Copyright © 2017-2022 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and permissive document license rules apply.
This document describes a formal information model and a common representation for a Web of Things (WoT) Thing Description 1.1. A Thing Description describes the metadata and interfaces of Things, where a Thing is an abstraction of a physical or virtual entity that provides interactions to and participates in the Web of Things. Thing Descriptions provide a set of interactions based on a small vocabulary that makes it possible both to integrate diverse devices and to allow diverse applications to interoperate. Thing Descriptions, by default, are encoded in a JSON format that also allows JSON-LD processing. The latter provides a powerful foundation to represent knowledge about Things in a machine-understandable way. A Thing Description instance can be hosted by the Thing itself or hosted externally when a Thing has resource restrictions (e.g., limited memory space) or when a Web of Things-compatible legacy device is retrofitted with a Thing Description. Furthermore, this document introduces the Thing Model, which allows authors to describe only the model or class of an Internet of Thing (IoT) entity. Thing Models can be seen as a template for Thing Description instances, but with reduced constraints such as no or few requirements for specific communication metadata.
This specification describes a superset of the features defined in Thing Description 1.0 [WOT-THING-DESCRIPTION]. Unless otherwise specified, documents created with version 1.0 of this specification remain compatible with Thing Description 1.1.
This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
This document was published by the Web of Things Working Group as a Working Draft using the Recommendation track.
Publication as a Working Draft does not imply endorsement by W3C and its Members.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document is governed by the 2 November 2021 W3C Process Document.
This section is non-normative.
The WoT Thing Description (TD) is a central building block in the W3C Web of Things (WoT) and can be considered as the entry point of a Thing (much like the index.html of a Web site). A TD instance has five main components: textual metadata about the Thing itself, a set of Interaction Affordances that indicate how the Thing can be used, schemas for the data exchanged with the Thing for machine-understandability, Security Definitions to provide metadata about the security mechanisms that must be used for interactions, and, finally, Web links to express any formal or informal relation to other Things or documents on the Web.
The Interaction Model of
W3C WoT
defines three types of Interaction
Affordances: Properties (PropertyAffordance
class) can be used for sensing and controlling parameters,
such as getting the current value or setting an operation
state. Actions (ActionAffordance
class)
model invocation of physical (and hence time-consuming)
processes, but can also be used to abstract RPC-like calls of
existing platforms. Events (EventAffordance
class)
are used for the push model of communication where
notifications, discrete events, or streams of values are sent
asynchronously to the receiver. See [WOT-ARCHITECTURE]
for details.
In general, the TD provides metadata for different
Protocol Bindings
identified by URI schemes [RFC3986]
(e.g., http
, coap
, etc.
[IANA-URI-SCHEMES]),
content types based on media types [RFC2046]
(e.g., application/json
,
application/xml
, application/cbor
,
application/exi
, etc. [IANA-MEDIA-TYPES]), and security
mechanisms (for authentication, authorization,
confidentiality, etc.). Serialization of TD instances is
based on JSON [RFC8259],
where JSON names refer to terms of the TD vocabulary, as
defined in this specification document. In addition the JSON
serialization of TDs follows the syntax of JSON-LD 1.1
[JSON-LD11] to
enable extensions and rich semantic processing.
Example 1 shows a TD instance and illustrates the Interaction Model with Properties, Actions, and Events by describing a lamp Thing with the title MyLampThing.
From this TD example, we know there exists one Property affordance with the title
status. In addition, information is provided to
indicate that this Property is accessible via (the secure
form of) the HTTP protocol with a GET method at the URI
https://mylamp.example.com/status
(announced
within the forms
structure by the
href
member), and will return a string-based
status value. The use of the GET method is not stated
explicitly, but is one of the default assumptions defined by
this document.
In a similar manner, an Action
affordance is specified to toggle the switch status using
the POST method on the
https://mylamp.example.com/toggle
resource,
where POST is again a default assumption for invoking
Actions.
The Event affordance
enables a mechanism for asynchronous messages to be sent by a
Thing. Here, a subscription to
be notified upon a possible overheating event of the lamp can
be obtained by using HTTP with its long polling subprotocol
on https://mylamp.example.com/oh
.
This example also specifies the basic
security scheme, requiring a username and password for
access. Note that a security scheme is first given a name in
securityDefinitions
and then activated by
specifying that name in a security
section. In
combination with the use of the HTTP protocol this example
demonstrates the use of HTTP Basic Authentication.
Specification of at least one security scheme at the top
level is mandatory, and gives the default access requirements
for every resource. However, security schemes can also be
specified per-form, with configurations given at the form
level overriding configurations given at the
Thing
level, allowing for the specification of
fine-grained access control. It is also possible to use a
special nosec
security scheme to indicate that
no access control mechanisms are used. Additional examples
will be provided later.
The Thing Description offers the possibility to add
contextual definitions in some namespace. This mechanism can
be used to integrate additional semantics to the content of
the Thing Description instance, provided that formal
knowledge, e.g., logic rules for a specific domain of
application, can be found under the given namespace.
Contextual information can also help specify some
configurations and behavior of the underlying communication
protocols declared in the forms
field. Example 2 extends
the TD sample from Example 1 by introducing a second
definition in the @context
to declare the prefix
saref
as referring to SAREF, the Smart Appliance
Reference Ontology [SMARTM2M].
This IoT ontology includes terms interpreted as semantic
labels that can be set as values of the @type
field, giving the semantics of Things and their Interaction
Affordances. In the example below, the Thing is labelled with
saref:LightSwitch
, the status
Property is labelled
with saref:OnOffState
and the
toggle
Action
with saref:ToggleCommand
.
The declaration mechanism inside some
@context
is specified by JSON-LD. A TD instance
complies to version 1.1 of that specification
[json-ld11].
Hence, a TD instance can be also processed as an RDF document
(for details about semantic processing, please refer to
Appendix C. JSON-LD Context
Usage and the documentation under the namespace IRIs,
e.g., https://www.w3.org/2019/wot/td).
One of the main intentions of a Thing Description is to provide a Consumer with all the details necessary to successfully interact with a Thing. In some IoT application scenarios, a fully detailed Thing Description, e.g., with communication metadata is not necessary (e.g., IoT ecosystems may implicitly handle communication separately), or may not be available because a new entity has not yet been deployed (e.g., IP address is not yet known). Sometimes, also a kind of class definition is required that forces capability definitions that should be available for all created instances (e.g., large-scale production of new devices).
In order to address the above-mentioned scenarios or others, the Thing Model can be used that mainly provides the data model definitions within Things' Properties, Actions, and/or Events and can be potentially used as template for creating Thing Description instances. In the following a sample Thing Model is presented that can be seen as a model for the Thing Description instance in Example 1.
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Lamp Thing Model",
"properties": {
"status": {
"description": "current status of the lamp (on|off)",
"type": "string",
"readOnly": true
}
},
"actions": {
"toggle": {
"description": "Turn the lamp on or off"
}
},
"events": {
"overheating": {
"description": "Lamp reaches a critical temperature (overheating)",
"data": {"type": "string"}
}
}
}
Thing
Model definitions are identified by the "@type":
"tm:ThingModel"
. As the example shows, it does not
provide details about a single Thing instance due to the lack of
communication and security metadata. This specification
presents a mechanism for deriving valid Thing
Description instances from such Thing Model definitions.
In addition, other design concepts are specified, including
how to override, extend, and reuse existing Thing Model
definitions.
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, RECOMMENDED, SHOULD, and SHOULD NOT in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
A Thing Description instance complies with this specification if it follows the normative statements in 5. TD Information Model and 6. TD Representation Format regarding Thing Description serialization.
A JSON Schema [JSON-SCHEMA] to validate Thing Description instances is provided in Appendix B. JSON Schema for TD Instance Validation.
This section is non-normative.
The fundamental WoT terminology such as Thing, Consumer, Thing Description (TD), Partial TD, Thing Model (TM), Interaction Model, Interaction Affordance, Property, Action, Event, Protocol Binding, Servient, Vocabulary, Term, Vocabulary Term, WoT Interface, and WoT Runtime are defined in Section 3 of the WoT Architecture specification [WOT-ARCHITECTURE].
In addition, this specification introduces the following definitions:
Thing
class. For that purpose, a TD Processor
may compute forms of Thing Descriptions
in which all possible Default Values are
assigned. A TD
Processor is typically a sub-system of a WoT Runtime.
Implementations of a TD Processor may be a TD producer only
(able to serialize to TD Documents) or a TD consumer only
(able to deserialize from TD Documents).
These definitions are further developed in 5.2 Preliminaries.
The version of the TD Information Model defined in 5. TD Information Model of this specification is identified by the following IRI:
https://www.w3.org/2022/wot/td/v1.1
This IRI [RFC3987], which is also a URI [RFC3986], can be dereferenced to obtain a JSON-LD context file [json-ld11], allowing the compact strings in TD Documents to be expanded to full IRI-based Vocabulary Terms. However, this processing is only required when transforming JSON-based TD Documents to RDF, an optional feature of TD Processor implementations.
In the present specification, Vocabulary Terms are always presented in their compact form. Their expanded form can be accessed under the namespace IRI of the Vocabulary they belong to. These namespaces follow the structure of 5.3 Class Definitions. Each Vocabulary used in the TD Information Model has its own namespace IRI, as follows:
Vocabulary | Namespace IRI |
---|---|
Core | https://www.w3.org/2019/wot/td# |
Data Schema |
https://www.w3.org/2019/wot/json-schema# |
Security |
https://www.w3.org/2019/wot/security# |
Hypermedia Controls |
https://www.w3.org/2019/wot/hypermedia# |
All vocabularies that are additionally used for Thing Model definitions have the following namespace IRI:
Vocabulary | Namespace IRI |
---|---|
Thing Model | https://www.w3.org/2022/wot/tm# |
The Vocabularies are independent
from each other. They may be reused and extended in other
W3C
specifications. Every breaking change in the design of a
Vocabulary will require
the assignment of a new year-based namespace URI. Note that to
maintain the general coherence of the TD Information Model, the
associated JSON-LD context file is versioned such that every
version has its own URI (v1
, v1.1
,
v2
, ...) to also identify non-breaking changes, in
particular the addition of new Terms.
Because a Vocabulary under some namespace IRI can only undergo non-breaking changes, its content can be safely cached or embedded in applications. One advantage of exposing relatively static content under a namespace IRI is to optimize payload sizes of messages exchanged between constrained devices. It also avoids any privacy leakage resulting from devices accessing publicly available vocabularies from private networks (see also 10. Privacy Considerations).
This section introduces the TD Information Model. The TD Information Model serves as the conceptual basis for the processing of Thing Descriptions and their serialization, which is described separately in 6. TD Representation Format.
The TD Information Model is built upon the following, independent Vocabularies:
Each of these Vocabularies is essentially a set of Terms that can be used to build data structures, interpreted as objects in the traditional object-oriented sense. Objects are instances of classes and have properties. In the context of W3C WoT, they denote Things and their Interaction Affordances. A formal definition of objects is given in 5.2 Preliminaries. The main elements of the TD Information Model are then presented in 5.3 Class Definitions. Certain object properties may be omitted in a TD when Default Values exist. A list of defaults is given in 5.4 Default Value Definitions.
The UML diagram shown next gives an overview of the
TD Information Model.
It represents all classes as tables and the associations that
exist between classes, starting from the class Thing
, as directed arrows. For the
sake of readability, the diagram was split in four parts, one
for each of the four base Vocabularies.
To provide a model that can be easily processed by both, simple rules on a tree-based document (i.e., raw JSON processing) and rich Semantic Web tooling (i.e., JSON-LD processing), this document defines the following formal preliminaries to construct the TD Information Model accordingly.
