data-models

NGSI-LD FAQ

This FAQ compilation is intended to clarify the NGSI-LD specification by providing answers to common questions.

A tutorial that can complement this FAQ can be found at https://github.com/FIWARE/tutorials.Linked-Data.

Q: What implementations of NGSI-LD are available?

• Orion-LD: https://github.com/Fiware/context.Orion-LD
• Scorpio: https://github.com/ScorpioBroker/ScorpioBroker
• Djane: https://github.com/sensinov/djane/
• Stellio: https://github.com/stellio-hub/stellio-context-broker

Q: What are the main (essential) differences between NGSI v2 and NGSI-LD?

In summary, the main differences are the following:

• The underlying Data Model is the Property Graph Data Model.
• Entity IDs shall be URIs (URLs or URNs).
• The metadata dictionary disappears. Metadata are represented by nested Properties of Properties.
• There is some “metadata” standardised (unitCode for expressing units, observedAt for expressing timestamps, …)
• There is a new type of Attribute Relationship intended to link one Entity to another Entity. That is done through the object member.
• Geospatial properties are represented using the Attribute type GeoProperty.
• The type of Attributes can only be Property, Relationship or GeoProperty.
• A JSON-LD @context (a hash map used to map member names to URIs) can be added to Entities to provide Fully Qualified Names (URIs) associated to terms. That is somewhat “similar” to the concept of XML namespaces.
• Overall the REST API is quite similar (even simpler) than NGSI v2, although subscription and registration payloads change a bit (but they are the same in essence).

Q: Could you give me some examples of NGSI-LD payloads?

{
    "id": "urn:ngsi-ld:AirQualityObserved:RZ:Obsv4567",
    "type": "AirQualityObserved",
    "dateObserved": {
        "type": "Property",
        "value": {
            "@type": "DateTime",
            "@value": "2018-08-07T12:00:00Z"
        }
    },
    "NO2": {
        "type": "Property",
        "value": 22,
        "unitCode": "GP",
        "accuracy": {
            "type": "Property",
            "value": 0.95
        }
    },
    "refPointOfInterest": {
        "type": "Relationship",
        "object": "urn:ngsi-ld:PointOfInterest:RZ:MainSquare"
    },
    "@context": [
        "https://schema.lab.fiware.org/ld/context",
        "https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context.jsonld"
    ]
}

For each FIWARE Data Model there is an example Entity encoding it in NGSI-LD. For instance, here

Q: Could you give me some examples of a JSON-LD @context?

You can find an example here.

Q: What URI should I use to reference the FIWARE Data Models @context?

The sources of truth for the FIWARE Data Models @context are https://fiware.github.io/data-models/context.jsonld and https://fiware.github.io/data-models/full-context.jsonld. The latter includes both the FIWARE Data Models @context and the Core @context.

The full @context is also available at https://schema.lab.fiware.org/ld/context

When it comes to referencing the FIWARE Data Models @context in your NGSI-LD Entities, there are three different options:

• Use the FIWARE Lab URI https://schema.lab.fiware.org/ld/context.
• Use the GitHub URI https://fiware.github.io/data-models/full-context.jsonld.
• Host a copy of the Data Models @context file within your own premises and use a URI referencing a local server

The main advantage of the first method, is that the FIWARE Lab URI is convenient, as it is shorter and cleaner, however it does depend on a redirection made by the FIWARE Lab Infrastructure, which does not have 100% availability guaranteed. The GitHub URI is longer, but in general GitHub has higher guaranteed availability than FIWARE Lab. The self-hosting method, ensures that the @context will only be updated when you chose to do and an on-premise URI can help to solve issues faced by applications that are deployed behind firewalls, however your local copy of the @context file will not include recent changes made to the standard models defined by the @context.

Q: What is a Property of a Property / Relationship and all the combinations?

It is similar to NGSI v2 metadata. In NGSIv2, in the example above, the Property accuracy would have been represented as a member of the metadata dictionary.

Q: But, Property and Relationship can be arbitrarily nested?

Yes, but only one or two nesting levels could make sense in a real world scenario.

Q: What is observedAt?

It is a “timestamp” associated to a Property or Relationship. See the example below. In NGSI v2 it is usually specified using the timestamp metadata attribute.

