I had to run a very large accounting Excel file inside a web service about ten years ago. The service was born as manually operated Excel sheet plus interaction

I had to run a very large accounting Excel file inside a web service about ten years ago. The service was born as manually operated Excel sheet plus interaction by email/phone and they wanted to scale it on the web. I run it on Windows Server and paid somebody to write the Windows service to manage the Excel sheet. I can't remember which language I used for the web service itself. They eventually gave up, commercial problems, not technical ones.