtime nodes.|[link](../development/extensions-core/kafka-eight-firehose.html)| |druid-kafka-extraction-namespace|Kafka-based namespaced lookup. Requires namespace lookup extension.|[link](../development/extensions-core/kafka-extraction-namespace.html)| |druid-kafka-indexing-service|Supervised exactly-once Kafka ingestion for the indexing service.|[link](../development/extensions-core/kafka-ingestion.html)| |druid-kerberos|Kerberos authentication for druid nodes.|[link](../development/extensions-core/druid-kerberos.html)| |druid-lookups-cached-global|A module for [lookups](../querying/lookups.html) providing a jvm-global eager caching for lookups. It provides JDBC and URI implementations for fetching lookup data.|[link](../development/extensions-core/lookups-cached-global.html)| |druid-lookups-cached-single| Per lookup caching module to support the use cases where a lookup need to be isolated from the global pool of lookups |[link](../development/extensions-core/druid-lookups.html)| |druid-protobuf-extensions| Support for data in Protobuf data format.|[link](../development/extensions-core/protobuf.html)| |druid-s3-extensions|Interfacing with data in AWS S3, and using S3 as deep storage.|[link](../development/extensions-core/s3.html)| |druid-stats|Statistics related module including variance and standard deviation.|[link](../development/extensions-core/stats.html)| |mysql-metadata-storage|MySQL metadata store.|[link](../development/extensions-core/mysql.html)| |postgresql-metadata-storage|PostgreSQL metadata store.|[link](../development/extensions-core/postgresql.html)| |simple-client-sslcontext|Simple SSLContext provider module to be used by internal HttpClient talking to other nodes over HTTPS.|[link](../development/extensions-core/simple-client-sslcontext.html)| # Community Extensions