@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://webthings.io/schemas"

@type member

The @type member is an optional annotation which can be used to provide the names of schemas for types of capabilities a device supports, from a schema repository referred to in the @context member. The value of the @type member is a string or an array of strings which correspond to schema names from a schema repository.

Providing a @type annotation tells a client that it can expect the device to conform to the constraints of that type's schema. This may include certain types of property, action and event the device can be expected to support. A client may make use of this information to aid in automation of or to generate a user interface for known standard device capabilities.

"@type": ["Light", "OnOffSwitch"]

id member

The id member provides an identifier of the device in the form of a URI [[RFC3986]] (e.g. a URL or a URN). This may be the URL of the Thing Description itself, but can be any URI.

"id": "https://mywebthingserver.com/things/lamp"

title member

The title member is a human friendly string which describes the device. This can be set to any value by the device creator and may include a brand name or model number.

description member

The description member is a human friendly string which describes the device and its functions. This can be set to any value by the device creator.

properties member

The properties member is a map of Property objects which describe the attributes of the device.

actions member

The actions member is a map of Action objects which describe functions that can be carried out on a device. Actions are used when the setting of a property alone is not sufficient to affect a required change in state. This may be used to describe a function which takes a period of time to complete, manipulates multiple properties, or has a different outcome based on provided arguments or current state. An example might include a "fade" action which has a specified "duration", a "sequence" action which triggers a sequence of flashing lights or a "toggle" action on a switch.

events member

The events member is a map of Event objects which define the types of events which may be emitted by a device. Events can be used where subscribing to a change in property state is not sufficient. This may be used to monitor changes in multiple properties or events which describe something other than a property change. An example might be an event which is emitted when a device is about to reboot.

links member

The links member provides an array of Link Objects which link to other resources of a thing.

Examples of links include a link to a Properties resource ("rel":"properties"), an Actions resource ("rel":"actions") or an Events resource ("rel":"events"). It can also include links to alternate representations of a thing such as a WebSocket API endpoint or an HTML user interface ("rel":"alternate").

"links": [
  {
    "rel": "properties",
    "href": "/things/lamp/properties"
  },
  {
    "rel": "actions",
    "href": "/things/lamp/actions"
  },
  {
    "rel": "events",
    "href": "/things/lamp/events"
  },
  {
    "rel": "alternate",
    "href": "wss://mywebthingserver.com/things/lamp"
  },
  {
    "rel": "alternate",
    "mediaType": "text/html",
    "href": "/things/lamp"
  }
]
        

Property object

A property object describes an attribute of a Thing and is indexed by a property id. A property description may include:

• A primitive type (one of null, boolean, object, array, number, integer or string as per [[!json-schema]])
• A semantic @type (a string identifying a type from the linked @context)
• A unit ([[!SI]] unit)
• A title (A string providing a human friendly name)
• A description (A string providing a human friendly description)
• A links array (An array of Link objects linking to one or more representations of a Property resource, each with an implied default rel=property.)
• enum (an enumeration of possible values for the property)
• readOnly (A boolean indicating whether or not the property is read-only, defaulting to false)
• A minimum and maximum (numeric values)
• multipleOf (A number indicating what the value must be a multiple of)

"level" : {
  "title": "Level",
  "description": "The level of light from 0-100",
  "@type": "LevelProperty",
  "type": "integer",
  "unit": "percent",
  "minimum": 0,
  "maximum": 100,
  "readOnly": false,
  "links": [{"href": "/things/lamp/properties/level"}]
}
        

Action object

An action object describes a function which can be carried out on a device. An action definition may include:

• A title (A string providing a human friendly name)
• A description (A string providing a human friendly description)
• A links array (An array of Link objects linking to one or more representations of an Action resource, each with an implied default rel=action.)
• An input object with a:
  • A primitive type (one of null, boolean, object, array, number, integer or string as per [[!json-schema]])
  • A semantic @type (a string identifying a type from the linked @context)
  • A unit ([[!SI]] unit)
  • A minimum and maximum (numeric values)
  • multipleOf (A number indicating what the value must be a multiple of)