All definitions in this section refer to sets, which intuitively are collections of elements that can themselves be sets. All arbitrarily complex data structures can be defined in terms of sets. In particular, an Object is a data structure recursively defined as follows:
Though this definition does not prevent Objects to include multiple name-value pairs with the same name, they are generally not considered in this specification. An Object whose elements only have numbers as names is called an Array. Similarly, an Object whose elements only have Terms (that do not belong to any Vocabulary) as names is called a Map. All names appearing in some name-value pair in a Map are assumed to be unique within the scope of the Map.
Moreover, Objects can be instances of some Class. A Class, which is denoted by a Vocabulary Term, is first defined by a set of Vocabulary Terms called a Signature. A Class whose Signature is empty is called a Simple Type.
The Signature of a Class allows to construct two
functions that further define Classes: an Assignment Function and a Type Function. The Assignment Function
of a Class takes a Vocabulary Term of the
Class's Signature as input and returns
either true
or false
as output.
Intuitively, the Assignment Function
indicates whether an element of the Signature is mandatory or
optional when instantiating the Class. The Type Function of a
Class also takes a Vocabulary Term of the
Class's Signature as input and returns
another Class as
output. These functions are partial: their domain is
limited to the Signature of the Class being defined.
On the basis of these two functions, an Instance Relation can be defined for a pair composed of an Object and a Class. This relation is defined as constraints to be satisfied. That is, an Object is an instance of a Class if the two following constraints are both satisfied:
true
, the Object includes a name-value
pair with the Vocabulary Term as name.
According to the definition above, an Object would be an instance of
every Simple
Type, regardless of its structure. Instead, another
definition for the Instance Relation is
introduced for Simple
Types: an Object is
an instance of a Simple Type if it is a
Term with a given lexical form
(e.g., true
, false
for the
boolean
type, 1
, 2
,
3
, ... for the unsignedInt
type,
etc.).
Moreover, additional Classes, called Parameterized Classes, can be derived from the generic Map and Array structures. An Object is a Map of some Class, that is, an instance of the Map type parameterized with some Class, if it is a Map such that the value in all the name-value pairs it contains is an instance of this Class. The same applies to Arrays.
Finally, a Class is a Subclass of some other Class if every instance of the former is also an instance of the latter.
Given all definitions above, the TD Information Model is to be
understood as a set of Class definitions, which include a
Class name (a Vocabulary Term), a
Signature (a set of
Vocabulary Terms),
an Assignment Function,
and a Type
Function. These Class
definitions are provided as tables in 5.3 Class Definitions. For each table, the
values "mandatory" (respectively, "optional") in the
assignment column indicates that the Assignment Function
returns true
(respectively, false
)
for the corresponding Vocabulary Term.
By convention, Simple Types are denoted by
names starting with lowercase. The TD Information Model
references the following Simple Types defined in
XML Schema [XMLSCHEMA11-2-20120405]:
string
, anyURI
,
dateTime
, integer
,
unsignedInt
, double
, and
boolean
. Their definition (i.e., the
specification of their lexical form) is outside of the scope
of the TD
Information Model.
In addition, the TD Information Model defines a
global function on pairs of Vocabulary Terms. The
function takes a Class name
and another Vocabulary Term as input and
returns an Object.
If the returned Object is
different from null
, it represents the
Default Value for some assignment on the input
Vocabulary Term in
an instance of the input Class. This function allows to
relax the constraint defined above on the Assignment Function:
an Object is an instance of
a Class if it includes all
mandatory assignments or if Default Value exist for
the missing assignments. All Default Values are given in
the table of 5.4 Default Value
Definitions. In each table of 5.3 Class Definitions, the assignment
column contains the value "with default" if a Default Value is
available for the corresponding combination of Class and Vocabulary Term in the
TD Information
Model.
The formalization introduced here does not consider the possible relation between Objects as abstract data structures and physical world objects such as Things. However, care was given to the possibility of re-interpreting all Vocabulary Terms involved in the TD Information Model as RDF resources, so as to integrate them in a larger model of the physical world (an ontology). For details about semantic processing, please refer to C. JSON-LD Context Usage and the documentation under the namespace IRIs, e.g., https://www.w3.org/2019/wot/td.
A TD Processor MUST satisfy the Class instantiation constraints on all Classes defined in 5.3.1 Core Vocabulary Definitions, 5.3.2 Data Schema Vocabulary Definitions, 5.3.3 Security Vocabulary Definitions, and 5.3.4 Hypermedia Controls Vocabulary Definitions.
Please note that all vocabulary terms and values are case sensitive. This is also true for the serialization of the information model (Section 6. TD Representation Format).
An abstraction of a physical or a virtual entity whose metadata and interfaces are described by a WoT Thing Description, whereas a virtual entity is the composition of one or more Things.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@context |
JSON-LD keyword to define short-hand names called terms that are used throughout a TD document. | mandatory |
anyURI or Array
|
@type |
JSON-LD keyword to label the object with semantic tags (or types). | optional |
string or Array of
string
|
id |
Identifier of the Thing in form of a URI [RFC3986] (e.g., stable URI, temporary and mutable URI, URI with local IP address, URN, etc.). | optional |
anyURI
|
title |
Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | mandatory |
string
|
titles |
Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). Also see MultiLanguage. | optional |
Map of MultiLanguage
|
description |
Provides additional (human-readable) information based on a default language. | optional |
string
|
descriptions |
Can be used to support (human-readable) information in different languages. Also see MultiLanguage. | optional |
Map of MultiLanguage
|
version |
Provides version information. | optional |
VersionInfo
|
created |
Provides information when the TD instance was created. | optional |
dateTime
|
modified |
Provides information when the TD instance was last modified. | optional |
dateTime
|
support |
Provides information about the TD maintainer as
URI scheme (e.g., mailto
[RFC6068],
tel [RFC3966],
https ). |
optional |
anyURI
|
base |
Define the base URI that is used for all
relative URI references throughout a TD document.
In TD instances, all relative URIs are resolved
relative to the base URI using the algorithm
defined in [RFC3986].base does not affect the URIs used in
@context and the IRIs used within
Linked Data [LINKED-DATA]
graphs that are relevant when semantic processing
is applied to TD instances. |
optional |
anyURI
|
properties |
All Property-based Interaction Affordances of the Thing. | optional |
Map of PropertyAffordance
|
actions |
All Action-based Interaction Affordances of the Thing. | optional |
Map of ActionAffordance
|
events |
All Event-based Interaction Affordances of the Thing. | optional |
Map of EventAffordance
|
links |
Provides Web links to arbitrary resources that relate to the specified Thing Description. | optional |
Array of Link
|
forms |
Set of form hypermedia controls that describe how an operation can be performed. Forms are serializations of Protocol Bindings. Thing-level forms are used to describe endpoints for a group of interaction affordances. | optional |
Array of Form
|
security |
Set of security definition names, chosen from
those defined in securityDefinitions .
These must all be satisfied for access to
resources. |
mandatory |
string or Array of
string
|
securityDefinitions |
Set of named security configurations
(definitions only). Not actually applied unless
names are used in a security
name-value pair. |
mandatory |
Map of SecurityScheme
|
profile |
Indicates the WoT Profile mechanisms followed by this Thing Description and the corresponding Thing implementation. | optional |
anyURI or Array of
anyURI
|
schemaDefinitions |
Set of named data schemas. To be used in a
schema name-value pair inside an
AdditionalExpectedResponse
object. |
optional |
Map of DataSchema
|
uriVariables |
Define URI template variables according to
[RFC6570] as collection
based on DataSchema declarations. The Thing level
uriVariables can be used in
Thing-level forms or in Interaction
Affordances. The individual variables DataSchema
cannot be an ObjectSchema or an ArraySchema. If the
same variable is both declared in Thing-level
uriVariables and in Interaction
Affordance level, the Interaction Affordance level
variable takes precedence. |
optional |
Map of DataSchema
|
For @context
the following rules are
defined for Thing Description
instances:
@context
name-value pair MUST contain the anyURI
https://www.w3.org/2022/wot/td/v1.1
in
order to identify the document as a TD 1.1 which would
allow Consumers to use the newly
introduced terms.https://www.w3.org/2019/wot/td/v1
MUST be the first entry and the
https://www.w3.org/2022/wot/td/v1.1
MUST be the second
entry.@context
is an Array, the anyURI
https://www.w3.org/2022/wot/td/v1.1
MAY be followed by elements of type
anyURI
or type Map in any order, while it is
RECOMMENDED to include only one
Map with all the
name-value pairs in the @context
Array.@context
Array MAY contain
name-value pairs, where the value is a namespace
identifier of type anyURI
and the name a
Term or prefix
denoting that namespace.@context
Array SHOULD
contain a name-value pair that defines the default
language for the Thing Description, where the name is
the Term
@language
and the value is a well-formed
language tag as defined by [BCP47]
(e.g., en
, de-AT
,
gsw-CH
, zh-Hans
,
zh-Hant-HK
,
sl-nedis
).To determine the base direction of all human-readable text in Thing Description and Thing Model instances this specification recommends to follow the [STRING-META] guideline about string-specific directional information when no built-in mechanism for associating base direction metadata is available.
TD Processors should be aware of certain special cases when processing bidirectional text. They should take care to use bidi isolation when presenting strings to users, particularly when embedding in surrounding text (e.g., for Web user interface). Mixed direction text can occur in any language, even when the language is properly identified.
TD producers should attempt to provide mixed direction strings in a way that can be displayed successfully by a naive user agent. For example, if an RTL string begins with an LTR run (such as a number or a brand or trade name in Latin script), including an RLM character at the start of the string or wrapping opposite direction runs in bidi controls can assist in proper display.
Strings on the Web: Language and Direction Metadata [string-meta] provides some guidance and illustrates a number of pitfalls when using bidirectional text.
In addition to the
explicitly provided Interaction
Affordances in the properties
,
actions
, and events
Maps, a Thing can also provide
meta-interactions, which are indicated by
Form
instances in its optional
forms
Array. When the
forms
Array of a Thing instance contains
Form
instances, the string values assigned
to the name op
, either directly or within an
Array, MUST
be one of the following operation types:
readallproperties
,
writeallproperties
,
readmultipleproperties
,
writemultipleproperties
,
observeallproperties
,
unobserveallproperties
,
queryallactions
,
subscribeallevents
, or
unsubscribeallevents
. (See an example for an usage
of form
in a Thing instance.)
The data schema for each of the property
meta-interactions is constructed by combining the data
schemas of each PropertyAffordance
instance
in a single ObjectSchema
instance, where the
properties
Map of the
ObjectSchema
instance contains each data
schema of the PropertyAffordances
identified
by the name of the corresponding
PropertyAffordances
instance.
If not specified otherwise (e.g., through a TD Context
Extension), the request data of the
readmultipleproperties
operation is an
Array that contains
the intended PropertyAffordances
instance
names, which is serialized to the content type specified
by the Form
instance.
Metadata of a Thing that shows the possible choices to Consumers, thereby suggesting how Consumers may interact with the Thing. There are many types of potential affordances, but W3C WoT defines three types of Interaction Affordances: Properties, Actions, and Events.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type |
JSON-LD keyword to label the object with semantic tags (or types). | optional |
string or Array of
string
|
title |
Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | optional |
string
|
titles |
Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). Also see MultiLanguage. | optional |
Map of MultiLanguage
|
description |
Provides additional (human-readable) information based on a default language. | optional |
string
|
descriptions |
Can be used to support (human-readable) information in different languages. Also see MultiLanguage. | optional |
Map of MultiLanguage
|
forms |
Set of form hypermedia controls that describe how an operation can be performed. Forms are serializations of Protocol Bindings. | mandatory |
Array of Form
|
uriVariables |
Define URI template variables according to
[RFC6570] as collection
based on DataSchema declarations. The individual
variables DataSchema cannot be an ObjectSchema or
an ArraySchema. If the same variable is both
declared in Thing-level
uriVariables and in Interaction
Affordance level, the Interaction Affordance level
variable takes precedence. |
optional |
Map of DataSchema
|
The class InteractionAffordance
has the
following subclasses:
An Interaction Affordance that exposes state of the Thing. This state can then be retrieved (read) and/or updated (write). Things can also choose to make Properties observable by pushing the new state after a change.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
observable |
A hint that indicates whether Servients hosting
the Thing and Intermediaries should provide a
Protocol Binding that supports the
observeproperty and
unobserveproperty operations for this
Property. |
with default |
boolean
|
Property instances are also instances of
the class DataSchema. Therefore, it can contain the
type
, unit
,
readOnly
and writeOnly
members, among others.
