Thing Description

The WoT Thing Description defines 3 kinds of Interaction Affordances, named Property, Action and Event:

Property

An Interaction Affordance that exposes state of an IoT device. This state can then be retrieved (read) and optionally updated (write). Devices can also choose to make Properties observable by pushing the new state after a change.

Action

An Interaction Affordance that allows to invoke a function of an IoT device, which manipulates state (e.g., toggling a lamp on or off) or triggers a process on the device (e.g., dim a lamp over time).

Event

An Interaction Affordance that describes an event source, which asynchronously pushes event data to the subscribers of the event (e.g., overheating alerts).

Components and standard technologies

In general, the Thing Description is designed to reuse and rely on established Internet and Web standards, this includes:

• Serialization: The JSON serialization of the TD information model is aligned with the syntax of JSON-LD 1.1.^[7] JSON-LD opens the opportunity of context extension and enrich the Thing Description instances with additional (e.g., domain-specific) semantics such as from Schema.org. This also supports the extraction and the understanding of relevant information such as in discovery scenarios in IoT applications^[8]
• Security: The Thing Description comes with a framework that allows to provide metadata to follow existing security scheme such as OAuth2 or Digest Access Authentication.^[9] Due to the context extension concept of the Thing Description specific security schemes can be imported.
• Linking: Things may have references to other Things or like to link to further information. For defining such kind of relations the Thing Description relies on the well know Web Linking^[10] concept.
• Interactions: The offered data and/or functions of a Thing are assigned to the corresponding interaction affordance Properties, Actions, and Events.
• Data Schema: For modeling the Thing's data that can be exchanged, the concept of JSON schema^[11] is embedded in the Thing Description.
• Bindings: The binding templates define the specific protocol and serialization method for the properties, actions, and events of the Thing. As protocol identification and addressing method URI Identifier^[12] are used. For announcing the serialization encoding (e.g., JSON, CBOR, XML, Efficient XML Interchange) of the payload that is specified by the data schema, Media Type^[13] assignment is applied in the Thing Description.

Thing Description examples

Below is an example TD serialized in JSON-LD format, which has one property, one action and one event. The IoT device represented by this TD uses the HTTP protocol but a TD can represent any protocol with a

{
    "@context": "https://www.w3.org/2019/wot/td/v1",
    "id": "urn:dev:ops:32473-WoTLamp-1234",
    "title": "MyLampThing",
    "securityDefinitions": {
        "basic_sc": {"scheme": "basic", "in":"header"}
    },
    "security": ["basic_sc"],
    "properties": {
        "status" : {
            "type": "string",
            "forms": [{
                "href": "https://mylamp.example.com/status",
                "htv:methodName":"GET"
            }]
        }
    },
    "actions": {
        "toggle" : {
            "forms": [{
                "href": "https://mylamp.example.com/toggle",
                "htv:methodName":"POST"
            }]
        }
    },
    "events":{
        "overheating":{
            "data": {"type": "string"},
            "forms": [{
                "href": "https://mylamp.example.com/oh",
                "htv:methodName":"GET",
                "subprotocol": "longpoll"
            }]
        }
    }
}

This TD represents an Internet connected lamp, which could be thought as a simple version of a Philips Hue lamp.

From this TD example, a client knows that there exists one Property affordance with the title status (lines 10-16). In addition, information is provided in lines 13-14 that this Property is readable with an HTTP GET request to the

This example also specifies the basic security scheme, requiring a username and password for access. A security scheme is first given a name and its corresponding scheme in the securityDefinitions and then activated by specifying that name in a security section. In combination with the use of the HTTP this example demonstrates the use of Basic access authentication.

Below is the same connected lamp but using MQTT protocol and no security.

{
    "@context": [
        "https://www.w3.org/2019/wot/td/v1",
        {"mqv": "http://www.example.org/mqtt-binding#"}
    ],
    "id": "urn:dev:ops:32473-WoTLamp-1234",
    "title": "MyLampThing",
    "securityDefinitions": {
        "nosec_sc": {"scheme": "nosec"}
    },
    "security": ["nosec_sc"],
    "properties": {
        "status" : {
            "type": "string",
            "forms": [{
                "href": "mqtt://mylamp.example.com/status",
                "mqv:controlPacketValue": "SUBSCRIBE"
            }]
        }
    },
    "actions": {
        "toggle" : {
            "forms": [{
                "href": "mqtt://mylamp.example.com/toggle",
                "mqv:controlPacketValue": "PUBLISH"
            }]
        }
    },
    "events":{
        "overheating":{
            "data": {"type": "string"},
            "forms": [{
                "href": "mqtt://mylamp.example.com/oh",
                "mqv:controlPacketValue": "SUBSCRIBE"
            }]
        }
    }
}

Differently from the last TD, here the forms include MQTT protocol as specified by the WoT Binding Templates. More specifically, lines 17, 25 and 34 describe what message types should be used to use the affordances. For example, instead of

Implementations

Thing Description editing and validation tools

• Eclipse edi{TD}or:^[14] A tool for simply designing W3C Thing Descriptions and Thing Models
• TD Playground:^[15] Playground for validation of Thing Description instances

Implementations using Thing Description

• Eclipse node-wot:^[16] An implementation of W3C WoT technologies in Node.js
• WoTify:^[17] A library of WoT implementation that support a Thing Description
• wot-py:^[18] An implementation of W3C WoT technologies in Python
• Node-RED node-gen:^[19] Generates nodes based on a Thing Description in Node-RED
• SANE:^[20] An implementation of W3C WoT technologies in Java

See also

• Web of Things
• Digital Twin
• OpenAPI Specification

References