"fade": {
  "@type": "FadeAction",
  "title": "Fade",
  "description": "Fade the lamp to a given level",
  "input": {
    "type": "object",
    "properties": {
      "level": {
        "type": "integer",
        "minimum": 0,
        "maximum": 100
      },
      "duration": {
        "type": "integer",
        "minimum": 0,
        "unit": "milliseconds"
      }
    }
  },
  "links": [{"href": "/things/lamp/actions/fade"}]
}
        

Event object

An event object describes a kind of event which may be emitted by a device. This may include:

• A primitive type (one of null, boolean, object, array, number, integer or string as per [[!json-schema]])
• A semantic @type (a string identifying a type from the linked @context)
• A unit ([[!SI]] unit)
• A title (A string providing a human friendly name)
• A description (A string providing a human friendly description)
• A links array (An array of Link objects linking to one or more representations of an Event resource, each with an implied default rel=event).
• A minimum and maximum (numeric values)
• multipleOf (A number indicating what the value must be a multiple of)

"overheated": {
  "title": "Overheated",
  "description": "The lamp has exceeded its safe operating temperature",
  "@type": "OverheatedEvent",
  "type": "number",
  "unit": "degree celsius",
  "links": [{"href": "/things/lamp/events/overheated"}]
}
        

Link object

A link object represents a link relation ([[!web-linking]]) and may have a:

• href (a string representation of a URL)
• rel (a string describing a relationship)
• mediaType (a string identifying a media type)

{
  "href": "/things/lamp/properties",
  "rel": "properties",
  "mediaType": "application/json"
}
        

Media Type

The JSON serialization of the Web Thing Description is identified by the Media Type application/td+json.

Alternative Encodings

The default encoding for the Thing description is JSON, but other encodings such as XML, JSON-LD or CBOR may be used. Alternative encodings are beyond the scope of this specification but may be defined separately.

References to alternative available encodings may be provided using Link HTTP response headers with the alternate link relation. A client can express a preference for an alternative encoding using HTTP content negotiation.

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 the href member of Thing object in a Things Resource provided 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
{
  "id": "https://mywebthingserver.com/things/pi",
  "title": "WoT Pi",
  "description": "A WoT-connected Raspberry Pi",
  "properties": {
    "temperature": {
      "title": "Temperature",
      "type": "number",
      "unit": "degree celsius",
      "readOnly": true,
      "description": "An ambient temperature sensor",
      "links": [{"href": "/things/pi/properties/temperature"}]
    },
    "humidity": {
      "title": "Humidity",
      "type": "number",
      "unit": "percent",
      "readOnly": true,
      "links": [{"href": "/things/pi/properties/humidity"}]
    },
    "led": {
      "title": "LED",
      "type": "boolean",
      "description": "A red LED",
      "links": [{"href": "/things/pi/properties/led"}]
    }
  },
  "actions": {
    "reboot": {
      "title": "Reboot",
      "description": "Reboot the device"
    }
  },
  "events": {
    "reboot": {
      "description": "Going down for reboot"
    }
  },
  "links": [
    {
      "rel": "properties",
      "href": "/things/pi/properties"
    },
    {
      "rel": "actions",
      "href": "/things/pi/actions"
    },
    {
      "rel": "events",
      "href": "/things/pi/events"
    },
    {
      "rel": "alternate",
      "href": "wss://mywebthingserver.com/things/pi"
    },
    {
      "rel": "alternate",
      "mediaType": "text/html",
      "href": "/things/pi"
    }
  ]
}
          

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
}
          

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 can be defined by the links 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
{
  "temperature": 21
}
          

Example: Set a property

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

200 OK
{
  "led": true
}
          

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/things/lamp/actions/
Accept: application/json

