I have some experience in Fintech. I wrote the translation layer between financial cores and a RESTful API (essentially wrapper logic). D+H, Fiserv, BottomLine, all provided us integration documentation which allowed us to preform actions (transfer funds between accounts, get balances, etc.) and it wasn't _too_ bad.

I do remember one particular core that was surprising difficult. Everything was encoded in EBCDIC. You had to rotate telnet (!) connections depending on the day of the week. There was one person (a man named Earl) who could help you if you had a problem with the continuous stationary documentation (luckily it was scanned).

Oh the memories...