[Question] @context and "name" attribute

[EliottPaillard]

Hello.
I'm using Stellio 2.13.0

I POST this entity :
{
"id": "urn:ngsi-ld:Building:001",
"type": "Building",
"name": {
"type": "Property",
"value": "Eiffel Tower"
},
"subCategory": {
"type": "Property",
"value": "tourism"
},
"airQualityLevel": {
"type": "Property",
"value": 4,
"unitCode": "C62",
"observedAt": "2020-09-09T16:40:00.000Z"
},
"almostFull": {
"type": "Property",
"value": false
},
"@context": [
"https://smartdatamodels.org/context.jsonld",
"https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context-v1.7.jsonld"
]
}
Then when I want to retrieve it using:
GET -H "Accept: application/ld+json"

The "name" element become "ngsi-ld:name".
However none of user or core context have such an extended syntax. I thought it would be instead "https://uri.etsi.org/ngsi-ld/name" as it is in the provided user context. Why is this not the case? I did not find why in the API Specifications.

Also I expected the "@context" element to be like the provided one. Instead I have "https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context-v1.8.jsonld". I believe this is related to Stellio and its version and not related to the context provided when the entity was created but I found it kinda curious actually. Again I did not find anything relevant about this behaviour in the specs but the provided examples with the same Accept header show the "@context" element as an array with an user and a core context.

At the same time, speaking about the "ngsi-ld:name" appearance, I experimented the aggregated methods for the temporal evolutions of entities. And the responses display them as "ngsi-ld:avg" or "ngsi-ld:max". I found it curious as well, and again I did not find in the specs apart one example showing "max" and "avg" in the response. Is this a normal behaviour though?

I hope I am understandable so you can enlighten me.
Thanks in advance

[bobeal]

Hi @EliottPaillard,

For the 1st point (name becoming ngsi-ld:name):

• from https://smartdatamodels.org/context.jsonld, name is expanded to https://uri.etsi.org/ngsi-ld/name at entity creation time
• when you get the entity without specifying any context, only the default core context is taken into account (https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context-v1.8.jsonld for Stellio)
• in the core context, there is a "ngsi-ld": "https://uri.etsi.org/ngsi-ld/" clause, so https://uri.etsi.org/ngsi-ld/name is compacted to ngsi-ld:name
• please note that this is JSON-LD expansion / compaction behavior, so not something specific to NGSI-LD specification

For the 2nd point (provided "@context" point):

• as described in 5.5.7: "For the avoidance of doubt, the @context used to perform compaction or expansion of terms shall be the one provided by each API call (or the default @context in its absence), and not any other @context which might have been supplied previously. For instance, when performing "Query Entity" operations (clause 5.7.2), the @context used to perform URI expansion and compaction shall be the one provided by the request.".
• so if you do a GET operation without specifying any specific JSON-LD context, the context broker will use its default core context (https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context-v1.8.jsonld for Stellio)
• and the context used at entity creation time is always discarded after expansion has been done

For the 3rd point (ngsi-ld:avg):

• can you give me the request you are doing (with the request headers)?
• are you doing it on the same entity provided in the issue?

[EliottPaillard]

Hello @bobeal,

About point 1 and 2 I understand well now, thank you for this!

Regarding point 3 :
I create this entity :
{
"id": "urn:ngsi-ld:Vehicle:temporalEvolution",
"type": "Vehicle",
"speed": [
{
"type": "Property",
"value": 120,
"observedAt": "2020-08-01T12:03:00Z"
},
{
"type": "Property",
"value": 80,
"observedAt": "2020-08-01T12:05:00Z"
},
{
"type": "Property",
"value": 100,
"observedAt": "2020-08-01T12:07:00Z"
}
],
"batteryLevel": [
{
"type": "Property",
"value": 67,
"observedAt": "2020-08-01T12:03:00Z"
},
{
"type": "Property",
"value": 53,
"observedAt": "2020-08-01T13:05:00Z"
},
{
"type": "Property",
"value": 40,
"observedAt": "2020-08-01T14:07:00Z"
}
],
"@context": [
"https://smartdatamodels.org/context.jsonld",
"https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context-v1.6.jsonld"
]
}

