Design a news subscription processing engine with four core capabilities: receive a news item, add a subscription, remove a subscription, and publish matching news to subscribers. Focus on the domain model, APIs, indexing, and correctness under concurrent updates. ### Constraints & Assumptions - A news item has a stable ID and attributes used for matching. - A subscriber may have several subscriptions. - The exact subscription predicate language is not prescribed; begin with a small, explicit model and explain how it could evolve. - Duplicate receipt or publish requests are possible. - Delivery may fail independently for each subscriber. ### Clarifying Questions to Ask - Does receipt automatically trigger publication, or are they separate operations? - What fields may a subscription filter on? - Should removal affect a news item already being published? - What delivery guarantee is required? - Must subscribers observe news in a total order? ### What a Strong Answer Covers ```premium-lock What a Strong Answer Covers ``` ### Follow-up Questions - Add compound subscription predicates. - Preserve per-subscriber ordering across workers. - Recover when publication stops halfway through a large fan-out.