PropertyAffordance
is a Subclass of the
InteractionAffordance
Class and the
DataSchema
Class. When a Form
instance is within a PropertyAffordance
instance, the value assigned to op
MUST be one of
readproperty
, writeproperty
,
observeproperty
,
unobserveproperty
or an Array containing a combination
of these terms.
It is considered to be good practice that
each observeproperty
has a corresponding
unobserveproperty
unless the protocol
supports implicit unsubscription mechanisms (e.g.,
heartbeat to detect connection loss).
The observation mechanism depends on the underlying protocol or sub-protocol. Having said that, it is not guaranteed that the current Property value will be provided once the subscription is initiated. Hence, it may be necessary to read the current Property value before/after the subscription to get a first value.
An Interaction Affordance that allows to invoke a function of the Thing, which manipulates state (e.g., toggling a lamp on or off) or triggers a process on the Thing (e.g., dim a lamp over time).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
input |
Used to define the input data schema of the Action. | optional |
DataSchema
|
output |
Used to define the output data schema of the Action. | optional |
DataSchema
|
safe |
Signals if the Action is safe (=true) or not. Used to signal if there is no internal state (cf. resource state) is changed when invoking an Action. In that case responses can be cached as example. | with default |
boolean
|
idempotent |
Indicates whether the Action is idempotent (=true) or not. Informs whether the Action can be called repeatedly with the same result, if present, based on the same input. | with default |
boolean
|
synchronous |
Indicates whether the action is synchronous (=true) or not. A synchronous action means that the response of action contains all the information about the result of the action and no further querying about the status of the action is needed. Lack of this keyword means that no claim on the synchronicity of the action can be made. | optional |
boolean
|
ActionAffordance
is a Subclass of the
InteractionAffordance
Class. When a Form
instance is within an ActionAffordance
instance, the value assigned to op MUST either be
invokeaction
, queryaction
,
cancelaction
or an Array containing a combination
of these terms.
An Interaction Affordance that describes an event source, which asynchronously pushes event data to Consumers (e.g., overheating alerts).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
subscription |
Defines data that needs to be passed upon subscription, e.g., filters or message format for setting up Webhooks. | optional |
DataSchema
|
data |
Defines the data schema of the Event instance messages pushed by the Thing. | optional |
DataSchema
|
dataResponse |
Defines the data schema of the Event response messages sent by the consumer in a response to a data message. | optional |
DataSchema
|
cancellation |
Defines any data that needs to be passed to cancel a subscription, e.g., a specific message to remove a Webhook. | optional |
DataSchema
|
EventAffordance
is a Subclass of the
InteractionAffordance
Class. When a Form
instance is within an EventAffordance
instance, the value assigned to op
MUST be either
subscribeevent
,
unsubscribeevent
, or both terms within an
Array.
It is considered to be good practice that
each subscribeevent
has a corresponding
unsubscribeevent
unless the protocol
supports implicit unsubscription mechanisms (e.g.,
heartbeat to detect connection loss).
Metadata of a Thing that provides version information about the TD document. If required, additional version information such as firmware and hardware version (term definitions outside of the TD namespace) can be extended via the TD Context Extension mechanism.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
instance |
Provides a version indicator of this TD. instance. | mandatory |
string
|
model |
Provides a version indicator of the underlying TM. instance. | optional |
string
|
It is recommended that the values within
instances
and model
of the
VersionInfo
Class follow the semantic
versioning pattern, where a sequence of three numbers
separated by a dot indicates the major version, minor
version, and patch version, respectively. See
[SEMVER] for
details.
A Map providing a set of human-readable texts in different languages identified by language tags described in [BCP47]. See 6.3.2 Human-Readable Metadata for example usages of this container in a Thing Description instance.
Each name of the
MultiLanguage
Map MUST be a language tag as defined in
[BCP47].
Each value of the
MultiLanguage
Map MUST be of type
string
.
A data schema is an abstract notation for data contained in data formats.
The data schema vocabulary definition reflects a very common subset of the terms defined by JSON Schema [JSON-SCHEMA]. It is noted that data schema definitions within Thing Description instances are not limited to this defined subset and may use additional terms found in JSON Schema using a TD Context Extension for the additional terms as described in 7. TD Context Extensions, otherwise these terms are semantically ignored by TD Processors (for details about semantic processing, please refer to C. JSON-LD Context Usage and the documentation under the namespace IRIs, e.g., https://www.w3.org/2019/wot/td).
In a TD, concrete data formats are specified in Forms
(see 5.3.4.2 Form
) using content
types. When the value of a content type in an instance of
the Form is application/json
, the data schema
can be processed directly by JSON Schema processors.
Otherwise, Web of Things (WoT) Binding Templates
[WOT-BINDING-TEMPLATES]
defines data schema's available mappings to other content
types such as XML [xml].
If the content type in an instance of the Form is not
application/json
and if no mapping is defined
for the content type, specifying a data schema does not
make sense for the content type.
The following table is at risk but contains content types which MAY use data schema.
Format | Content Type |
---|---|
JSON/CBOR | application/json application/ld+json application/senml+json application/cbor application/senml+cbor |
XML/EXI | application/xml application/senml+xml application/exi application/senml-exi |
Metadata that describes the data format used. It can be used for validation.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type |
JSON-LD keyword to label the object with semantic tags (or types) | optional |
string or Array of
string
|
title |
Provides a human-readable title (e.g., display a text for UI representation) based on a default language. | optional |
string
|
titles |
Provides multi-language human-readable titles (e.g., display a text for UI representation in different languages). Also see MultiLanguage. | optional |
Map of MultiLanguage
|
description |
Provides additional (human-readable) information based on a default language. | optional |
string
|
descriptions |
Can be used to support (human-readable) information in different languages. Also see MultiLanguage. | optional |
Map of MultiLanguage
|
const |
Provides a constant value. | optional | any type |
default |
Supply a default value. The value should validate against the data schema in which it resides. | optional | any type |
unit |
Provides unit information that is used, e.g., in international science, engineering, and business. To preserve uniqueness, it is recommended that the value of the unit points to a semantic definition (also see Section Semantic Annotations). | optional |
string
|
oneOf |
Used to ensure that the data is valid against one of the specified schemas in the array. | optional |
Array of DataSchema
|
enum |
Restricted set of values provided as an array. | optional | Array of any type |
readOnly |
Boolean value that is a hint to indicate whether a property interaction / value is read only (=true) or not (=false). | with default |
boolean
|
writeOnly |
Boolean value that is a hint to indicate whether a property interaction / value is write only (=true) or not (=false). | with default |
boolean
|
format |
Allows validation based on a format pattern such as "date-time", "email", "uri", etc. (Also see below.) | optional |
string
|
type |
Assignment of JSON-based data types compatible with JSON Schema (one of boolean, integer, number, string, object, array, or null). | optional | any type (one of object ,
array , string ,
number , integer ,
boolean , or null ) |
The class DataSchema
has the following
subclasses:
The format
string values are known from a
fixed set of values and their corresponding format rules
defined in [JSON-SCHEMA]
(Section 7.3 Defined Formats in particular). Servients MAY
use the format
value to perform additional
validation accordingly. When a value that is
not found in the known set of values is assigned to
format
, such a validation SHOULD succeed.
The format
term is not widely
implemented by JSON Schema tools. In addition, the term
format
is being discussed by the JSON
Schema standardisation community and may be replaced by
another mechanism or removed in a future JSON Schema
version.
Metadata describing data of type Array. This Subclass is indicated by
the value array
assigned to
type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
items |
Used to define the characteristics of an array. | optional |
DataSchema
or Array of DataSchema
|
minItems |
Defines the minimum number of items that have to be in the array. | optional |
unsignedInt
|
maxItems |
Defines the maximum number of items that have to be in the array. | optional |
unsignedInt
|
Metadata describing data of type boolean
.
This Subclass is indicated by the
value boolean
assigned to type
in DataSchema
instances.
Metadata describing data of type number
.
This Subclass is indicated by the
value number
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
minimum |
Specifies a minimum numeric value, representing an inclusive lower limit. Only applicable for associated number or integer types. | optional |
double
|
exclusiveMinimum |
Specifies a minimum numeric value, representing an exclusive lower limit. Only applicable for associated number or integer types. | optional |
double
|
maximum |
Specifies a maximum numeric value, representing an inclusive upper limit. Only applicable for associated number or integer types. | optional |
double
|
exclusiveMaximum |
Specifies a maximum numeric value, representing an exclusive upper limit. Only applicable for associated number or integer types. | optional |
double
|
multipleOf |
Specifies the multipleOf value number. The value must strictly greater than 0. Only applicable for associated number or integer types. | optional |
double
|
Metadata describing data of type integer
.
This Subclass is indicated by the
value integer
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
minimum |
Specifies a minimum numeric value, representing an inclusive lower limit. Only applicable for associated number or integer types. | optional |
integer
|
exclusiveMinimum |
Specifies a minimum numeric value, representing an exclusive lower limit. Only applicable for associated number or integer types. | optional |
integer
|
maximum |
Specifies a maximum numeric value, representing an inclusive upper limit. Only applicable for associated number or integer types. | optional |
integer
|
exclusiveMaximum |
Specifies a maximum numeric value, representing an exclusive upper limit. Only applicable for associated number or integer types. | optional |
integer
|
multipleOf |
Specifies the multipleOf value number. The value must strictly greater than 0. Only applicable for associated number or integer types. | optional |
integer
|
Metadata describing data of type Object. This Subclass is indicated by
the value object
assigned to
type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
properties |
Data schema nested definitions. | optional |
Map of DataSchema
|
required |
Defines which members of the object type are mandatory. | optional |
Array of
string
|
Metadata describing data of type string
.
This Subclass is indicated by the
value string
assigned to type
in DataSchema
instances.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
minLength |
Specifies the minimum length of a string. Only applicable for associated string types. | optional |
unsignedInt
|
maxLength |
Specifies the maximum length of a string. Only applicable for associated string types. | optional |
unsignedInt
|
pattern |
Provides a regular expression to express constraints of the string value. The regular expression must follow the [ECMA-262] dialect. | optional |
string
|
contentEncoding |
Specifies the encoding used to store the contents, as specified in RFC 2054. | optional |
string (e.g., 7bit ,
8bit , binary ,
quoted-printable , or
base64 )
|
contentMediaType |
Specifies the MIME type of the contents of a string value, as described in RFC 2046. | optional |
string (e.g.,
image/png , or
audio/mpeg )
|
Metadata describing data of type null
.
This subclass is indicated by the value null
assigned to type
in DataSchema
instances. This Subclass describes only one acceptable
value, namely null
. It is important to note
that null
does not mean the absence of a
value. It is analogous to null
in
JavaScript, None
in Python,
null
in Java and nil
in Ruby
programming languages. It can be used as part of a
oneOf
declaration, where it is used to
indicate, that the data can also be
null
.
This specification provides a selection of well-established security mechanisms that are directly built into protocols eligible as Protocol Bindings for W3C WoT or are widely in use with those protocols. The current set of HTTP security schemes is partly based on OpenAPI 3.0.1 (see also [OPENAPI]). However while the HTTP security schemes, Vocabulary, and syntax given in this specification share many similarities with OpenAPI, they are not compatible.
