From the outside, social features seem like something straightforward: follow a user, like a post, see a feed, but when you attempt to implement them at any real scale you find that every single one is something you've got to be aware of, a pattern, a well-documented query pattern with all its failure modes. Here's a step-by-step look at four of them: adjacency list, fan-out feed, mutual-friends join, and some graph traversal that SQL wasn't really designed for. The adjacency list an...