System design interview question
"Design a notification system for sports lineups. Users subscribe to arbitrary criteria — 'tell me when my player is confirmed in a starting lineup', 'any goalkeeper ruled out in this league', 'a player on my fantasy roster is benched'. Lineups are published in bursts an hour before kickoff, dozens of matches at once, and the notification is worthless if it arrives after the deadline."
Recognition that this is a search problem run backwards. Normally many documents are indexed and a query arrives; here millions of standing queries are held and documents stream in. Candidates who spot that inversion — and know that a search engine can index queries and match documents against them — are well ahead of those who plan to loop over subscriptions.
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