# Design Radius-Based Notifications from Mobile Locations Design a system in which mobile applications periodically send their users' locations to a server. Given a target latitude, longitude, and radius `x`, the server finds users whose latest non-stale locations fall within the radius and sends each of them a notification. Cover the location-update API, latest-location storage, geospatial candidate search, exact distance filtering, stale-data policy, and notification delivery. Explain how the design behaves when many users update at once. ### Constraints & Assumptions - Only a user's latest accepted location is needed for the proximity decision. - Location updates have timestamps and may arrive late or out of order. - A geospatial index may return a coarse candidate set that needs exact radius filtering. - Notification delivery is asynchronous and may be retried. ### Clarifying Questions to Ask - How often does the mobile app update location, and how long before a location is stale? - What notification latency is required after the radius query begins? - What range can `x` take, and what user count and update rate must the system support? - May one campaign notify the same user more than once? ```hint Index a coarse cell, verify exact distance A cell or bounding-box lookup narrows candidates quickly, but points near the boundary still require a distance calculation. ``` ```hint Decouple selection from delivery Persist the intended recipients before fan-out so a notification worker retry does not rerun a changing location query. ``` ### What a Strong Answer Covers - Authenticated, timestamp-aware location updates with stale-write rejection. - A latest-position store plus a geospatial index and exact radius filtering. - Expiration of stale locations and clear boundary semantics for `x`. - Durable recipient fan-out, idempotent notification attempts, and delivery observability. - Partitioning, hotspot handling, backpressure, privacy, and deletion controls. ### Follow-up Questions - How would you prevent a user near a cell boundary from being missed? - What changes if the notification must follow moving users continuously rather than run once? - How would users opt out and request deletion of stored location data?