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

201 Created

{
  "fade": {
    "input": {
      "level": 50,
      "duration": 2000
    },
    "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
    "status": "pending"
  }
}
        

Actions Queue

GET /things/lamp/actions
Accept: application/json
        

200 OK
[
  {
    "fade": {
      "input": {
        "level": 50,
        "duration": 2000
      },
      "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
      "timeRequested": "2017-01-25T15:01:35+00:00",
      "status": "pending"
    }
  },
  {
    "reboot": {
      "href": "/things/lamp/actions/reboot/124e4568-f89b-22d3-a356-427656",
      "timeRequested": "2017-01-24T13:13:33+00:00",
      "timeCompleted": "2017-01-24T13:15:01+00:00",
      "status": "completed"
    }
  }
]
        

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 can 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.

Action Request

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

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

201 Created

{
  "fade": {
    "input": {
      "level": 50,
      "duration": 2000
    },
    "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655"
    "status": "pending"
  }
}
        

Action Queue

GET /things/lamp/actions/fade
Accept: application/json
        

200 OK
[
  {
    "fade": {
      "input": {
        "level": 50,
        "duration": 2000
      },
      "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
      "timeRequested": "2017-01-25T15:01:35+00:00",
      "status": "pending"
    }
  },
  {
    "fade": {
      "input": {
        "level": 100,
        "duration": 2000
      },
      "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
      "timeRequested": "2017-01-24T11:02:45+00:00",
      "timeCompleted": "2017-01-24T11:02:46+00:00",
      "status": "completed"
    }
  }
]
        

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.

Action Request Status

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

200 OK
{
  "fade": {
    "input": {
      "level": 50,
      "duration": 2000
    },
    "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
    "timeRequested": "2017-01-25T15:01:35+00:00",
    "status": "pending"
  }
}
        

Cancel an Action Request

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

204 No Content

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 /things/lamp/events
Accept: application/json
        

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

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 can be defined by the links section of an Event object in a Thing Description.

Event Log

GET /things/lamp/events/overheated
Accept: application/json
        

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

Things resource

A things resource represents a collection of web things exposed by a particular Web of Things server. This resource is not needed for individual web things, for which the Thing Description is the entry point. It is used only by Web of Things gateways and services which expose a directory of web things.

Example: Get a list of things

GET /things
Accept: application/json
          

200 OK
[
  {
    "id": "https://mywebthingserver.com/things/pi",
    "title": "WoT Pi",
    "description": "A WoT-connected Raspberry Pi",
    "properties": {
      "temperature": {
        "title": "Temperature",
        "type": "number",
        "unit": "degree celsius",
        "readOnly": true,
        "description": "An ambient temperature sensor",
        "links": [{"href": "/things/pi/properties/temperature"}]
      },
      "humidity": {
        "title": "Humidity",
        "type": "number",
        "unit": "percent",
        "readOnly": true,
        "links": [{"href": "/things/pi/properties/humidity"}]
      },
      "led": {
        "title": "LED",
        "type": "boolean",
        "description": "A red LED",
        "links": [{"href": "/things/pi/properties/led"}]
      }
    },
    "actions": {
      "reboot": {
        "title": "Reboot",
        "description": "Reboot the device"
      }
    },
    "events": {
      "reboot": {
        "description": "Going down for reboot"
      }
    },
    "href": "/things/pi"
  }
]
          

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.

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": {},
  }
}
          

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": {
      "href": "/actions/grab/123e4567-e89b-12d3-a456-426655",
      "status": "completed",
      "timeRequested": "2017-01-24T11:02:45+00:00",
      "timeCompleted": "2017-01-24T11:02:46+00:00"
    }
  }
}
          

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 webthing over TCP, i.e. _webthing._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=/things/lamp. Additionally, a TXT record named tls with a value of 1 should be included if the Web Thing supports connections via HTTPS.