If a value of auto
is set for the
in
field of a SecurityScheme
,
then the name
field SHOULD
NOT be set. In this case, the application of the
SecurityScheme
MUST
follow the respective specification for the given protocol
(e. g. [RFC8288]
when using the BasicSecurityScheme
with
HTTP).
Metadata describing the configuration of a security
mechanism. The value assigned to the name
scheme
MUST be defined
within a Vocabulary included in
the Thing Description,
either in the standard Vocabulary defined in
§ 5. TD
Information Model or in a TD Context
Extension.
For all security schemes, any private keys, passwords, or other sensitive information directly providing access should be shared and stored out-of-band and MUST NOT be stored in the TD. The purpose of a TD is to describe how to access a Thing if and only if a Consumer already has authorization, and is not meant be used to grant that authorization.
Security schemes generally may require additional
authentication parameters, such as a password or key. The
location of this information is indicated by the value
associated with the name in
, often in
combination with the value associated with
name
. The value associated with
in
can take one of the following values:
header
:name
.query
:name
.body
:name
. When used in the context
of a body
security information location,
the value of name
MUST be in
the form of a JSON pointer [RFC6901]
relative to the root of the input
DataSchema
for each interaction it is used
with. Since this value is not a fragment
identifier, and is not relative to the root of the TD
but to whichever data schemas the security scheme is
bound to, this value should not start with
#
; it is a "pure" JSON pointer. Since this
value is not a fragment identifier, it also does not
need to URL-encode special characters. The targeted
element may or may not already exist at the specified
location in the referenced data schema. If it does not,
it will be inserted. This avoids having to duplicate
definitions in the data schemas of every interaction.
When an element
of a data schema indicated by a JSON pointer indicated
in a body
locator does not already exist
in the indicated schema, it MUST be possible
to insert the indicated element at the location
indicated by the pointer. The JSON pointer
used in the body
locator MAY
use the "-
" character to indicate a
non-existent array element when it is necessary to
insert an element after the last element of an existing
array. The element
referenced (or created) by a body
security
information location MUST be required
and of type "string
". If
name
is not given, it is assumed the
entire body is to be used as the security
parameter.cookie
:name
.uri
:name
. This is more general than the
query
mechanism but more complex.
The value
uri
SHOULD be
specified for the name in
in a security
scheme only if query
is not
applicable. The URIs provided in
interactions where a security scheme using
uri
as the value for in
MUST be a URI template including
the defined variable.combo
security scheme and allOf
. In some cases
parameters may not actually be secret but a user may wish
to leave them out of the TD to help protect privacy. As
an example of this, some security mechanisms require both
a client identifier and a secret key. In theory, the
client identifier is public however it may be hard to
update and pose a tracking risk. In such a case it can be
provided as an additional security parameter so it does
not appear in the TD.
The names of URI
variables declared in a SecurityScheme
MUST be distinct from all other URI
variables declared in the TD.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
@type |
JSON-LD keyword to label the object with semantic tags (or types). | optional |
string or Array of
string
|
description |
Provides additional (human-readable) information based on a default language. | optional |
string
|
descriptions |
Can be used to support (human-readable) information in different languages. Also see MultiLanguage. | optional |
Map of MultiLanguage
|
proxy |
URI of the proxy server this security configuration provides access to. If not given, the corresponding security configuration is for the endpoint. | optional |
anyURI
|
scheme |
Identification of the security mechanism being configured. | mandatory |
string (e.g.,
nosec , combo ,
basic , digest ,
bearer , psk ,
oauth2 , or apikey )
|
The class SecurityScheme
has the
following subclasses:
A security configuration corresponding to identified
by the Vocabulary Term
nosec
(i.e., "scheme":
"nosec"
), indicating there is no authentication or
other mechanism required to access the resource.
An auto authentication security configuration
identified by the term auto
(i.e.,
"scheme": "auto"
). This scheme can be used
to indicate that the security parameters are going to be
negotiated by the underlying protocols at runtime.
This section is at risk.
A combination of other security schemes identified by
the Vocabulary Term
combo
(i.e., "scheme":
"combo"
). Elements of this scheme define various
ways in which other named schemes defined in
securityDefinitions
, including other
ComboSecurityScheme
definitions, are to be combined to create a new scheme
definition. Exactly one
of either oneOf
or allOf
MUST be included. Only
security scheme definitions which can be used together
can be combined with allOf
. For example, it
is not possible in general to combine different OAuth 2.0
flows together using allOf
unless one
applies to a proxy and one to the endpoint. Note that
when multiple named security scheme definitions are
listed in a security
field the same
semantics apply as in an allOf
combination
(and the same limitations on allowable combinations). The
oneOf
combination is equivalent to using
different security schemes on forms that are otherwise
identical. In this sense a oneOf
scheme is
not an essential feature but it does avoid redundancy in
such cases.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
oneOf |
Array of two or more strings identifying other named security scheme definitions, any one of which, when satisfied, will allow access. Only one may be chosen for use. | mandatory |
string or Array of
string
|
allOf |
Array of two or more strings identifying other named security scheme definitions, all of which must be satisfied for access. | mandatory |
string or Array of
string
|
Basic Authentication [RFC7617]
security configuration identified by the Vocabulary Term
basic
(i.e., "scheme":
"basic"
), using an unencrypted username and
password. This scheme should be used with some other
security mechanism providing confidentiality, for
example, TLS.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
name |
Name for query, header, cookie, or uri parameters. | optional |
string
|
in |
Specifies the location of security authentication information. | with default |
string (one of
header , query ,
body , cookie , or
auto )
|
Digest Access Authentication [RFC7616]
security configuration identified by the Vocabulary Term
digest
(i.e., "scheme":
"digest"
). This scheme is similar to basic
authentication but with added features to avoid
man-in-the-middle attacks.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
name |
Name for query, header, cookie, or uri parameters. | optional |
string
|
in |
Specifies the location of security authentication information. | with default |
string (one of
header , query ,
body , cookie , or
auto )
|
qop |
Quality of protection. | with default |
string (one of
auth , or auth-int )
|
API key authentication security configuration
identified by the Vocabulary Term
apikey
(i.e., "scheme":
"apikey"
). This scheme is to be used when the
access token is opaque, for example when a key in an
unknown or proprietary format is provided by a cloud
service provider. In this case the key may not be using a
standard token format. This scheme indicates that the key
provided by the service provider needs to be supplied as
part of service requests using the mechanism indicated by
the "in"
field.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
name |
Name for query, header, cookie, or uri parameters. | optional |
string
|
in |
Specifies the location of security authentication information. | with default |
string (one of
header , query ,
body , cookie ,
uri , or auto )
|
Bearer Token [RFC6750]
security configuration identified by the Vocabulary Term
bearer
(i.e., "scheme":
"bearer"
) for situations where bearer tokens are
used independently of OAuth2. If the oauth2
scheme is specified it is not generally necessary to
specify this scheme as well as it is implied. For
format
, the value jwt
indicates
conformance with [RFC7519],
jws
indicates conformance with
[RFC7797],
cwt
indicates conformance with
[RFC8392], and
jwe
indicates conformance with
[RFC7516], with
values for alg
interpreted consistently with
those standards. Other formats and
algorithms for bearer tokens MAY be specified in
vocabulary extensions.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
authorization |
URI of the authorization server. | optional |
anyURI
|
name |
Name for query, header, cookie, or uri parameters. | optional |
string
|
alg |
Encoding, encryption, or digest algorithm. | with default |
string (e.g.,
ES256 , or ES512-256 )
|
format |
Specifies format of security authentication information. | with default |
string (e.g., jwt ,
cwt , jwe , or
jws )
|
in |
Specifies the location of security authentication information. | with default |
string (one of
header , query ,
body , cookie , or
auto )
|
Pre-shared key authentication security configuration
identified by the Vocabulary Term
psk
(i.e., "scheme": "psk"
).
This is meant to identify that a standard is used for
pre-shared keys such as TLS-PSK [RFC4279],
and that the ciphersuite used for keys will be
established during protocol negotiation.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
identity |
Identifier providing information which can be used for selection or confirmation. | optional |
string
|
OAuth 2.0 authentication security configuration for
systems conformant with [RFC6749],
[RFC8252] and (for
the device
flow) [RFC8628],
identified by the Vocabulary Term
oauth2
(i.e., "scheme":
"oauth2"
).
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
authorization |
URI of the authorization server. In the case of
the device flow, the URI provided for
the authorization value refers to the
device authorization endpoint [RFC8628]. |
optional |
anyURI
|
token |
URI of the token server. | optional |
anyURI
|
refresh |
URI of the refresh server. | optional |
anyURI
|
scopes |
Set of authorization scope identifiers provided
as an array. These are provided in tokens returned
by an authorization server and associated with
forms in order to identify what resources a client
may access and how. The values associated with a
form should be chosen from those defined in an
OAuth2SecurityScheme active on that
form. |
optional |
string or Array of
string
|
flow |
Authorization flow. | mandatory |
string (e.g., code ,
client , or device )
|
For the code
flow both authorization
and
token
MUST be
included. For the
client
flow token
MUST be included. For the
client
flow authorization
MUST NOT be included.
For the
device
flow both authorization
and token
MUST be
included. In the case of the device
flow the value provided for authorization
refers to the device authorization endpoint defined in
[RFC8628].
The mandatory elements for each flow are summarized in
the following table:
Element | code |
client |
device |
---|---|---|---|
authorization |
mandatory | omit | mandatory; refers to device authorization endpoint |
token |
mandatory | mandatory | mandatory |
refresh |
optional | optional | optional |
The present model provides a representation for (typed)
Web links and Web forms exposed by a Thing. The Link
class definition reflects a very common subset of the terms
defined in Web Linking [RFC8288]. The defined terms can be
used, e.g., to describe the relation to another Thing such as a Lamp
Thing is controlled by a Switch Thing. The
Form
class corresponds to a newly introduced
form of hypermedia control to manipulate the state of
Things (and other Web
resources).
A link can be viewed as a statement of the form "link context has a relation type resource at link target", where the optional target attributes may further describe the resource.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
href |
Target IRI of a link or submission target of a form. | mandatory |
anyURI
|
type |
Target attribute providing a hint indicating what the media type [RFC2046] of the result of dereferencing the link should be. | optional |
string
|
rel |
A link relation type identifies the semantics of a link. | optional |
string
|
anchor |
Overrides the link context (by default the
Thing itself identified by its id )
with the given URI or IRI. |
optional |
anyURI
|
sizes |
Target attribute that specifies one or more sizes for the referenced icon. Only applicable for relation type "icon". The value pattern follows {Height}x{Width} (e.g., "16x16", "16x16 32x32"). | optional |
string
|
hreflang |
The hreflang attribute specifies the language of a linked document. The value of this must be a valid language tag [BCP47]. | optional |
string or Array of
string
|
The hreflang
attribute is
allowed to be a string
or
array
in this version of the spec.
Depending on the result of [LINKSET-MEDIA-TYPES]
the values of hrefLang
can be restricted
to array
only.
Link relations can be used to describe relations such as to other Things (e.g., a Switch Thing controls a Lamp Thing), to a specific kind of Thing Models (e.g., a Thing Description is an instance of a specific Thing Model), or to further documentations information (e.g., device manual of a Thing). It is recommended to reuse existing and established Link Relation definitions from IANA.
In the following a best practice relation type table is introduced that is recommended to use within WoT Thing Description or Thing Model instances.
Value | Occurrence | Explanation | Source of value origin |
---|---|---|---|
icon |
0..* | Imports an icon associated to the Thing (e.g., for UI purposes). | IANA Link Relation |
service-doc |
0..* | Relation to a resource that provide (human-readable) documentation or descriptions. | IANA Link Relation |
alternate |
0..* | Point to alternative representation of the Thing (i.e. RDF-Turtle, human-readable HTML document, ...). | IANA Link Relation |
type |
0..1 | Indicate that the Thing is an instance of the target resource such as to a Thing Model. | IANA Link Relation |
tm:extends |
0..1 | Extends an existing definition of the target resource such as a Thing Model. Only applicable for Thing Model definitions. | W3C WoT Thing Model |
tm:submodel |
0..* | Used to compose one or multiple Thing Models. Only applicable for Thing Model definitions. | W3C WoT Thing Model |
manifest |
0..* | Point to the web app manifest of a web application which provides, e.g., a user interface with which a user can interact with the Thing (also see [APPMANIFEST]). | IANA Link Relation |
proxy-to |
0..* | Target resource provide the address of a proxy. | W3C WoT Security and WoT Binding Template |
collection |
0..1 | Points to a collections of Things. | IANA Link Relation |
item |
0..* | Points to a Thing that is member of the current Thing collections. | IANA Link Relation |
predecessor-version |
0..1 | Points to a previous Thing Description or Thing Model version. | IANA Link Relation |
controlledBy |
0..* | Refers to a Thing that controls the context Thing. | W3C Thing Description |
A form can be viewed as a statement of "To perform an operation type operation on form context, make a request method request to submission target" where the optional form fields may further describe the required request. In Thing Descriptions, the form context is the surrounding Object, such as Properties, Actions, and Events or the Thing itself for meta-interactions.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
href |
Target IRI of a link or submission target of a form. | mandatory |
anyURI
|
contentType |
Assign a content type based on a media type
(e.g., text/plain ) and potential
parameters (e.g., charset=utf-8 ) for
the media type [RFC2046]. |
with default |
string
|
contentCoding |
Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Examples of content coding include "gzip", "deflate", etc. . | optional |
string
|
security |
Set of security definition names, chosen from
those defined in securityDefinitions .
These must all be satisfied for access to
resources. |
optional |
string or Array of
string
|
scopes |
Set of authorization scope identifiers provided
as an array. These are provided in tokens returned
by an authorization server and associated with
forms in order to identify what resources a client
may access and how. The values associated with a
form should be chosen from those defined in an
OAuth2SecurityScheme active on that
form. |
optional |
string or Array of
string
|
response |
This optional term can be used if, e.g., the output communication metadata differ from input metadata (e.g., output contentType differ from the input contentType). The response name contains metadata that is only valid for the primary response messages. | optional |
ExpectedResponse
|
additionalResponses |
This optional term can be used if additional expected responses are possible, e.g. for error reporting. Each additional response needs to be distinguished from others in some way (for example, by specifying a protocol-specific error code), and may also have its own data schema. | optional |
AdditionalExpectedResponse
or Array of AdditionalExpectedResponse
|
subprotocol |
Indicates the exact mechanism by which an
interaction will be accomplished for a given
protocol when there are multiple options. For
example, for HTTP and Events, it indicates which of
several available mechanisms should be used for
asynchronous notifications such as long polling
(longpoll ), WebSub [websub]
(websub ), Server-Sent Events
(sse ) [html] (also known as
EventSource). Please note that there is no
restriction on the subprotocol selection and other
mechanisms can also be announced by this
subprotocol term. |
optional |
string (e.g.,
longpoll , websub , or
sse )
|
op |
Indicates the semantic intention of performing the operation(s) described by the form. For example, the Property interaction allows get and set operations. The protocol binding may contain a form for the get operation and a different form for the set operation. The op attribute indicates which form is for which and allows the client to select the correct form for the operation required. op can be assigned one or more interaction verb(s) each representing a semantic intention of an operation. | with default |
string or Array of
string (one of
readproperty ,
writeproperty ,
observeproperty ,
unobserveproperty ,
invokeaction ,
queryaction ,
cancelaction ,
subscribeevent ,
unsubscribeevent ,
readallproperties ,
writeallproperties ,
readmultipleproperties ,
writemultipleproperties ,
observeallproperties ,
unobserveallproperties ,
subscribeallevents ,
unsubscribeallevents , or
queryallactions )
|
Possible values for the contentCoding
property can be found, e.g., in the
IANA HTTP content coding registry.
The list of possible operation types of a form is fixed. As of this version of the specification, it only includes the well-known types necessary to implement the WoT interaction model described in [WOT-ARCHITECTURE]. Future versions of the standard may extend this list but operations types SHOULD NOT be arbitrarily set by servients and be restricted to the values in the table below.
Operation Type | Description |
---|---|
readproperty | Identifies the read operation on Property Affordances to retrieve the corresponding data. |
writeproperty | Identifies the write operation on Property Affordances to update the corresponding data. |
observeproperty | Identifies the observe operation on Property Affordances to be notified with the new data when the Property is updated. |
unobserveproperty | Identifies the unobserve operation on Property Affordances to stop the corresponding notifications. |
invokeaction | Identifies the invoke operation on Action Affordances to perform the corresponding action. |
queryaction | Identifies the querying operation on Action Affordances to get the status of the corresponding action. |
cancelaction | Identifies the cancel operation on Action Affordances to cancel the ongoing corresponding action. |
subscribeevent | Identifies the subscribe operation on Event Affordances to be notified by the Thing when the event occurs. |
unsubscribeevent | Identifies the unsubscribe operation on Event Affordances to stop the corresponding notifications. |
readallproperties | Identifies the readallproperties operation on a Thing to retrieve the data of all Properties in a single interaction. |
writeallproperties | Identifies the writeallproperties operation on a Thing to update the data of all writable Properties in a single interaction. |
readmultipleproperties | Identifies the readmultipleproperties operation on a Thing to retrieve the data of selected Properties in a single interaction. |
writemultipleproperties | Identifies the writemultipleproperties operation on a Thing to update the data of selected writable Properties in a single interaction. |
observeallproperties | Identifies the observeallproperties operation on Properties to be notified with new data when any Property is updated. |
unobserveallproperties | Identifies the unobserveallproperties operation on Properties to stop notifications from all Properties in a single interaction. |
queryallactions | Identifies the queryallactions operation on a Thing to get the status of all Actions in a single interaction. |
subscribeallevents | Identifies the subscribeallevents operation on Events to subscribe to notifications from all Events in a single interaction. |
unsubscribeallevents | Identifies the unsubscribeallevents operation on Events to unsubscribe from notifications from all Events in a single interaction. |
A Thing Description of a WoT producer may have multiple forms entries with, e.g., different protocol and/or content types declarations that a Consumer could possibly support. In that case the Consumer may choose any form entry that works (e.g., the protocol and content type is supported) for them. When one form is chosen, it is expected that the Consumer will continue to use it as long as possible for every new interaction with the WoT producer.
This section is non-normative.
Protocols that can be used with TDs follow
request-response or eventing mechanisms. The Data
Schema of an affordance generally correlates with the
op
keywords used in forms
.
The table below informatively summarizes the available
data schema related terms with the op
keywords.
Operation Type | Consumer to Thing DataSchema Correlation | Thing to Consumer DataSchema Correlation |
---|---|---|
readproperty | No correlation. | All fields in the Property Affordance without
"writeOnly":true . |
writeproperty | All fields in the Property Affordance without
"readOnly":true . |
No correlation.
additionalResponses can be used in
the form level. |
observeproperty | No correlation. | All fields in the Property Affordance without
"writeOnly":true . |
unobserveproperty | No correlation. | No correlation. |
invokeaction | Value of the input key. |
Value of the output key. |
queryaction | No correlation. | No correlation.
additionalResponses can be used in
the form level. |
cancelaction | No correlation. | No correlation.
additionalResponses can be used in
the form level. |
subscribeevent | Value of the subscription key
with all fields without
"readOnly":true |
Value of the subscription key
with all fields without
"writeOnly":true |
unsubscribeevent | Value of the subscription key
with all fields without
"readOnly":true |
Value of the subscription key
with all fields without
"writeOnly":true |
The data schemas for meta operations such
as readallproperties
is currently under
discussion.
The optional response
name-value pair
can be used to provide metadata for the expected
response message. With the core vocabulary, it only
includes content type information, but TD Context
Extensions could be applied. If no
response
name-value pair is provided, it
MUST be assumed that the content
type of the response is equal to the content type
assigned to the Form instance. Note that
contentType
within an
ExpectedResponse
Class does not have a
Default Value. For
instance, if the value of the content type of the form
is application/xml
the assumed value of
the content type of the response will be also
application/xml
. In some cases additional
responses might be possible. One example of this is
error responses but in some cases there might also be
additional successful responses. In this case the
response
name-value pair is still used for
the primary response but
additionalResponses
may also be provided,
whose value is an array of
AdditionalExpectedResponse
objects. Each
additional response must be distinguished in some way
from the primary response, either by
contentType
or by protocol-specific
settings such as error code header values. Each
additional response may also have a data schema which
can differ from the normal output data schema for the
interaction.
In some use cases, input and output data might be
represented in a different form, for instance an Action
that accepts JSON, but returns an image. In such a
case, the optional response
name-value
pair can describe the content type of the expected
response. If the content type
of the expected response differs from the content type
of the form, the Form
instance MUST include a name-value pair with
the name response
. For instance, an
ActionAffordance
could only accept
application/json
for its input data, while
it will respond with an image/jpeg
content
type for its output data. In that case the content
types differ and the response
name-value
pair has to be used to provide response content type
(image/jpeg
) information to the Consumer. Similar
considerations apply to additional responses, although
in this case the contentType
is optional
if it is the same as the input content Type (e.g.
JSON). If the
content type of an additional expected response differs
from the content type of the form, the
Form
instance MUST include an
entry in the array associated with the name
additionalResponses
that includes a value
for the name contentType
.
If the data
schema of an additional expected response differs from
the output data schema of the interaction, the
Form
instance MUST include an
entry in the array associated with the name
additionalResponses
that includes a value
for the name schema
.
Communication metadata describing the expected response message for the primary response.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
contentType |
Assign a content type based on a media type
(e.g., text/plain ) and potential
parameters (e.g., charset=utf-8 ) for
the media type [RFC2046]. |
mandatory |
string
|
Communication metadata describing the expected response message for additional responses.
Vocabulary term | Description | Assignment | Type |
---|---|---|---|
success |
Signals if an additional response should not be considered an error. | with default |
boolean
|
contentType |
Assign a content type based on a media type
(e.g., text/plain ) and potential
parameters (e.g., charset=utf-8 ) for
the media type [RFC2046]. |
with default |
string
|
schema |
Used to define the output data schema for an
additional response if it differs from the default
output data schema. Rather than a
DataSchema object, the name of a
previous definition given in a
schemaDefinitions map must be
used. |
optional |
string
|
When assignments in a TD are missing, a TD Processor MUST follow the Default Value assignments expressed in the table of Default Value Definitions.
The following table gives all Default Values defined in the TD Information Model.
Class | Vocabulary Term | Default Value | Comment |
---|---|---|---|
PropertyAffordance |
readOnly |
false |
The default value for this vocabulary term applies
only to the PropertyAffordance level
definition. In other contexts, such as
DataSchema definitions, the vocabulary
term is optional. |
PropertyAffordance |
writeOnly |
false |
The default value for this vocabulary term applies
only to the PropertyAffordance level
definition. In other contexts, such as
DataSchema definitions, the vocabulary
term is optional. |
PropertyAffordance |
observable |
false |
|
ActionAffordance |
safe |
false |
|
ActionAffordance |
idempotent |
false |
|
AdditionalExpectedResponse |
success |
false |
|
AdditionalExpectedResponse |
contentType |
value of the contentType of the
Form element it belongs to. |
|
Form |
contentType |
application/json |
|
Form |
op |
Array of
string with the elements
readproperty and
writeproperty when readOnly
and writeOnly are set to
false or Array of
string with the element
readproperty when readOnly
is set to true or Array of
string with the element
writeproperty when
writeOnly is set to
true . |
If defined within an instance of
PropertyAffordance |
Form |
op |
invokeaction |
If defined within an instance of
ActionAffordance |
Form |
op |
Array of
string with the elements
subscribeevent and
unsubscribeevent
|
If defined within an instance of
EventAffordance |
BasicSecurityScheme |
in |
header |
|
DigestSecurityScheme |
in |
header |
|
DigestSecurityScheme |
qop |
auth |
|
APIKeySecurityScheme |
in |
query |
|
BearerSecurityScheme |
in |
header |
|
BearerSecurityScheme |
alg |
ES256 |
|
BearerSecurityScheme |
format |
jwt |
WoT Thing Descriptions represent Things and are modeled and
structured based on 5. TD Information Model.
