midPoint's tech stack

runtime:

databases — PostgreSQL (the RDBMS of choice), and there was support for MySQL/MariaDB/MS SQL/Oracle/H2 prior to v4.4
scripting languages — Groovy (the default one; documentation), Python (used very little), and JavaScript (almost never used)
integration — a REST API is provided (there used to be a SOAP API as well, but it was removed in v4.2)

development: