Design a personalized news-feed system that ingests articles from many sources and builds a home feed. Users may follow topics and sources. A new user with no preferences should receive a reasonable default mix. Breaking news must be eligible for prominent placement for all users without permanently overriding personalization. Compare fan-out-on-read with fan-out-on-write and select a hybrid strategy. ### Constraints & Assumptions - An article may arrive more than once from syndicated sources. - Feed results need deterministic pagination while new articles continue to arrive. - Source and topic preferences can change. ### Clarifying Questions to Ask - What defines breaking news and who is allowed to assign that status? - How fresh must ordinary and breaking items be? - Do users need strict exclusion or only ranking preferences for sources and topics? ```hint Treat the feed as a merge Candidate sets can come from subscriptions, precomputed defaults, and a small breaking-news lane before ranking and deduplication. ``` ### What a Strong Answer Covers - Ingestion, normalization, canonical article identity, enrichment, and indexing. - Candidate generation for followed topics and sources plus a cold-start default pool. - A justified fan-out strategy, ranking, stable cursors, caching, and invalidation. - Controlled breaking-news insertion, expiry, deduplication, and abuse safeguards. ### Follow-up Questions - How would you correct a falsely labeled breaking story? - How would you avoid showing five syndicated copies of the same event? - What changes for a user following thousands of sources?