z/TPF [1], is basically a database system OS that runs almost all VISA, AMEX, Citibank (authorizations); Holiday Inn, Amtrak, Mariott, Air Canada, Delta, Japan Airlines, Travelport, Trenitalia (reservations). But it's IBM proprietary and runs only on IBM licensed hardware.

It's always in my programming dreams to be involved in building a database OS with I/O capabilities since I worked with TPF and MUMPS.

[1] https://www.ibm.com/it-infrastructure/z/transaction-processi...