As a frontend developer, I've been wanting to expand my skills as a server-side developer, and have been a little lost on where to get started in building an auth system, and how I'd approach deeply understanding it enough to do it right (regardless of whether it's the right business choice or not, it seems likely worthwhile to be able to do it). Does anyone have any good resource that would provide a robust exploration into building this kind of system, either in a specific language or just a general one? I'm talking sane abstractions that you'd see Fowler talking about, and something that I could actually follow while writing code to do it.