Geofencing — Notes

Functional

• Register polygonal / circular fences.
• Watch devices; emit ENTER / EXIT / DWELL events.
• Deliver via webhook / push / in-app.
• Per-device per-fence state.

Non-functional

• p99 event latency < 5 s.
• Tens of millions of fences; 100M+ devices.
• 99.9% delivery for webhooks.

Capacity

• Location updates: millions/s.
• Per update: index lookup + state update + maybe event emit.

Schema

• fences(id, owner, geometry, type, attrs)
• subscriptions(fence_id, target, type)
• device_state(device_id, fence_id, in_or_out, last_changed)

Trade-offs

• Server-side vs client-side geofencing: client saves bandwidth and battery (OS APIs); server centralizes business logic. Hybrid common.
• R-tree vs S2/H3 cells: cells parallelize beautifully across shards; R-tree better for small fence counts.
• At-most-once vs at-least-once with idempotency: usually at-least-once.

Refs

• Esri / PostGIS spatial indexing; Uber H3 docs; Mapbox / Radar geofencing platform docs; ByteByteGo "Design geofencing".