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Programming is all about input and output, but what that means has changed over the years. You go back a few decades and the idea was simple. The only programs we used were what were called batch programs. You started the program like it was a food processor. You'd be giving a file of data for input, you'd let the program process all of it and spit out the results to another file or even directly to the printer, and then the program stopped. You did not interact with the program as it was running. The closest thing to this for most people now is a few command line programs or the occasional utility like doing a full virus scan of a hard drive.
Things which are designed to run and finish as fast as possible, but instead the shift is to programs that run with graphical user interfaces. Things like a web browser that don't just start and run all the way through and stop as quickly as possible, but they are programs that stay up. We can interact with these continuously and unpredictably over hours or even days or even weeks. And with these input and output is a bit vague. Well, yes, a mouse-click is input, something typed in a webpage is input, and changing an image on the screen is output.
But that doesn't really feel the same as saving and loading files, because sometimes we still want to be able to save documents to the hard drive or load files from the hard drive or save information to the web. So describing just everything as input and output becomes a little generic. So when we talk about really saving information, running a program that then saves information to our documents folder or saves to a database or saves to the web or saves to a cloud, and I mean data that outlives the currently running program, meaning that we could close the program and that data would still be on the drive or in the database or in the cloud, then we use the term persistence. And we also refer to this as saving the state of a program. Because without deciding to persist, our data it all just disappears.
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