The Metadata Storage is an external dependency of Druid. Druid uses it to store various metadata about the system, but not to store the actual data. There are a number of tables used for various purposes.
Derby is the default metadata store for Druid, however, it is not suitable for production. MySQL and PostgreSQL are more production suitable metadata stores. Druid provides extensions to support MySQL and PostgreSQL as metadata storage. But there may be cases, where you already have a Microsoft SQL Server installed in your local or production environment. So it is not feasible to deploy one additional cluster of MySQL/PostgreSQL if you can use your existing sql server as the metadata storage. Druid has a wonderful community and they have provided an extension to support this out of the box. In this post, I will show you how to configure druid to use SQL Server as the metadata storage.