Designing a Geofencing System

Geofencing detects when a device crosses the boundary of a defined geographic zone and triggers actions accordingly.

Use Cases

Marketing:      Send coupon when user enters store area
Logistics:      Alert when truck leaves delivery zone
Parental:       Notify when child leaves school zone
Fraud:          Alert when card used outside home region
Field service:  Track technician arrival at job site
Smart home:     Turn on lights when owner approaches

Geofence Types

Circle:   Center point + radius (simple, common)
Polygon:  Custom shape (precise building footprint)
Route:    Corridor along a path (logistics)
Grid:     H3 hexagonal cells (analytics)

Architecture

Device Location Updates (every 5-30 sec)
          ↓
  Location Ingestion (Kafka)
          ↓
  Geofence Evaluation Service
  ├── Load applicable fences
  ├── Point-in-polygon check
  └── State change detection
          ↓
  Event: ENTER / EXIT / DWELL
          ↓
  Action Service (notification, webhook)

Spatial Index

Efficient zone lookup — don't check every zone for every update:

Approach 1: R-Tree index (PostGIS)
  geofence table indexed spatially
  SELECT * FROM geofences
  WHERE ST_Contains(zone, ST_Point(:lng, :lat))

Approach 2: H3 hexagonal grid
  Each zone: list of H3 cells it covers
  Device at H3 cell C? → Query zones that include C
  → Simple hash lookup, very fast

State Tracking

Track per (device_id, fence_id):
  OUTSIDE → (enter) → INSIDE
  INSIDE  → (exit)  → OUTSIDE
  INSIDE  → (dwell after 5min) → DWELL event

Store state in Redis:
  geofence:state:{device_id} → { fence_123: INSIDE, ... }

Point-in-Polygon (Ray Casting)

For each geofence polygon:
  Cast a ray from point eastward
  Count how many edges it crosses
  Odd crossings → INSIDE
  Even crossings → OUTSIDE

Optimization: Bounding box pre-check before full algorithm

Event Deduplication

Prevents spam:
  ENTER event fires once until EXIT detected
  DWELL fires once per dwell_period

Idempotency key: {device_id}:{fence_id}:{event_type}:{date}

Conclusion

Geofencing combines spatial indexing (H3/R-tree), stateful enter/exit tracking, and event-driven alerting. H3 cell-based lookup provides sub-millisecond fence matching at scale.