Multi-Company e-Archive Invoice Integration

I developed REST API wrappers to manage the e-archive invoicing processes for over 10 subsidiaries within the holding company from a single central location. There are two separate implementations—one in Python/FastAPI and one in C#/.NET—where each company’s GİB portal accounts are managed within the central system using JWT authentication and company-specific credential mapping. Invoice creation, approval, deletion and reporting are all provided via a single API. I resolved GİB server connection issues through HttpClient optimisation and authentication errors via service instance caching. Integration with Windows Server and a low-code platform has been completed.