Apache Druid is an open source distributed data store. Druid’s core design combines ideas from data warehouses, timeseries databases, and search systems to create a high performance real-time analytics database for a broad range of use cases. Druid merges key characteristics of each of the 3 systems into its ingestion layer, storage format, querying layer, and core architecture.
Key features of Druid include:
Druid is complementary to many open source data technologies in the Apache Software Foundation including Apache Kafka, Apache Hadoop, Apache Flink, and more.
Druid typically sits between a storage or processing layer and the end user, and acts as a query layer to serve analytic workloads.
Druid supports both streaming and batch ingestion. Druid connects to a source of raw data, typically a message bus such as Apache Kafka (for streaming data loads), or a distributed filesystem such as HDFS (for batch data loads).
Druid converts raw data stored in a source to a more read-optimized format (called a Druid “segment”) in a process calling “indexing”.
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Like many analytic data stores, Druid stores data in columns. Depending on the type of column (string, number, etc), different compression and encoding methods are applied. Druid also builds different types of indexes based on the column type.
Similar to search systems, Druid builds inverted indexes for string columns for fast search and filter. Similar to timeseries databases, Druid intelligently partitions data by time to enable fast time-oriented queries.
Unlike many traditional systems, Druid can optionally pre-aggregate data as it is ingested. This pre-aggregation step is known as rollup, and can lead to dramatic storage savings.
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Druid supports querying data through JSON-over-HTTP and SQL. In addition to standard SQL operators, Druid supports unique operators that leverage its suite of approximate algorithms to provide rapid counting, ranking, and quantiles.
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Druid has a microservice-based architecture can be thought of as a disassembled database. Each core service in Druid (ingestion, querying, and coordination) can be separately or jointly deployed on commodity hardware.
Druid explicitly names every main service to allow the operator to fine tune each service based on the use case and workload. For example, an operator can dedicate more resources to Druid’s ingestion service while giving less resources to Druid’s query service if the workload requires it.
Druid services can independently fail without impacting the operations of other services.
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Druid is designed to power applications that need to be up 24 hours a day, 7 days a week. As such, Druid possesses several features to ensure uptime and no data loss.
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