A database. To be clear though, it's usually a library plus a service. It could be library + sqlite etc though I guess. Look here for example, page 8:

https://aws-quickstart.s3.amazonaws.com/quickstart-microfocu...

It's pretty amazing, big companies make these migrations on important systems. There are 10's (or 100's maybe) of billions of lines of COBOL still being deployed and companies move them into a cloud (ie aws etc, x86 + cloud services) environment for various purposes.