Remember that in NGSI-LD timestamps must always be expressed using UTC i.e. a trailing ‘Z’ must always be present.

{
    "id": "urn:ngsi-ld:WasteContainer:RZ:Obsv4567",
    "type": "WasteContainer",
    "fillingLevel": {
        "type": "Property",
        "value": 0.85,
        "observedAt": "2017-02-07T16:00:00Z"
    },
    "location": {
        "type": "GeoProperty",
        "value": {
            "type": "Point",
            "coordinates": [-2, 35]
        }
    },
    "@context": [
        "https://schema.lab.fiware.org/ld/context",
        "http://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context.jsonld"
    ]
}

Q: How geo-location is represented?

See the example above. In essence an Attribute of type GeoProperty plus a GeoJSON Geometry value.

Q: How DateTime is represented (e.g. timestamps, dates, time)?

{
    "id": "urn:ngsi-ld:WeatherObserved:RZ:Obsv4567",
    "type": "WeatherObserved",
    "dateObserved": {
        "type": "Property",
        "value": {
            "@type": "DateTime",
            "@value": "2018-08-07T12:00:00Z"
        }
    },
    "temperature": {
        "type": "Property",
        "value": 22
    },
    "@context": [
        "https://schema.lab.fiware.org/ld/context",
        "http://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context.jsonld"
    ]
}

Q: Is application/json a supported MIME type?

Yes. However, when using it the LD @context has to be externally provided, or no JSON-LD @context at all. In the latter case Entities will be under the Default @context. You can see an example here

Q: What happens if I only use application/json content without worrying about the @context member?

Nothing, i.e. if you are working in your own application and your data model is somewhat “private” that is perfectly OK. It is somewhat similar as using XML content without namespaces.

However, we recommend to use JSON-LD @context and that can be easily abstracted out by a convenience library.

Q: What is the JSON-LD Link header?

It is a standard HTTP Link Header intended to provide a @context in two scenarios:

• when application/json is used as MIME type.
• in GET and DELETE operations to specify what is the @context to be used for mapping types or attribute names to Fully Qualified Names (URIs).

Q: Could you put an example of a JSON-LD HTTP Link Header?

For instance, the Link header to address the FIWARE Data Models would be:

Link: <https://schema.lab.fiware.org/ld/context>; rel="http://www.w3.org/ns/json-ld#context"; type="application/ld+json"

Please note that only one JSON-LD Link header is allowed per HTTP Request

Q: Is the @context mandatory?

For JSON-LD content (application/ld+json), yes it shall accompany each Entity payload as a @context member. For JSON content (application/json) it can only be specified through the JSON-LD HTTP Link header.

Please note that only one JSON-LD Link header is allowed per HTTP Request

Q: If I have a @context which references multiple URIs how can I reference it through the HTTP Link header?

As the Link header can only reference one JSON-LD @context it is necessary to create a wrapper @context.

Below you can see an example of JSON-LD @context wrapping, in which FIWARE Data Models and schema.org @context are put together.

{
   "@context": [
      "http://schema.lab.fiware.org/ld/context",
      "http://schema.org"
   ]
}

If you set up an endpoint URI to serve the content above (serving it with MIME type application/ld+json) then you can reference it from a HTTP Link header. Please note that in many cases that would not be necessary as , for instance, the FIWARE Data Models @context already contains the proper references to schema.org.

Q: What happens if an Entity ID is a URL and I use it in a resource like /entities/{entityId}?

Nothing. Entity IDs have to be percent encoded as mandated by IETF specifications.

Q: What is the Core @context?

It is the JSON-LD @context where all the NGSI-LD API Core terms are defined. It can be found at https://uri.etsi.org/ngsi-ld/v1/ngsi-ld-core-context.jsonld

The Core @context terms cannot be overwritten by applications

Q: What is the Default @context?

Actually, the role of Default @contextis played by the Core @context itself, which does include a default @vocab rule to map unknown terms (i.e. those for which no correspondance is found in the user @context) to a default URI.

Q: Do I always need to provide the Core @context when invoking API operations?

It is not necessary. The Core @context is always implicit when processing API requests. However, when generating API responses the Core @context is always included to facilitate the work of JSON-LD processors that might be upstream.

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