And I do this request :
curl -iX GET 'http://localhost:1080/ngsi-ld/v1/temporal/entities/urn:ngsi-ld:Vehicle:temporalEvolution/?aggrMethods=avg,max&aggrPeriodDuration=PT1H&options=aggregatedValues' -H 'Accept: application/json' -H 'Link: https://smartdatamodels.org/context.jsonld; rel="http://www.w3.org/ns/json-ld#context"; type="application/ld+json"'

And the response is:
{
"id": "urn:ngsi-ld:Vehicle:temporalEvolution",
"type": "Vehicle",
"batteryLevel": {
"type": "Property",
"ngsi-ld:avg": {
"@list": [
{
"@list": [
67.0,
"2020-08-01T12:03:00Z",
"2020-08-01T13:03:00Z"
]
},
{
"@list": [
53.0,
"2020-08-01T13:03:00Z",
"2020-08-01T14:03:00Z"
]
},
{
"@list": [
40.0,
"2020-08-01T14:03:00Z",
"2020-08-01T15:03:00Z"
]
}
]
},
"ngsi-ld:max": {
"@list": [
{
"@list": [
67.0,
"2020-08-01T12:03:00Z",
"2020-08-01T13:03:00Z"
]
},
{
"@list": [
53.0,
"2020-08-01T13:03:00Z",
"2020-08-01T14:03:00Z"
]
},
{
"@list": [
40.0,
"2020-08-01T14:03:00Z",
"2020-08-01T15:03:00Z"
]
}
]
}
},
"speed": {
"type": "Property",
"ngsi-ld:avg": {
"@list": [
{
"@list": [
100.0,
"2020-08-01T12:03:00Z",
"2020-08-01T13:03:00Z"
]
}
]
},
"ngsi-ld:max": {
"@list": [
{
"@list": [
120.0,
"2020-08-01T12:03:00Z",
"2020-08-01T13:03:00Z"
]
}
]
}
}
}

[bobeal]

Hi @EliottPaillard,

The provided context (https://smartdatamodels.org/context.jsonld) contains pieces of the NGSI-LD core context (look for all the terms starting with http://uri.etsi.org/ngsi-ld/).

When Stellio receives the query, it looks at the provided context and thinks the NGSI-LD core context is already included in it since it finds many terms from the NGSI-LD core context. So it does not add it at the end as it should do when the core context is not provided and the JSON-LD compaction is not what you are expecting since some important definitions are missing. For instance, this one:

  "avg": {
      "@id": "ngsi-ld:avg",
      "@container": "@list"
  }

These contexts including (often outdated) pieces from the core context are really dangerous :(

I'll think of a way we can better handle this in Stellio...

[EliottPaillard]

Hey,

I tried to manage this context problem by removing the incriminated elements.
However, I met again the 503 code error I was already talking about in #1162
But this time I made the context available through a github page as you recommended.
I don't understand why the 503 appears again.

For example while trying to create this entity:
{
"id": "urn:ngsi-ld:Building:002",
"type": "Building",
"name": {
"type": "Property",
"value": "Eiffel Tower"
},
"subCategory": {
"type": "Property",
"value": "tourism"
},
"location": {
"type": "GeoProperty",
"value": {
"type": "Point",
"coordinates": [
13.3986,
52.5547
]
}
},
"@context": "https://raw.githubusercontent.com/EliottPaillard/context/main/test.jsonld"
}

[bobeal]

Hi,

The link you are using is not pointing to a GitHub page.

For instance, for this raw context file that we have in one of our repository: https://raw.githubusercontent.com/easy-global-market/ngsild-api-data-models/master/authorization/jsonld-contexts/authorization-compound.jsonld, the GitHub page version is: https://easy-global-market.github.io/ngsild-api-data-models/authorization/jsonld-contexts/authorization-compound.jsonld

Tried to find it for your context but https://eliottpaillard.github.io/context/test.jsonld is giving me a 404.