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Good question! And few people get to work at this scale, so it's not an unreasonable guess. I'll join you in speculating wildly about this, since, hey, it's kind of fun. IMHO sharding traffic by subreddit doesn't pass the smell-test, though. Different subreddits have very different traffic patterns, so the system would likely end up with hotspots continuously popping up, and it'd probably be toilsome to constantly ba…
A good, but wrong, assumption is to assume Reddit's engineers know what they're doing. The founding Reddit team was non-technical (Even Spez. I've been to UVA; it's not an engineering school; and spez had never done any real engineering before coming onto Reddit). They ran a skeleton crew of some 10ish people for a long time (none from any exceptional backgrounds. One of them was from Sun & Oracle, after their prime)…
I've been parts of 2 different engineering teams that wrote crap that I would cuss out but were super succesfuly, and most recently I joined a team that was super anal about every little detail. I think business success only gets hindered on the extremes now. If you're somewhere in the middle, it's fine. I'd rather have a buggy product that people use than a perfect codebase that only exists on github.