This section defines a JSON-based representation format for
Things, a serialization of
instances of the Class
Thing
defined by the TD Information Model.
A TD Processor MUST be able to serialize Thing Descriptions into the JSON format [RFC8259] and/or deserialize Thing Descriptions from that format, according to the rules noted in 6.1 Mapping to JSON Types and 6.3 Information Model Serialization.
The JSON serialization of the TD Information Model is aligned with the syntax of JSON-LD 1.1 [json-ld11] in order to streamline semantic evaluation. Hence, the TD representation format can be processed either as raw JSON or with a JSON-LD 1.1 processor (for details about semantic processing, please refer to C. JSON-LD Context Usage and the documentation under the namespace IRIs, e.g., https://www.w3.org/2019/wot/td).
In order to support interoperable internationalization, TDs MUST be serialized according to the requirements defined in Section 8.1 of RFC8259 [RFC8259] for open ecosystems. In summary, this requires the following:
The TD Information Model is constructed, so that there is an easy mapping between model Objects and JSON types. Every Class instances maps to a JSON object, where each name-value pair of the Class instance is a member of the JSON object.
Every Simple
Type mentioned in 5.3 Class Definitions
(i.e., string
, anyURI
,
dateTime
, integer
,
unsignedInt
, double
, and
boolean
) maps to a primitive JSON type (string,
number, boolean), as per the rules listed below. These rules
apply to values in name-value pairs:
string
or anyURI
MUST be serialized as JSON
strings.dateTime
MUST be
serialized as JSON strings following the "date-time" format
specified by [RFC3339].
Examples would include 2019-05-24T13:12:45Z
and 2015-07-11T09:32:26+08:00
. Values that are of type
dateTime
SHOULD use
the literal Z
representing the UTC time zone
instead of an offset.integer
or unsignedInt
MUST be serialized as JSON numbers without a
fraction or exponent part.double
MUST be
serialized as JSON number.boolean
MUST be
serialized as JSON boolean.Every complex type of the TD Information Model (i.e., Arrays, Maps, and Class instances) maps to a structured JSON type (array and object), as per the rules listed below:
A Thing Description serialization may omit Vocabulary Term for which Default Values are defined, as listed in the table given in 5.4 Default Value Definitions.
The following example shows the TD instance from Example 1 with a checkbox to also include the members with Default Values (=checkbox checked). These members can be omitted (=checkbox unchecked) to simplify the TD serialization. Note that a TD Processor interprets these omitted members identically as if they were explicitly present with a given Default Value.
Please note that, depending on the Protocol Binding used, additional protocol-specific Vocabulary Terms may apply. They may also have associated Default Values, and hence can also be omitted as explained in this subsection. Further information can be found in 8.3 Protocol Bindings.
A Thing Description is a data structure rooted at an
Object of type
Thing
. In turn, a JSON
serialization of the Thing Description is a JSON object,
which is the root of a syntax tree constructed from the
TD
Information Model.
The root
element of a TD Serialization
MUST be a JSON object that
includes a member with the name @context
and a
value of type string or array that equals or respectively
contains
https://www.w3.org/2022/wot/td/v1.1
.
In general, this URI is used to identify the TD
representation format version defined by this
specification. For JSON-LD processing [json-ld11], this URI
specifies the Thing Description context file. An
@context
of type array indicates TD Context Extensions
(see 7. TD Context
Extensions for details).
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
...
}
All name-value pairs of an instance
of Thing
, where the name is a Vocabulary Term in the
Signature of
Thing
, MUST be
serialized as JSON members of the root object.
A TD snippet for a serialized root object including all mandatory and optional members is given below:
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"@type": "Thing",
"id": "urn:dev:ops:32473-Thing-1234",
"title": "MyThing",
"titles": {...},
"description": "Human readable information.",
"descriptions": {...},
"support": "mailto:support@example.com",
"version": {...},
"created": "2018-11-14T19:10:23.824Z",
"modified": "2019-06-01T09:12:43.124Z",
"securityDefinitions": {...},
"security": ...,
"base": "https://servient.example.com/",
"properties": {...},
"actions": {...},
"events": {...},
"links": [...],
"forms": [...]
}
All
values assigned to version
,
securityDefinitions
, properties
,
actions
, and events
in an
instance of the Class
Thing
MUST be
serialized as JSON objects.
All
values assigned to links
, and
forms
in an instance of the Class Thing
MUST be serialized as JSON arrays
containing JSON objects as defined in 6.3.8 links
and 6.3.9
forms
, respectively.
The value assigned to
security
in an instance of Class Thing
MUST be serialized as
JSON string or as JSON array whose elements are JSON
strings.
JSON members named title
and
description
are used within a TD document to
provide human-readable metadata. They can be used as
comments for developers inspecting a TD document or as
display texts for user interface.
As defined in 5.3.1.1
Thing
, the base text direction used to
display human-readable metadata can either be estimated
using heuristics such as the first-strong rule or inferred
from language information. In TD documents the default
language is defined by a value assigned to
@language
in the @context
, and
this, along with a script subtag if necessary, can be used
to determine a base text direction. However,
when interpreting human-readable text, each human-readable
string value MUST be processed
independently. In other words, a TD Processor
cannot carry forward changes in direction from one string
to another, or infer direction for one string from another
one elsewhere in the TD.
A TD snippet using title
and
description
is shown below. The default
language is set to en
through the definition
of the @language
member within a JSON object
in the @context
array.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{ "@language" : "en" }
],
"title": "MyThing",
"description": "Human readable information.",
...
"properties": {
"on": {
"title": "On/Off",
"type": "boolean",
"forms": [...]
},
"status": {
"title": "Status",
"type": "object",
...
"forms": [...]
}
},
...
}
Strings on the Web [STRING-META]
recommends the use of metadata to determine the
base
direction of string values. Given that
the Thing Description format is based on JSON-LD 1.1
[json-ld11],
@direction
with the string values
"ltr"
, "rtl"
and null value
null
MAY be used
inside the @context
to indicate the default
text direction for the human readable strings in the entire
TD document. When metadata such as
@direction
is not present, TD Consumers
SHOULD use
first-strong detection as a fallback.
For the MultiLanguage
Map, TD Consumers MAY infer the base
direction from the language tag of the individual
strings. The example below illustrates the use of
the @direction
term. See [json-ld11] and
[string-meta]
for more detailed information.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"@language": "ar-EG",
"@direction": "rtl"
}
],
"title": "شيء يخصني يقيس درجة الحرارة",
"description":"شيء يقيس درجة الحرارة و يظهر حالته",
...
"properties": {
"temp": {
"title": "درجة الحرارة",
"type": "boolean",
"forms": [...]
},
"status": {
"title": "حالة",
"type": "object",
...
"forms": [...]
}
},
...
}
The JSON members named titles
and
descriptions
are used within the TD document
to provide human-readable metadata in multiple languages
within a single TD document. All name-value
pairs of a MultiLanguage
Map MUST be serialized as members of a JSON
object, where the name is a valid language tag as defined
by [BCP47] (also
see
W3C I18N Glossary) and the value is a
human-readable string in the language indicated by the
tag. See 5.3.1.7
MultiLanguage
for details. All
MultiLanguage
object within a TD document
SHOULD contain the same set of
language members.
A TD snippet using titles
and
descriptions
at different levels is given
below:
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"title": "MyThing",
"titles": {
"en":"MyThing",
"de": "MeinDing",
"ja": "私の物",
"zh-Hans": "我的东西",
"zh-Hant": "我的東西"
},
"descriptions": {
"en":"Human readable information.",
"de": "Menschenlesbare Informationen.",
"ja": "人間が読むことができる情報",
"zh-Hans": "人们可阅读的信息",
"zh-Hant": "人們可閱讀的資訊"
},
...
"properties": {
"on": {
"titles": {
"en": "On/Off",
"de": "An/Aus",
"ja": "オンオフ",
"zh-Hans": "开关",
"zh-Hant": "開關" },
"type": "boolean",
"forms": [...]
},
"status": {
"titles": {
"en": "Status",
"de": "Zustand",
"ja": "状態",
"zh-Hans": "状态",
"zh-Hant": "狀態" },
"type": "object",
...
"forms": [...]
}
},
...
}
TD instances may also combine the use of
title
and description
with
titles
and descriptions
.
When title
and
titles
or description
and
descriptions
are present within the same JSON
object, the values of title
and
description
MAY be
seen as the default text. When
title
and titles
or
description
and descriptions
are
present in a TD document, each title
and
description
member SHOULD have a corresponding
titles
and descriptions
member,
respectively. The language of the default text is
indicated by the default language, which is usually set by
the creator of the Thing Description instance.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{ "@language" : "de" }
],
"title": "MeinDing",
"titles": {
"en": "MyThing",
"de": "MeinDing",
"ja": "私の物",
"zh-Hans": "我的东西",
"zh-Hant": "我的東西"
},
"description": "Menschenlesbare Informationen.",
"descriptions": {
"en": "Human readable information.",
"de": "Menschenlesbare Informationen.",
"ja": "人間が読むことができる情報",
"zh-Hans": "人们可阅读的信息",
"zh-Hant": "人們可閱讀的資訊"
},
...
"properties": {
"on": {
"title": "An/Aus",
"titles": {
"en": "On/Off",
"de": "An/Aus",
"ja": "オンオフ",
"zh-Hans": "开关",
"zh-Hant": "開關" },
"type": "boolean",
"forms": [...]
},
"status": {
"title": "Zustand",
"titles": {
"en": "Status",
"de": "Zustand",
"ja": "状態",
"zh-Hans": "状态",
"zh-Hant": "狀態" },
"type": "object",
...
"forms": [...]
}
},
...
}
Another possibility to set the default language is
through a language negotiation mechanism, such as the
Accept-Language
header field of HTTP.
In cases where
the default language has been negotiated, an
@language
member MUST
be present to indicate the result of the negotiation and
the corresponding default language of the returned
content. When the
default language has been negotiated successfully, TD
documents SHOULD include the
appropriate matching values for the members
title
and description
in
preference to MultiLanguage
objects in
titles
and descriptions
members. Note
however that Things MAY choose to
not support such dynamically-generated TDs nor to support
language negotiation (e.g., because of resource
constraints).
There is no guarantee that strings in TDs will be displayed in an HTML rendering context. In fact, to mitigate the XSS security risk described in 9.5 Script Injection, HTML tags embedded in strings sourced from TDs should be sanitized (and so not interpreted as HTML) in applications embedding these strings in web pages or web applications. Therefore HTML embedded in strings is not an appropriate mechanism for specifying text rendering direction.
All
name-value pairs of an instance of
VersionInfo
, where the name is a Vocabulary Term included
in the Signature of
VersionInfo
, MUST be
serialized as JSON members with the Vocabulary Term as
name.
A TD snippet of a version information object is given below:
{
...
"version": { "instance": "1.2.1" },
...
