What database does MettleCI use to store its metadata?

MettleCI does not use a database to store any of its information.

User’s are authenticated against your Information Server installation, and all user information is managed by Information Server
Unit Test specifications and data are maintained using industry-standard flat files (CSV, JSON, JUnit-compliant XML, etc.)
Connection information to your Git and Work Item Management systems are stored in various YAML files in your MettleCI home directory on your MettleCI Workbench host (most often the DataStage Engine host).