Answer the following ML system design questions: - Describe the machine learning system you know best. Walk through the problem definition, data sources, feature generation, offline training pipeline, online inference path, model refresh strategy, monitoring, and the trade-offs between the offline and online parts of the system. - Design a ranking system for a personalized feed, search results page, or recommendation surface. Specify the objective function, candidate generation, ranking model, feature store, training data, online serving stack, latency budget, experimentation plan, and how you would handle cold start, exploration, feedback loops, and model drift.