Proximity / Nearby Service — Simple

Problem statement (interviewer prompt)

Design a generic proximity / nearby service: given a point (lat, lng) and a radius (or k), return the nearest POIs in <200ms. Cover the spatial index choice (quadtree / geohash / S2 / H3 / R-tree), sharding, and how to handle hot cells (e.g. city centres).

flowchart LR
  U([User])
  Q[Nearby Query]
  IDX[(Spatial Index<br/>quadtree / S2 / geohash)]
  POI[(Places)]
  U --> Q --> IDX --> POI --> U

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class U client;
    class Q service;
    class IDX,POI datastore;