This document discusses how Apache Calcite makes it easier to write database management systems (DBMS) by decomposing them into modular components like a query parser, catalog, algorithms, and storage engines. It presents Calcite as a framework that allows these components to be mixed and matched, with a core relational algebra and rule-based optimization. Calcite powers systems like Apache Hive, Drill, Phoenix, and Kylin by translating SQL and other queries to relational algebra and optimizing queries using over 100 rules before executing them using configurable engines and data sources.