Start with introduction to core code and the server-side source code repositories.

The Okapi Guide and Reference provides the necessary background understanding.

The FOLIO-Sample-Modules guide further explains what is a server-side module and how to develop one. Importantly, these examples show that any programming language is possible.

The RAML Module Builder (RMB) framework, is a special FOLIO module that abstracts much functionality and enables the developer to focus on implementing business functions. Define the APIs and objects in RAML files and schema files, then the RMB generates code and provides tools to help implement the module. (Note that at this stage of the FOLIO project, only this Java-based framework is available.)

The Primer for RAML and JSON Schema provides some guidance.