Design a recommendation system end-to-end

Design a production recommendation service that outputs a personalized feed—for instance, a short-video “For You” stream or an e-commerce “recommended for you” shelf. The service must rank content for several hundred million monthly users, onboard users and items with no prior history, react to new behavior within seconds, and optimize for long-term engagement while actively controlling harmful feedback loops. Walk through the full pipeline: retrieval and ranking, batch and…