The target technology for the database is fhirbase, itself installed on a Postgres database and making use of JSONB feature to implement the expected entity extensibility mechanism.
fhirbase is open source, under MIT license. Yet extensions like fhirpath seem not to be in MIT license fhirbase extensions.
Other database support:
Existing implementation of openIMIS make use of SQL-Server database.
The migration strategy puts the database migration to final stages (to ensure an iterative approach).
Early version of openIMIS will thus remain on SQL-Server, but phase 2 of the Transformation methodology is dedicated to switch to Postgres.
The first database connector to be found for django to MSSQL is django-mssql.
However that connector is not maintained anymore (and only supports django up to 1.8).
The connector to use is https://pypi.org/project/django-pyodbc-azure/
FHIR implementation (fhirbase) only supports Postgres as database platform. No other database engine (Oracle, DB2, MongoDB,...) will be supported.