Kylin is an open source distributed data analytics engine on top of Hadoop/Spark. It offers SQL interface to do OLAP on large datasets. Unlike massive parallel processing engines like Hive and Presto, Kylin pre-calculates a set of data cubes, stores them in HBase, and directly looks up the results in them. If a query cannot be answered by the data cubes, it will be executed by the underlying process engine. In this way, Kylin is usually used as an accelerator of traditional parallel data processing engines. Kylin is a pure OLAP engine, so it only supports `SELECT` queries. `INSERT`, `UPDATE` and `DELETE` are not supported.
The Kylin project was started in 2013, from eBay's R&D in Shanghai, China. It was open sourced on Github as "KylinOLAP" in Oct 2014. In Nov 2015, Kylin joined Apache Software Foundation incubator; In Dec 2015, Apache Kylin became a Top Level Project.
Kylin applies dictionary encoding to all dimension values in data cubes. Kylin's dictionary is order-preserving and supports mapping both from keys to values and vice versa. The dictionary is implemented as a radix tree. Besides, Kylin also supports naive compression in HBase and Hive.