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Externals Model (Linking UDMI to Other Ontologies)

The externals block within a UDMI device’s metadata.json file provides a standard mechanism to link the device to external ontologies or building information models. This allows UDMI to integrate with other industry-standard semantic models without needing to duplicate all their structural data.

Each entry within the externals block corresponds to a specific ontology or schema (e.g., dbo, haystack, brick, bim), and typically provides properties like ext_id (the external identifier) and type (the external class/type).

Below are conceptual examples of how different external models could be linked to a UDMI device using the externals block.

Digital Buildings Ontology (DBO)

Google’s Digital Buildings Ontology defines building assets. In DBO, the type refers to a specific equipment class, and ext_id is a UUID identifying the entity.

"externals": {
  "dbo": {
    "ext_id": "c773b86d-b0c0-46fc-bd3f-d726fadd5f1e",
    "type": "HVAC/VAV_SD_DSP"
  }
}

Project Haystack

Project Haystack is a semantic modeling ontology based on tags. In Haystack, entities are typically identified by an opaque Ref (reference) string, and their type is implicitly defined by their marker tags (e.g., vav, equip).

"externals": {
  "haystack": {
    "ext_id": "2180b666-7032054c",
    "type": "vav equip"
  }
}

Note: The ext_id here represents the Haystack Ref (without the leading @). The type might contain a space-separated list of the primary marker tags identifying the equipment type.

Brick Schema

Brick Schema is an open-source ontology for building assets built on RDF/OWL. In Brick, entities are nodes in a graph identified by URIs, and their types are Brick Classes.

"externals": {
  "brick": {
    "ext_id": "bldg:VAV-1",
    "type": "brick:VAV"
  }
}

Note: The ext_id is typically the entity URI or a namespaced local name (e.g. bldg:VAV-1). The type refers to a class in the Brick ontology.

BIM / IFC (Industry Foundation Classes)

Building Information Modeling (BIM) often uses the IFC open standard to exchange data. In IFC, objects are uniquely identified by an IFC GlobalId (a 22-character Base64 encoded string) and categorized by IFC Classes.

"externals": {
  "bim": {
    "ext_id": "1K1$W$h3v0$Q1mO6O$uR5R",
    "type": "IfcUnitaryEquipment"

  }
}

Note: The ext_id is the 22-character IFC GlobalId. The type is the specific IFC class of the equipment.

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