}
The version
member is intended as container
for additional application- and/or device-specific version
information based on TD Context Extensions. See
7.1 Semantic Annotations for details.
In a Thing
instance, the value assigned to
securityDefinitions
is a Map of instances of
SecurityScheme
. All name-value pairs
of a Map of
SecurityScheme
instances MUST be serialized as members of the
JSON object that results from serializing the Map; the name of a pair MUST be serialized as a JSON string and
the value of the pair, an instance of
SecurityScheme
, MUST
be serialized as a JSON object.
All name-value pairs of an instance
of one of the Subclasses of
SecurityScheme
, where the name is a Vocabulary Term included
in the Signature of that Subclass or in the
Signature of
SecurityScheme
, MUST
be serialized as members of the JSON object that results
from serializing the SecurityScheme
Subclass's instance, with
the Vocabulary Term as
name.
The following TD snippet shows a simple security
configuration specifying basic username/password
authentication in the header. The value given for
in
is actually the Default Value
(header
) and could be omitted. A named
security configuration (basic_sc
) is given in
the securityDefinitions
map. In this example,
that definition is activated by including its JSON name in
the security
member.
...
"securityDefinitions": {
"basic_sc": {
"scheme": "basic",
"in": "header"
}
},
"security": "basic_sc",
...
Security configuration in the TD is mandatory.
At least one security definition
MUST be activated through the
security
member at the Thing level (i.e., in
the TD root object). This configuration can be seen
as the default security mechanism required to interact with
the Thing.
Security definitions MAY also be activated at the form level by
including a security
member in form objects,
which overrides (i.e., completely replace) all definitions
activated at the Thing level.
The nosec
security scheme is provided for
the case that no security is needed. The minimal security
configuration for a Thing is activation of the
nosec
security scheme at the Thing level, as
shown in the following example:
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"id": "urn:dev:ops:32473-Thing-1234",
"title": "MyThing",
"description": "Human readable information.",
"support": "https://servient.example.com/contact",
"securityDefinitions": { "nosec_sc": { "scheme": "nosec" }},
"security": "nosec_sc",
"properties": {...},
"actions": {...},
"events": {...},
"links": [...]
}
To give a more complex example, suppose we have a
Thing where all
Interaction
Affordances require basic authentication except for
one, for which no authentication is required. For the
status
Property and the toggle
Action, basic
authentication is required and
defined at the Thing level. For the
overheating
Event, however, no
authentication is required, and hence the security
configuration is overridden at the form level.
{
...
"securityDefinitions": {
"basic_sc": {"scheme": "basic"},
"nosec_sc": {"scheme": "nosec"}
},
"security": "basic_sc",
...
"properties": {
"status": {
...
"forms": [{
"href": "https://mylamp.example.com/status"
}]
}
},
"actions": {
"toggle": {
...
"forms": [{
"href": "https://mylamp.example.com/toggle"
}]
}
},
"events": {
"overheating": {
...
"forms": [{
"href": "https://mylamp.example.com/oh",
"security": "nosec_sc"
}]
}
}
}
TDs can specify a combination of security schemes as
well. Below is a TD snippet showing digest authentication
on a proxy combined with bearer token authentication on
the Thing.
In the digest
scheme, the Default Value
of in
(i.e., header
) is
omitted, but still applies. Note that the corresponding
private security configuration such as username/password
and tokens need to be configured in the Consumer to interact
successfully. When activating multiple security
definitions, the security
member becomes an
array.
...
"securityDefinitions": {
"proxy_sc": {
"scheme": "digest",
"proxy": "https://portal.example.com/"
},
"bearer_sc": {
"scheme": "bearer",
"in":"header",
"format": "jwt",
"alg": "ES256",
"authorization": "https://servient.example.com:8443/"
}
},
"security": ["proxy_sc", "bearer_sc"],
...
However, the use of an
array with multiple elements to combine security schemes
in a security
element is now deprecated,
instead a ComboSecurityScheme
SHOULD be used. In the
following example, which is exactly equivalent to the one
above, this is demonstrated:
...
"securityDefinitions": {
"proxy_sc": {
"scheme": "digest",
"proxy": "https://portal.example.com/"
},
"bearer_sc": {
"scheme": "bearer",
"in":"header",
"format": "jwt",
"alg": "ES256",
"authorization": "https://servient.example.com:8443/"
},
"combo_sc": {
"scheme": "combo",
"allOf": ["proxy_sc", "bearer_sc"]
}
},
"security": "combo_sc",
...
Security configurations can also be specified for
different forms within the same Interaction
Affordance. This may be required for devices that
support multiple protocols, for example HTTP and CoAP
[RFC7252],
which support different security mechanisms. This is also
useful when alternative authentication mechanisms are
allowed. Here is a TD snippet demonstrating three
possible ways to activate a Property affordance: via
HTTPS with basic authentication, with digest
authentication, with bearer token authentication. In
other words, the use of different security configurations
within multiple forms provides a way to combine security
mechanisms in an "OR" fashion. In contrast, putting
multiple security configurations in the same
security
member combines them in an "AND"
fashion, since in that case they would all need to be
satisfied to allow activation of the Interaction
Affordance. Note that activating one (default)
configuration at the Thing level is still mandatory.
{
...
"securityDefinitions": {
"basic_sc": { "scheme": "basic" },
"digest_sc": { "scheme": "digest" },
"bearer_sc": { "scheme": "bearer" }
},
"security": "basic_sc",
...
"properties": {
"status": {
...
"forms": [{
"href": "https://mylamp.example.com/status"
}, {
"href": "https://mylamp.example.com/status",
"security": "digest_sc"
}, {
"href": "https://mylamp.example.com/status",
"security": "bearer_sc"
}]
}
},
...
}
To avoid redundancy in this case, e.g. repeating the
details of the form
elements, a ComboSecurityScheme
with oneOf
can be used instead.
{
...
"securityDefinitions": {
"basic_sc": { "scheme": "basic" },
"digest_sc": { "scheme": "digest" },
"bearer_sc": { "scheme": "bearer" },
"combo_sc": {
"scheme": "combo",
"oneOf": [ "basic_sc", "digest_sc", "bearer_sc" ]
}
},
"security": "combo_sc",
...
"properties": {
"status": {
...
"forms": [{
"href": "https://mylamp.example.com/status"
}]
}
},
...
}
As another more complex example, OAuth 2.0 makes use
of scopes. These are identifiers that may appear in
tokens and must match with corresponding identifiers in a
resource to allow access to that resource (or Interaction
Affordance in the case of W3C WoT). For example,
in the following, the status
Property can be
read by Consumers using bearer
tokens containing the scope limited
, but the
configure
Action can only be invoked with a
token containing the special
scope. Scopes
are not identical to roles, but are often associated with
them; for example, perhaps only those in an
administrative role are authorized to perform "special"
interactions. Tokens can have more than one scope and are
issued by dedicated web services to users. In this
example, an administrator could be issued tokens with
both the limited
and special
scopes, while ordinary users could be provided with
tokens with the limited
scope.
{
...
"securityDefinitions": {
"oauth2_sc": {
"scheme": "oauth2",
"flow": "client",
"token": "https://example.com/token",
"scopes": ["limited", "special"]
}
},
"security": "oauth2_sc",
...
"properties": {
"status": {
...
"forms": [{
"href": "https://scopes.example.com/status",
"scopes": ["limited"]
}]
}
},
"actions": {
"configure": {
...
"forms": [{
"href": "https://scopes.example.com/configure",
"scopes": ["special"]
}]
}
},
...
}
A Thing can require an onboarding process that results in the Consumer requiring an API key to interact with the Thing. This API key can be included in the request to the Thing in different ways as the API key scheme specifies. Below is an example of how it can be used as a URI template where the API key should be replaced in the URI by the Consumer when sending an HTTPS request.
{
...
"securityDefinitions": {
"apikey_key": {
"scheme": "apikey",
"in": "uri",
"name": "adminKey"
}
},
"security": "apikey_key",
"properties": {
"status": {
...
"forms": [{
"href": "https://example.com/{adminKey}/status",
...
}]
}
},
...
}
To give another example of the use of the ComboSecurityScheme
in addition to the use of URI templates example shown
above, suppose there is a security scheme where a client
ID and a "secret" key provided by a cloud service
provider must both be embedded in the URL. Technically,
only the key is actually secret and must be handled
out-of-band, and the client ID, which is not secret,
could be embedded in the TD. However, if the client ID
cannot be easily rotated we may want to avoid embedding
it in the TD to enhance privacy. In this case we can
combine two instances of APIKeySecurityScheme
,
both using the uri
value for the
in
location specifier, to declare two URI
variables. These can then (in fact, they must) be used in
the href
in a Form
where the
security scheme is active. An example follows:
{
...
"securityDefinitions": {
"apikey_key": {
"scheme": "apikey",
"in": "uri",
"name": "secKey"
},
"apikey_id": {
"scheme": "apikey",
"in": "uri",
"name": "secClientID"
},
"apikey_combo": {
"scheme": "combo",
"allOf": ["apikey_key","apikey_id"]
}
},
"security": "apikey_combo",
...
"properties": {
"status": {
...
"forms": [{
"href": "https://example.com/{secClientID}/status/{secKey}",
...
}]
}
},
...
}
While not shown in this example, it is legal to
declare additional URI template variables using
uriVariables
and include them in the same
URI template, although the names cannot conflict with
those declared in security schemes. Using a specific
prefix as in the above example for URI variables declared
in security schemes can make it easier to avoid name
conflicts.
API Key in Body: Security parameters might also
be included along with the payload in some systems. For
example, suppose a system requires every payload to be a
JSON object including a member named auth
whose value is an object containing a member called
key
containing an access key. Depending on
the interaction, however, other elements of the JSON
object might vary. This situation can be dealt with using
the body
security information location. Note
that for this location, the name
parameter
is actually a JSON pointer evaluated relative to the root
of the DataSchema
for each interaction it is
bound with, which allows it to be used with payloads that
vary in other respects. As an example, here is a light
that has a property to set its brightness and color and
two separate actions to turn it on and off. Although the
JSON payloads are different for these actions the
/auth/key
element occurs in the same
relative location so single JSON pointer can be used.
Note: if the security key occurs in different
inconsistent locations, it will be necessary to use
multiple security scheme definitions.
{
...
"securityDefinitions": {
"apikey_body": {
"scheme": "apikey",
"in": "body",
"name": "/auth/key"
}
},
"security": "apikey_body",
...
"properties": {
"color": {
...
"type": "object",
"properties": {
"brightness": {
"type": "number",
...
},
"rgb": {
"type": "array",
...
},
"auth": {
"type": "object",
"properties": {
"key": {
"type": "string"
}
},
"required": ["key"]
}
},
"required": ["brightness", "rgb", "auth"],
"forms": [{
"href": "https://example.com/color",
...
}]
}
},
"action": {
"on": {
...
"input": {
"auth": {
"type": "object",
"properties": {
"key": {
"type": "string"
}
},
"required": ["key"]
}
},
"required": ["auth"],
"forms": [{
"href": "https://example.com/on",
...
}]
},
"off": {
...
"input": {
"auth": {
"type": "object",
"properties": {
"key": {
"type": "string"
}
},
"required": ["key"]
}
},
"required": ["auth"],
"forms": [{
"href": "https://example.com/off",
...
}]
}
},
...
}
body
location will be automatically inserted
if it does not exist. In this case the above example can
be simplified to the following. Note that in fact a data
schema will effectively be created for the
actions on
and off
to hold just
the security information.
{
...
"securityDefinitions": {
"apikey_body": {
"scheme": "apikey",
"in": "body",
"name": "/auth/key"
}
},
"security": "apikey_body",
...
"properties": {
"color": {
...
"type": "object",
"properties": {
"brightness": {
"type": "number",
...
},
"rgb": {
"type": "array",
...
