@context member

The @context member is an optional annotation which can be used to provide a URI for a schema repository which defines standard schemas for common "types" of device capabilities. These types can then be referred to using @type annotations elsewhere in the Thing Description.

"@context": ["https://www.w3.org/2019/wot/td/v1", "https://iot.mozilla.org/schemas"]

Thing resource

A thing Resource provides a Thing Description for a device. A Thing Resource is considered the root resource of a Thing.

The URL of a Thing Resource may be enumerated by a gateway or directory, or may be discovered by some other means.

A Thing description is usually read only. An HTTP GET request can be used to retrieve a Thing Description for a Thing.

Example: Get a description of a Thing

GET http://mythingserver.com/things/pi
Accept: application/json

200 OK
          

        

Property resource

A property resource represents a single property of a device. Some property resources may be read only and some may be writable. The value of a property can be retrieved with an HTTP GET request and updated with an HTTP PUT request.

The URL of a Property resource must be defined by the forms section of a Property object in a Thing Description.

If a Property object does not define a primitive type it may not have a JSON serialisation and may instead return a binary file or stream in response to an HTTP GET request (e.g. an image file or video stream).

Example: Get a property

GET http://mythingserver.com/things/pi/properties/temperature
Accept: application/json

200 OK
21

Example: Set a property

PUT http://mythingserver.com/things/pi/properties/led
true
Accept: application/json

200 OK
true

Properties resource

A properties resource represents all the properties of a device. The value of all properties can be retrieved with an HTTP GET request.

The URL of a Properties resource can be defined by a properties link relation in the links member of a Thing Description.

Example: Get all properties

GET http://mythingserver.com/things/pi/properties
Accept: application/json

200 OK
{
  "temperature": 21,
  "humidity": 50,
  "led": true
}

Action resource

An action resource represents a queue of actions of a single action type. A new action is created in the queue with an HTTP POST request and a list of action requests in the queue can be requested with an HTTP GET request.

The URL of an Action resource must be defined by the links section of an Action object in a Thing Description.

If a client tries to request an action of another type via this resource, the server should respond with a 400 Bad Request response.

If an action request is successful, the server should respond with a 201 Created response, containing an ActionRequest resource in the response body.

Action Request

POST https://mythingserver.com/actions/fade
Accept: application/json

{
  "level": 50,
  "duration": 2000
}
      

201 Created
        

      

Action Queue

GET /actions/fade
Accept: application/json
      

200 OK
        

      

ActionRequest resource

An action request resource represents an individual action request for a given action. The current status of an action request can be retrieved with an HTTP GET request, updated with an HTTP PUT request and deleted with an HTTP DELETE request.

The URL of an Action request resource can be defined by the href member of an action request object in an action queue.

An ActionRequest object should conform to the following JSON schema:

Action Request Status

GET /actions/fade/123e4567-e89b-12d3-a456-426655
Accept: application/json
      

200 OK
        

      

Cancel an Action Request

DELETE /actions/fade/123e4567-e89b-12d3-a456-426655
      

204 No Content
      

Actions resource

An actions resource represents a queue of actions to be executed on a device. A new action is created in the queue with an HTTP POST request and a list of action requests in the queue can be requested with an HTTP GET request.

The URL of an actions resource can be defined by an actions link relation in the links member of a Thing Description.

Any action supported by the thing can be requested via this top level action queue. If an unsupported action type is requested, the server should respond with a 400 Bad Request response.

Action Request

POST https://mythingserver.com/actions/
Accept: application/json

{
  "fade": {
    "input": {
      "level": 50,
      "duration": 2000
    }
  }
}
      

201 Created
        

      

Actions Queue

GET /actions
Accept: application/json
      

200 OK
        

      

Event resource

An event resource provides a log of events recently emitted by a device for a particular event type. An event resource is usually read only.

The URL of an Event resource must be defined by the forms section of an Event object in a Thing Description.

Event Log

GET /events/overheated
Accept: application/json
      

200 OK
[
  {
    "event": "overheated",
    "data": 102,
    "timestamp": "2017-01-25T15:01:35+00:00"
  },
  {
    "event": "overheated",
    "data": 101,
    "timestamp": "2017-01-24T13:02:45+00:00"
  }
]

Events resource

An events resource provides a log of all events recently emitted by a device. An events resource is usually read only.

The URL of an Events resource can be defined by an events link relation in the links member of a Thing Description.

