FIWARE - Orion Context Broker as Context Provider - fiware

Question

I'm having a hard time understanding how context providers work in the Orion Context Broker.
I followed the examples in the step-by-step guide written by Jason Fox. However, I still do not exactly get what happens in the background and how the context broker exactly creates the POST from the registration. Here is what I am trying to do:
I do have a WeatherStation that provides sensor data for a neighborhood.
{
"id": "urn:ngsi-ld:WeatherStation:001",
"type": "Device:WeatherStation",
"temperature": {
"type": "Number",
"value": 20.5,
"metadata": {}
},
"windspeed": {
"type": "Number",
"value": 60.0,
"metadata": {}
}
}
Now I like the WeatherStation to be a context provider for all buildings.
{
"id": "urn:ngsi-ld:building:001",
"type": "Building"
}
Here is the registration that I try to use.
{
"id": null,
"description": "Random Weather Conditions",
"provider": {
"http": {
"url": "http://localhost:1026/v2"
},
"supportedForwardingMode": "all"
},
"dataProvided": {
"entities": [
{
"id": "null",
"idPattern": ".*",
"type": "Building",
"typePattern": null
}
],
"attrs": [
"temperature",
"windspeed"
],
"expression": null
},
"status": "active",
"expires": null,
"forwardingInformation": null
}
The context broker accepts both entities and the registration without any error.
Since I have a multi-tenant setup I use one fiware_service for the complete neighborhood but every building would later have a seperate fiware_servicepath. Hence, the weatherstation has a different servicepath than the building. Although I also tried to put them both on the same path.
For now I used the same headers for all entities.
{
"fiware-service": "filip",
"fiware-servicepath": "/testing"
}
Here is the log of the context broker (version: 3.1.0):
INFO#2021-09-23T19:17:17.944Z logTracing.cpp[212]: Request forwarded (regId: 614cd2b511c25270060d873a): POST http://localhost:1026/v2/op/query, request payload (87 bytes): {"entities":[{"idPattern":".*","type":"Building"}],"attrs":["temperature","windspeed"]}, response payload (2 bytes): [], response code: 200
INFO#2021-09-23T19:17:17.944Z logTracing.cpp[130]: Request received: POST /v2/op/query?options=normalized%2Ccount&limit=1000, request payload (55 bytes): {"entities": [{"idPattern": ".*", "type": "Building"}]}, response code: 200
The log says that it receives the request and forwards it as expected. However, as I understand it this would simply point to the same building entity again. Hence, it is somehow a circular forwarding. I also cannot tell anything about the headers of the request.
I do not understand how the forwarded request from the building can actually query the weather station for information. When I query my building I still only receive the entity with no own properties:
{
"id": "urn:ngsi-ld:building:001",
"type": "Building"
}
I also tried to vary the url of the registration but with no success.
Is this scenario actually possible with the current implementation? It would be very useful
Is there any example for this including also the headers?
I know that I could simply use reference but that would put more work on the user.
Thanks for any help on this.

Answer 1

It is messy, but you could achieve this via a subscription. Hold the weather station as a separate entity in the context broker and poll or push updates into the entity. The subscription would fire whenever the data changes and make two NGSI requests:
Find all entities which have a Relationship servicedBy=WeatherStationX
Run an upsert on all entities to add a Property to each entity:
{
"temperature" : {
"type" : "Property",
"value" : 7,
"unitCode": "CEL",
"observedAt": "XXXXX",
"providedBy": "WeatherStation1"
}
}
Where observedAt comes either from the payload of the weather station or the notification timestamp.
Within the existing IoT Agents, provisioning the link attribute allows a device to propagate measures to a second entity (e.g. this Thermometer entity is measuring temperature for an associated Building entity)
{
"entity_type": "Device",
"resource": "/iot/d",
"protocol": "PDI-IoTA-UltraLight",
..etc
"attributes": [
{"object_id": "l", "name": "temperature", "type":"Float",
"metadata":{
"unitCode":{"type": "Text", "value" :"CEL"}
}
}
],
"static_attributes": [
{
"name": "controlledAsset",
"type": "Relationship",
"value": "urn:ngsi-ld:Building:001",
"link": {
"attributes": ["temperature"],
"name": "providedBy",
"type": "Building"
}
}
]
}
At the moment the logic just links direct one-to-one, but it would be possible to raise a PR to check for an Array and update multiple entities in an upsert - the relevant section of code is here