The Weird OS Built Around a Database [video]
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My uncle, a very tech-savvy accountant type, used it a lot back in the day and always had nice things to say about it.
They can also make use of raw partitions, thus having a server that is all about the RDMS and nothing else.
Is also a very productive way to program!
The video called me out when it mentioned that the query language might seem scary or dirty to some of us - and it does! My eyelid twitched when I saw the example on screen - but it's still really interesting to me to think about a database as the source of the OS primitives, rather than an application running within a more standard OS and using the OS's conceptual framing.
SQLite callbacks let you implement anything you please inside the scope of transactions, triggers, other events etc. To make an app run this way all you need is a starting event. You could implement an OS this way, heheh.