Extended relation information for Semantic Definition Format (SDF)

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Table of Contents

1. Introduction

The Semantic Definition Format (SDF) [SDF] is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things. The SDF specification defines a generic data model that can be used as a meta model when converting between other data models, such as IPSO Smart Objects or Digital Twins Definition Language (DTDL) [DTDL]. SDF model defines a set of affordances, describing the interfaces for the Object. These can be mapped to corresponding affordances in other data models.¶

The base specification defines ways to represent parent-child relations between two definitions. However, sometimes there is a need to describe also more complex relations to support arbitrary connections between definitions and also referring to definitions outside of the SDF models. These could be, for example, defining possible location of a device inside a room, how a device is controlled by another device, or physical topology between devices. This enables defining more complex systems using SDF models.¶

The basic parent-child relations between SDF Objects and Things can be defined by including a definition of a child in the definition of the parent. This covers a large share of simple data models defining, e.g., simple sensors, or more complex devices containing a set of sensors. On the other hand, SDF can be used also to describe even more complex entities, such as buildings with rooms and other related objects inside a building. When we extend the SDF usage, the simple parent-child relation is often not enough, but more complex relations may be needed to describe the connections between the definitions. These relations can be for example physical (e.g., an object is inside another object), functional (e.g., an object can control another object), or semantic (e.g., an object is similar to a term defined in another ontology).¶

This document extends the base SDF specification by adding a new keyword to describe also other relations between physical or logical objects than plain parent-child relations. This new keyword is needed to describe, without loss of information, models from ecosystems that are using complex relation information in their definitions.¶

This extension enables describing relations from SDF models to various (SDF or other) definitions. For a link data type for affordances, e.g., for a link property that can be accesses and modified during runtime, the "sdfType for links" extension [I-D.bormann-asdf-sdftype-link] can be used instead.¶

NOTE: This extension is now defined based on the Relationships feature in the DTDL specification. There may be other kind of definitions for relationships in other data models that must be taken into account and this specification may need to be extended to cover also those requirements.¶

2. Terminology

This specification uses the terminology specified in [SDF], in particular "Class Name Keyword", "Object", and "Affordance".¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. SDF Relation Extension

This section defines a new SDF Class Name Keyword, sdfRelation, that can be used to describe complex relations. The relationship definitions are on class-level, i.e., the sdfRelation keyword does not describe instance specific information about the relation, but describes how different models and definitions relate to each other.¶

{
  "namespace": {
    "saref": "https://saref.etsi.org/saref4bldg/v1.1.2/"
  }
}

  "namespace": {
    "exont": "https://example.com/relationOntology",
    "saref": "https://saref.etsi.org/core/v3.1.1/"
  },
  sdfObject: {
    "temperature": {
      "description": "Example temperature object",
      "sdfProperty": {
        ...
      },
      "sdfRelation": {
        "sameAs": {
          "relType": "exont:same-as",
          "target": "saref:TemperatureSensor"
        }
      }
    }
    ...

  "namespace": {
    "exont": "https://example.com/relationOntology"
  },
  sdfObject: {
    "first-object": {
      "description": "Example object",
      "sdfProperty": {
        "adjacent-node": {
          "type": "object",
          "sdfType": "link"
        }
      },
      "sdfRelation": {
        "next-to": {
          "description": "This object is adjacent to the second
                          object",
          "relType": "exont:next-to",
          "target": "#/sdfObject/second-object"
        }
      }
    },
    "second-object": {
      "description": "Example object, next to the first object",
      "sdfProperty": {
        "adjacent-node": {
          "type": "object",
          "sdfType": "link"
        }
        ...
      },
      "sdfRelation": {
        "next-to": {
          "relType": "exont:next-to",
          "target": "#/sdfObject/first-object"
        }
      }
    }
  }

4. DTDL - SDF conversion

This section (to be removed) discusses the mapping between SDF and DTDL qualities.¶

...
"sdfRelation": {
  "name-from-DTDL": {
    ...
  }
}

5. Security Considerations

TODO Security¶

6. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

The author wants to thank Ari Keranen, Mikko Saarisalo, and Christer Holmberg for their feedback and comments.¶

Author's Address