Event Log

GET /events
Accept: application/json
      

200 OK
[
  {
    "event": "overheated",
    "data": 102,
    "timestamp": "2017-01-25T15:01:35+00:00"
  },
  {
    "event": "reboot",
    "timestamp": "2017-01-24T13:02:45+00:00"
  }
]

Alternative Protocol Bindings

The default protocol binding for the Web Thing REST API is HTTP(S). Bindings to alternative application protocols (e.g. CoAP) may be used, but these bindings are beyond the scope of this specification. A Web Thing API protocol binding may also be layered on top of a non-Internet application protocol by use of a gateway.

Alternative Encodings

The default content encoding for the Web Thing HTTP(S) REST API is JSON. Alternative content encodings (e.g. CBOR) may be used, but these bindings are beyond the scope of this specification.

Protocol Handshake

To open a WebSocket on a Thing, an HTTP GET request is upgraded to a WebSocket using a standard WebSocket protocol handshake [[WEBSOCKETS-PROTOCOL]] and the "webthing" subprotocol. The WebSocket URL for a Web Thing is specified in the links member of the Web Thing Description.

GET wss://mythingserver.com/things/robot
Host: mythingserver.com
Origin: https://mythingserver.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: webthing
Sec-WebSocket-Version: 13

HTTP 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: webthing

A WebSocket can be opened from a web page using the JavaScript WebSocket API which will take care of the handshake detailed above and allow messages to be sent and received.

const socket = new WebSocket('wss://mywebthingserver/things/robot', 'webthing');

setProperty message

The setProperty message type is sent from a web client to a Web Thing in order to set the value of one or more of its properties. This is equivalent to a PUT request on a Property resource URL using the REST API, but with the WebSocket API a property value can be changed multiple times in quick succession over an open socket and multiple properties can be set at the same time.

{
  "messageType": "setProperty",
  "data": {
    "leftMotor": 100
  }
}

requestAction message

The requestAction message type is sent from a web client to a Web Thing to request an action be carried out on a Web Thing. This is equivalent to a POST request on an Actions resource URL using the REST API, but multiple actions can be requested at the same time or in quick succession over an open socket.

{
  "messageType": "requestAction",
  "data": {
    "goForward": {
      "steps": 100
    },
  }
}

addEventSubscription message

The addEventSubscription message type is sent from a web client to a Web Thing to allow a client to subscribe to a particular event type, defined by the events member of a Web Thing description. This is similar to adding an event listener in JavaScript, but events are received as an event message over the Web Thing WebSocket API.

{
  "messageType": "addEventSubscription",
  "data": {
    "motion": {}
  }
}

propertyStatus message

The propertyStatus message type is sent from a Web Thing to a web client whenever a property of a Web Thing changes. The payload data of this message is in the same format as a response to a GET request on Property resource using the REST API, but the message is pushed to the client whenever a property changes and can include multiple properties at the same time.

{
  "messageType": "propertyStatus",
  "data": {
    "led": true
  }
}

actionStatus message

The actionStatus message type is sent from a Web Thing to a web client when the status of a requested action changes. The payload data is consistent with the format of an Action resource in the REST API, but messages are pushed to the client as soon as the status of an action changes.

{
  "messageType": "actionStatus",
  "data": {
    "grab": {
      "action": "goForward",
      "id": "123e4567-e89b-12d3-a456-426655",
      "href": "/actions/go-forward/123e4567-e89b-12d3-a456-426655",
      "timeRequested": "2017-01-24T11:02:45+00:00",
      "timeCompleted": "2017-01-24T11:02:46+00:00",
      "status": "completed",
      "input": {
        "steps": 100
      },
      "output": {},
    }
  }
}

event message

The event message type is sent from a Web Thing to a web client when an event occurs on the Web Thing. The payload data is consistent with the format of an Event resource in the REST API but messages are pushed to the client as soon as an event occurs.

{
  "messageType": "event",
  "data": {
    "motion": {
      "timestamp": "2017-01-24T13:02:45+00:00"
    }
  }
}

mDNS/DNS-SD

A Web Thing can make itself discoverable on a local network by advertising itself via DNS-SD, with a service type of labthing over TCP, i.e. _labthing._tcp. The service should define a TXT record named path, with its value denoting the URI of the thing description on the Web Thing's web server, e.g. path=/api/. Additionally, a TXT record named tls with a value of 1 should be included if the Web Thing supports connections via HTTPS.