how_to_track_moving_objects.md•5.5 kB
# How to Track Moving Objects with PubNub
PubNub is frequently used for real-time location tracking applications, such as delivery services, asset tracking, ride-sharing apps, and collaborative mapping. While PubNub provides the real-time messaging infrastructure, building a robust tracking system involves several considerations.
## Core Mechanism: Publishing Location Updates
The basic principle is straightforward:
1. **Object (Publisher):** The moving object (e.g., a vehicle with a GPS device, a mobile app user) periodically determines its current geographic coordinates (latitude, longitude) and other relevant data (e.g., speed, heading, timestamp, object ID).
2. **Publish to PubNub:** The object publishes this location data as a message to a specific PubNub channel.
* The channel could be specific to the object (e.g., `vehicle_location:<vehicleID>`) or a more general channel if many objects are being tracked for a common view.
* **Message Payload Example:**
```json
{
"objectId": "vehicle_123",
"lat": 34.0522,
"lng": -118.2437,
"speed": 45.5, // km/h
"heading": 90, // degrees
"timestamp": 1617000000000 // milliseconds UTC
}
```
3. **Viewers (Subscribers):** Client applications (e.g., a map display in a web or mobile app) subscribe to the relevant PubNub channel(s) to receive these location updates in real-time.
4. **Display Updates:** Upon receiving a new location message, the subscribing client updates the object's marker on a map or its position in a list.
## Key Considerations for Robust Object Tracking
While PubNub ensures fast message delivery, real-world conditions like network latency and potential message loss (though rare with TCP) mean you should build resilience into your tracking logic.
1. **Message Ordering:**
* PubNub messages published on a single channel are time-ordered by their publish timetoken. When you retrieve history or receive messages in a subscribe loop, they generally arrive in the order they were published.
* **Client-Side Timestamp:** Always include a client-generated timestamp in your location payload. This is crucial because network latency can cause messages to arrive at the subscriber slightly out of order relative to when they were *sent*, even if PubNub delivers them in order of *receipt*. The client timestamp allows subscribers to re-sequence or correctly interpret updates.
2. **Handling Potential Message Gaps (Interpolation):**
* Network issues might cause a publisher to be temporarily unable to send updates, or a subscriber to miss a few updates.
* **Embed a Counter:** Include a sequence number or counter in each message from a specific object. Subscribers can check this counter to detect if any messages were missed.
* **Interpolation:** If a gap is detected or if updates arrive infrequently, subscribers can interpolate the object's position between two known location updates. This involves estimating the path and speed based on the last known points and timestamps to create a smoother visual movement on the map, rather than having the marker jump abruptly.
* **Dead Reckoning / Physics Model:** For more advanced scenarios (common in gaming), you can use a simple physics model on the client side. Based on the last known velocity (speed and heading), the client can predict the object's next position. When a new update arrives, it corrects this prediction. This helps mask latency and provides smoother animation.
3. **Update Frequency vs. Bandwidth/Battery:**
* **Higher Frequency:** More frequent updates provide more accurate, real-time tracking but consume more bandwidth (for the publisher and subscribers) and more battery (for mobile publishers).
* **Lower Frequency:** Less frequent updates save bandwidth and battery but can result in less smooth movement on the map (requiring more interpolation).
* **Adaptive Frequency:** Consider an adaptive approach. For example, publish more frequently when the object is moving fast or changing direction, and less frequently when it's stationary or moving slowly.
4. **Channel Design:**
* **Individual Channels:** `object_location:<objectID>`. Pros: Easy for a specific viewer to track a specific object. Cons: Many channels if tracking many objects for a central dashboard.
* **Group Channels:** `region_updates:<regionID>` or `fleet_updates:<fleetID>`. Pros: Fewer channels for a dashboard to subscribe to if it needs a broad view. Cons: Higher message volume on these channels; clients will need to filter for objects they care about.
* **Channel Groups/Wildcard:** Can be used to efficiently subscribe to multiple individual object channels.
5. **Historical Tracking (Breadcrumbs):**
* Enable PubNub Message Persistence on your location channels.
* Subscribers can then use the History API to fetch past location data to draw a "breadcrumb" trail of the object's path.
6. **Geofencing and Proximity Alerts:**
* Use PubNub Functions to process incoming location data. A Function can check if an object enters or exits predefined geographical areas (geofences) and then publish alerts or trigger other actions.
PubNub provides the real-time messaging layer. The sophistication of your object tracking (smoothness, handling of network issues, historical data) depends on the logic you build into your publishing and subscribing clients.