Geofencing Service — Simple

Problem statement (interviewer prompt)

Design a geofencing service: register polygonal fences for millions of users / vehicles. Stream location updates and emit ENTER / EXIT / DWELL events to webhook + push consumers. Avoid event spam at fence boundaries (hysteresis) and tolerate noisy GPS.

flowchart LR
  D([Device])
  LOC[Location Update]
  GIDX[(Geofence Index<br/>R-tree / S2)]
  EVT([Event Generator])
  HOOK[Webhooks / Push]
  D --> LOC --> GIDX --> EVT --> HOOK

    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 D client;
    class LOC,HOOK service;
    class GIDX datastore;
    class EVT compute;