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However, as a recommended practice, the Attributes and Actions related to the interactions with an external thing (sensors, actuators, machines) are often defined in a reusable structure called ThingFacet, and combined into a ThingModel. This relationship was explained in the concept of Models and Model Facets. Application related information are often defined directly as attributes of the ThingModel.

Purpose of ThingModels:

• ThingModels are normally defined by users when interacting with Physical Things. The behavior of Physical Things are modeled via ThingFacet.
• ThingModels are driver for ThingFacets. ThingFacet (s) are reusable models and TQL CRUD operations can not be performed on ThingFacet. ThingFacet(s) are put to use only when combined within a ThingModel.
• ThingModels can contain application related information that are related to things.

Simplest form of defining a ThingModel is to provide a Name and Attributes. Example of a PhidgetServoModel ThingModel with one attribute PhidServoId In the following example, the ThingModel PhidgetServoModel combines the ThingFacet PhidgetServoFacet, where: 

• PhidgetServoFacet contains the device interaction specific Attributes (e.g. ServoProtocolURL) and Actions (e.g. PhidgetServoAction);
• PhidgetServoModel contains application related Attributes: PhidServoId and InstalledAtZone (assuming the Phidget servo motor is installed in a particular zone within a Greenhouse environment, which has multiple zones).
• Combines is a modifier of ThingModel

In the most simple case, a ThingModel may only have one single directly defined attribute of type Sid.

<ThingModel Name="PhidgetServoModel" Combines="PhidgetServoFacet">
	<Sid Name="PhidServoId"/>
	<String Name="InstalledAtZone"/>
</ThingModel>
<ThingFacet Name="PhidgetServoFacet">
	<String Name="ServoProtocolURL" Default="phid://" />
	<String Name="DeviceInterfaceIndex" />
	<String Name="PhidgetDeviceType" />
	<Integer Name="ServoAngle" KnownBy="PhidgetServoAction" />
	<Action Name="PhidgetServoAction" Documentation="Control servo motor">
		...
	</Action>
</ThingFacet>

This pattern allows the ThingFacet to become a reusable artifact, and potentially combined by multiple ThingModels.

Purpose of ThingModels:

• ThingModels are normally defined by users when interacting with Physical Things. The behavior of Physical Things are modeled via ThingFacet.
• ThingModels are driver for ThingFacets. ThingFacet (s) are reusable models and TQL CRUD operations can not be performed on ThingFacet. ThingFacet(s) are put to use only when combined within a ThingModel.
• ThingModels can contain application related information that are related to things.

ThingsFacets are used within ThingModels using Combines modifier. Example of a PhidgetServoModel ThingModel combining a PhidgetServoFacet ThingFacet

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