HSQLDB is a pure java RDBMS which supports more SQL features than we need for xplanner.

It can be deployed in 4 modes:

1. Standalone (embedded): database runs inside client VM and is only accessible from it, i.e. no multi-user capability. Simple persistence to regular file(s) (By default one, multiple if cached tables are selected).
2. In-memory (embedded): Same as standalone but with no persistence (potentially good to speed up acceptance testing)
3. Server: runs as a regular external server process
4. Web: Runs as a web server or as a servlet in a container. Queries are submitted through http. No transaction.

Guide

Standalone mode

hibernate.connection.url=jdbc:hsqldb:file:/c:/tmp/xplanner

In the default standalone mode everything is kept in memory. In order to save everything to disk a sql shutdown must be issued before shutting the hosting VM to flush it correctly.