System design interview question
"Design 'Nearby Friends'. A user opts in and continuously sees which of their friends are within a few miles, updated in near real time as everyone moves. 100 million users have it enabled."
continuous spatial queries on moving points. Unlike "find restaurants near me" — a query against a static index — here both the querier and the results are moving, and every user is simultaneously a subject and an object. The candidate must discover that you cannot re-run a radius query per user per second.
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