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Overriding extension

Version 1

Usable in DTDL version 3

Overriding is a DTDL language feature that enables a model property of a Property or a Telemetry to be overridden by a value in an instance of the model. If a service supports the Overriding extension, it recognizes and understands the Override adjunct type and its overrides property if the Overriding context is specified. Overriding may be used only in conjunction with the Annotation feature extension.

Overriding context

The context specifier for version 1 of the Overriding extension is “dtmi:dtdl:extension:overriding;1”.

Override adjunct type

The Override adjunct type can co-type a Property in DTDL version 3, but only if the Property is also co-typed ValueAnnotation.

The chart below lists the additional properties that may be part of an element that is co-typed Override.

Property	Required	Data type	Limits	Description
overrides	required	IRI	must match an extension property of the sibling element identified by the annotates property	Term or IRI of an extension property that is overridden.

When a Property in a model is co-typed ValueAnnotation and Override, the value of the annotates property specifies the name of another Property or Telemetry within the contents of the same Interface, and the value of the overrides property specifies an extension property on this other Property or Telemetry. This indicates that the ValueAnnotation/Override should be regarded as metadata for the element referenced by annotates, and further that this metadata overrides the value of the extension property indicated by overrides.

Override examples

The following example shows an Interface with two contents elements. The element named “currentTemp” has type Telemetry and co-type Temperature; this element provides a stream of temperature readings from the Sensor. The semantic type Temperature is defined in the QuantitativeTypes feature extension.

The element named “currentTempUnit” has type Property and co-types ValueAnnotation and Override; this element provides a metadata value that overrides the unit property on the element named “currentTemp”. The adjunct type ValueAnnotation indicates that the Property provides metadata, and the value of property annotates indicates that the metadata applies to the element named “currentTemp”. The adjunct type Override indicates that the Property overrides a value of the annotated element, and the value of property overrides indicates that the overridden property is unit.

The schema of “currentTempUnit” is TemperatureUnit, which — like the Temperature type — is defined in the QuantitativeTypes feature extension. Because “currentTempUnit” is overriding the unit property of “currentTemp”, which has co-type Temperature, the schema of “currentTempUnit” must be approriate for the Temperature type. The table in the QuantitativeTypes documentation indicates which unit type (e.g., TemperatureUnit) corresponds to each semantic type (e.g., Temperature). This table also indicates the allowed unit values that can be specified in an instance of the model when overriding the unit property of the annotated element.

Note that the Overriding extension is not dependent upon the QuantitativeTypes extension, although all current use cases for Overriding involve QuantitativeTypes. The Overriding extension enables any property defined by any extension to be overridden by a Property. The schema of the overriding Property must match the schema type of the overridden property, just as the schema of “currentTempUnit” matches the schema type of the “currentTemp” unit property in the following example.

{
  "@context": [
      "dtmi:dtdl:context;3",
      "dtmi:dtdl:extension:quantitativeTypes;1",
      "dtmi:dtdl:extension:annotation;1",
      "dtmi:dtdl:extension:overriding;1"
  ],
  "@id": "dtmi:com:example:Sensor;1",
  "@type": "Interface",
  "contents": [
    {
      "@type": [ "Telemetry", "Temperature" ],
      "name": "currentTemp",
      "schema": "double",
      "unit": "degreeFahrenheit"
    },
    {
      "@type": [ "Property", "ValueAnnotation", "Override" ],
      "name": "currentTempUnit",
      "annotates": "currentTemp",
      "overrides": "unit",
      "schema": "TemperatureUnit",
      "writable": true
      }
  ]
}

To override the “currentTemp” unit property, an instance of this model specifies a value for “currentTempUnit”. The allowed values are indicated by the table in the QuantitativeTypes documentation. The row for semantic type Temperature lists the unit values that may be specified in the instance, which are the same values that may be specified for the unit property in the model. The instance might, for example, contain the following content:

{
  "currentTempUnit": "degreeCelsius"
}

The presence of a “currentTempUnit” value in the instance indicates that the value “degreeFahrenheit” specified by the model’s unit property no longer applies. Instead, the value for the unit of “currentTemp” is “degreeCelsius” because this is specified in the instance, and the instance value overrides the model value.

As the following example shows, the Property or Telemetry whose property is overridden need not be defined directly in the same Interface as the Override. When an Interface extends another Interface, the former can override a property of a contents element that is defined in the latter, because the extends property imports the Property or Telemetry into the former’s contents.

[
  {
    "@context": [
      "dtmi:dtdl:context;3",
      "dtmi:dtdl:extension:quantitativeTypes;1"
    ],
    "@id": "dtmi:com:example:BaseSensor;1",
    "@type": "Interface",
    "contents": [
      {
        "@type": [ "Telemetry", "Temperature" ],
        "name": "currentTemp",
        "schema": "double",
        "unit": "degreeFahrenheit"
      }
    ]
  },
  {
    "@context": [
        "dtmi:dtdl:context;3",
        "dtmi:dtdl:extension:quantitativeTypes;1",
        "dtmi:dtdl:extension:annotation;1",
        "dtmi:dtdl:extension:overriding;1"
    ],
    "@id": "dtmi:com:example:Sensor;1",
    "@type": "Interface",
    "extends": "dtmi:com:example:BaseSensor;1",
    "contents": [
      {
        "@type": [ "Property", "ValueAnnotation", "Override" ],
        "name": "currentTempUnit",
        "annotates": "currentTemp",
        "overrides": "unit",
        "schema": "TemperatureUnit",
        "writable": true
      }
    ]
  }
]

Feature versions

The chart below lists the versions of the Overriding extension that are currently available.

Extension	Context	DTDL versions
Overriding v1	dtmi:dtdl:extension:overriding;1	3

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