Apache Calcite is a dynamic data management framework.
It contains many of the pieces that comprise a typical database management system but omits the storage primitives. It provides an industry standard SQL parser and validator, a customisable optimizer with pluggable rules and cost functions, logical and physical algebraic operators, various transformation algorithms from SQL to algebra (and the opposite), and many adapters for executing SQL queries over Cassandra, Druid, Elasticsearch, MongoDB, Kafka, and others, with minimal configuration.
For more details, see the home page.