Design Content Moderation

Before any of the mechanics, get the shape of Reddit's moderation right, because it is not one system making one decision. Reddit is a collection of communities called subreddits, and each one is governed by its own volunteer moderators who set their own rules. So moderation happens on two layers that run at the same time. The first is community moderation. The mods of r/science ban low-effort posts that r/aww would happily allow. They encode those rules in AutoModerator, a rule engine that reads a community's configuration and acts on content the moment it is submitted. The second is site-wide safety, which Reddit itself runs across every community. It catches what no single mod can, like spam rings, coordinated abuse, and people evading a ban with new accounts.

That two-layer split is the whole design, so keep it in front of you. A weak answer treats moderation as a single classifier that stamps content good or bad. Reddit runs a pipeline. A per-community rule engine, a site-wide machine-learning scorer, and human moderators each handle the slice they are good at. The hard parts are the seams between them. How does the fast rule check stay cheap when it runs on every item for every community? How does an automated removal that turns out wrong get undone? How do you enforce against a bad actor without handing them a signal they can adapt to? Build the pipeline first, because the interviewer expects the skeleton. Then spend the back half on those seams, because that is where a Reddit answer looks different from a generic spam filter.

The Reddit angle

Reddit hands most moderation to the communities themselves, so the system's job is to give volunteer mods a fast, programmable rule engine, not to centrally judge every post. This follows straight from Reddit's mission of empowering communities and making their knowledge accessible. AutoModerator exists because a single subreddit can see hundreds of flagged items a day, more than any volunteer can read. The mods write rules once and the engine enforces them around the clock.