I remember using an AS/400 mainframe as a kid (I have fond memories of essentially IM'ing back and forth with my father when I was 4 in his office). I used one

I remember using an AS/400 mainframe as a kid (I have fond memories of essentially IM'ing back and forth with my father when I was 4 in his office). I used one helping him with some database migration/upgrade at the same office when I was in my teens. So I've touched mainframes at least, but now all of that is in the fog of time and memory. Now, I'm a Ruby developer and I honestly don't know what I'd use a mainframe for. I don't know when I'd deploy one, what an appropriate task is for them, etc. I get the sense they are good for when you have "big data" stuff to deal with, but it feels more like a 1980's definition of "big data" (MILLIONS of rows, omg!) than datasets in the billions/trillions of rows (or huge datasets like a human genome sequence). I assume on a technical level, being turing complete machines, that I can do 'anything' with them, but realistically what can I do? Can I run a web server? Do data analysis using Hadoop across a huge dataset? Can I easily setup background tasks and queues? Do they do 64 bit instructions? What are the limitations? Can I run Lisp on them? What is the toolset like? Do they support Unicode? What is their database, and what is it capable of? And what's the best place to learn about this stuff? I can go to Barnes and Noble and get a book on C, Ruby, Java or PHP... but finding good modern documentation on mainframes? I dunno where to look, or what would be considered 'too old' for documentation. Anything about Ruby from 2002 is ancient, but perhaps that's the new hip stuff when it comes to COBOL?