}
},
"required": ["brightness", "rgb"],
"forms": [{
"href": "https://example.com/color",
...
}]
}
},
"action": {
"on": {
...
"required": ["auth"],
"forms": [{
"href": "https://example.com/on",
...
}]
},
"off": {
...
"forms": [{
"href": "https://example.com/off",
...
}]
}
},
...
}
The value assigned to properties
in a
Thing
instance is a Map of instances of
PropertyAffordance
. All name-value pairs
of a Map of
PropertyAffordance
instances MUST be serialized as members of the
JSON object that results from serializing the Map; the name of a pair MUST be serialized as a JSON string and
the value of the pair, an instance of
PropertyAffordance
, MUST be serialized as a JSON
object.
All name-value pairs of an instance of
PropertyAffordance
, where the name is a
Vocabulary Term included in
(one of) the Signatures of
PropertyAffordance
,
InteractionAffordance
, or
DataSchema
, MUST be
serialized as members of the JSON object that results from
serializing the PropertyAffordance
instance,
with the Vocabulary Term as
name. See 6.3.10 Data
Schemas for details on serializing DataSchema
instances.
The value assigned to
forms
in an instance of
PropertyAffordance
MUST be serialized as a JSON array
containing one or more JSON object serializations as
defined in 6.3.9
forms
.
A snippet for two Property affordances is given below:
In a Thing
instance, the value assigned to
actions
is a Map of instances of
ActionAffordance
. All name-value pairs of
a Map of
ActionAffordance
instances MUST be serialized as members of the
JSON object that results from serializing the Map; the name of a pair MUST be serialized as a JSON string and
the value of the pair, an instance of
ActionAffordance
, MUST be serialized as a JSON
object.
All
name-value pairs of an instance of
ActionAffordance
, where the name is a Vocabulary Term included
in (one of) the Signatures of
ActionAffordance
or
InteractionAffordance
, MUST be serialized as members of the JSON
object that results from serializing the
ActionAffordance
instance, with the Vocabulary Term as
name.
The values assigned to
input
and output
in an instance
of ActionAffordance
MUST be serialized as JSON objects.
They rely on the Class
DataSchema
, whose serialization
is defined in 6.3.10 Data
Schemas.
The value assigned to forms
in an instance of ActionAffordance
MUST be serialized as a JSON array
containing one or more JSON object serializations as
defined in 6.3.9
forms
.
A TD snippet of an Action affordance is given below:
In a Thing
instance, the value assigned to
events
is a map of instances of
EventAffordance
. All name-value pairs of
a Map of
EventAffordance
instances MUST be serialized as members of the
JSON object that results from serializing the Map; the name of a pair MUST be serialized as a JSON string and
the value of the pair, an instance of
EventAffordance
, MUST
be serialized as a JSON object.
All
name-value pairs of an instance of
EventAffordance
, where the name is a Vocabulary Term included
in (one of) the Signatures of
EventAffordance
or
InteractionAffordance
, MUST be serialized as members of the JSON
object that results from serializing the
EventAffordance
instance, with the Vocabulary Term as
name.
The values assigned to
subscription
, data
, and
cancellation
in an instance of
EventAffordance
MUST
be serialized as JSON objects. They rely on the
Class DataSchema
,
whose serialization is defined in 6.3.10 Data
Schemas.
The
value assigned to forms
in an instance of
EventAffordance
MUST
be serialized as a JSON array containing one or more JSON
object serializations as defined in 6.3.9 forms
.
A TD snippet of an Event object is given below:
Event affordances have been defined in a flexible
manner, in order to adopt existing (e.g., WebSub
[websub]) or
customer-oriented event mechanisms (e.g., Webhooks). For
this reason, subscription
and
cancellation
can be defined according to the
desired mechanism. Please find further details in
[WOT-BINDING-TEMPLATES].
Example A.3 Webhook Event
Example illustrates how Events can use
subscription
and cancellation
to
describe Webhooks.
All
name-value pairs of an instance of Link
, where
the name is a Vocabulary Term included in
the Signature of
Link
, MUST be
serialized as members of the JSON object that results from
serializing the Link
instance, with the
Vocabulary Term as
name.
It is recommended to follow the link relation values as
provided in Section 5.3.4.1
Link
. The examples provided below
demonstrate the use of different link relation types.
A reference can be provided that points to a Thing (e.g., a controller) that
controls the underlying unit (e.g., a lamp). For this
controlledBy
can be used:
To point to a developer documentation of a Thing the value
service-doc
can be used:
...
"links": [{
"rel": "service-doc",
"href": "https://example.com/howTo",
"type": "application/pdf",
"hreflang" : "en"
}]
...
A superordinate Thing can collect a group of Things and
refer to them by using the item
value:
"title" : "Electric Drive",
...
"links": [{
"rel": "item",
"href": "coaps://motor1.example.com",
"type": " application/td+json"
},
{
"rel": "item",
"href": "coaps://motor2.example.com",
"type": " application/td+json"
}
]
...
A Thing refers to a group in which it is collected with
the collection
value:
"title" : "Electric Motor 1",
"base": "coaps://motor1.example.com",
...
"links": [{
"rel": "collection",
"href": "coaps://drive.example.com",
"type": " application/td+json"
}]
...
All
name-value pairs of an instance of Form
, where
the name is a Vocabulary Term included in
the Signature of
Form
, MUST be
serialized as members of the JSON object that results from
serializing the Form
instance, with the
Vocabulary Term as
name.
If required, form objects MAY be supplemented with protocol-specific Vocabulary Terms identified with a prefix. See also 8.3 Protocol Bindings.
A TD snippet of a form object in the forms
array is given below:
href
may also carry a URI that contains
dynamic variables such as lat
and
lon
in
http://example.org/weather/?lat=35&lon=139
.
In that case the URI can be defined as template as
defined in [RFC6570]:
http://example.org/weather/{?lat,long}
.
In such a case, the URI Template
variables MUST be collected in
the JSON-object based uriVariables
member
either in the Thing level or in Interaction Affordance
level with the associated (unique) variable names as JSON
names.
The serialization of each
value in the map assigned to uriVariables
in
an instance of Form
MUST rely on the Class DataSchema
, whose
serialization is defined in 6.3.10 Data
Schemas.
A TD snippet using a URI Template for query parameters
and uriVariables
in the Interaction
Affordance level is given below:
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
...
"properties": {
"weather": {
...
"uriVariables": {
"lat": {
"type": "number",
"minimum": 0,
"maximum": 90,
"description": "Latitude for the desired location in the world" },
"long": {
"type": "number",
"minimum": -180,
"maximum": 180,
"description": "Longitude for the desired location in the world" }
},
"forms": [{
"href": "http://example.org/weather/{?lat,long}",
"htv:methodName": "GET"
}]
},
...
},
...
}
Alternatively, as defined in [RFC6570],
uriVariables
can be used for replacing the
href
structure. An example TD is provided
below where a valid request to get the forecast of
Bogota, Colombia would be an HTTP GET request to
http://example.org/weather/bogota
:
{
"@context": "http://www.w3.org/ns/td",
...
"properties": {
"weather": {
...
"uriVariables": {
"city": {
"type": "string",
"description": "City name to find the weather information for"
}
},
"forms": [{
"href": "http://example.org/weather/{city}",
"htv:methodName": "GET"
}]
},
...
},
...
}
The two examples below can be also combined, while
using the same uriVariables
feature. An HTTP
GET request to
http://example.org/weather/bogota/?unit=Celsius
can be described as follows:
{
"@context": "http://www.w3.org/ns/td",
...
"properties": {
"weather": {
...
"uriVariables": {
"city": {
"type": "string",
"description": "City name to find the weather information for"
},
"unit": {
"type": "string",
"enum":["fahrenheit_value","celsius_value"],
"description": "Desired unit for the temperature value"
}
},
"forms": [{
"href": "http://example.org/weather/{city}/{?unit}",
"htv:methodName": "GET"
}]
},
...
},
...
}
uriVariables
are mainly for properties
and events. When retrofitting an existing system, it may
be necessary to use uriVariables
for
actions. In general, it is recommended to avoid
uriVariables
as much as possible when a new
WoT-based system is designed.
The contentType
member is used to assign
a media type [RFC2046]
including media type parameters as attribute-value pairs
separated by a ;
character. Example:
...
"contentType": "text/plain; charset=utf-8",
...
In some use cases, the form metadata of the Interaction
Affordance not only describes the request, but also
provides metadata for the expected response. For
instance, an Action takePhoto
defines an
input
schema to submit parameter settings of
a camera (aperture priority, timer, etc.) using JSON for
the request payload (i.e., "contentType":
"application/json"
). The output of this action is
the photo taken, which is available in JPEG format, for
example. In such cases, the response
member
is used to indicate the representation format of the
response payload (e.g., "contentType":
"image/jpeg"
). Here no output
schema
is required, as the content type fully specifies the
representation format.
If present, the value assigned
to response
in an instance of
Form
MUST be a JSON
object. If present, the response object
MUST contain a
contentType
member as defined in the
Class definition of
ExpectedResponse
.
A form
snippet with the
response
member is shown below based on the
takePhoto
Action described above:
{
...
"actions": {
"takePhoto": {
...
"forms": [{
"op": "invokeaction",
"href": "http://camera.example.com/api/snapshot",
"contentType": "application/json",
"response": {
"contentType": "image/jpeg"
}
}]
}
},
...
}
In some cases binary data is embedded in text-based
values, e.g., a JSON string-based value embeds a base64
encoded image. The terms contentMediaType
and contentEncoding
can be used to clarify
the context and encoding format of such name-value pairs.
A sample usage of contentMediaType
and
contentEncoding
is shown below:
{
...
"properties": {
"image": {
"description": "Provides latest image",
"type": "string",
"contentMediaType": "image/png",
"contentEncoding": "base64",
"forms": [{
"op": "readproperty",
"href": "coaps://mylamp.example.com/lastPicture",
"cov:methodName": "GET",
"contentType": "application/json"
}]
}
},
...
}
When forms
is present at the top level,
it can be used to describe meta interactions offered by a
Thing. For example,
the operation types readallproperties
and
writeallproperties
are for meta interactions
with a Thing
by which Consumers can read, write
or observe all properties at once. In the example below,
a forms
member is included in the TD root
object and the Consumer can use the
submission target
https://mylamp.example.com/properties
both
to read or write all Properties (i.e., on
,
brightness
, and timer
) of the
Thing in a single
protocol transaction.
{
...
"properties": {
"on": {
"type": "boolean",
"forms": [...]
},
"brightness": {
"type": "number",
"forms": [...]
},
"timer": {
"type": "integer",
"forms": [...]
}
},
...
"forms": [{
"op": "readallproperties",
"href": "https://mylamp.example.com/properties",
"contentType": "application/json",
"htv:methodName": "GET"
},
{
"op": "writeallproperties",
"href": "https://mylamp.example.com/properties",
"contentType": "application/json",
"htv:methodName": "PUT"
}]
}
Thing-level uriVariables
can be used here
to supply further variables to the operation or to
specify a list of Property Affordance names for a
readmultipleproperties
operation. In the
example below, the unit for the properties can be set via
such a variable and the desired list of properties can be
set:
{
...
"properties": {
"temperature": {
"type": "number",
"forms": [...]
},
"brightness": {
"type": "number",
"forms": [...]
},
"humidity": {
"type": "integer",
"forms": [...]
}
},
"uriVariables":{
"propertyNames":{
"type": "string",
"description": "Comma separated list of property names to select."
},
"unitSystem":{
"type": "string",
"enum":["metric_value","imperial_value","uscustomary_value"],
"description":"System of Measurement that will be used for the values"
}
}
"forms": [{
"op": "readallproperties",
"href": "https://mything.example.com/properties{?unitSystem}",
"contentType": "application/json",
"htv:methodName": "GET"
},
{
"op": "readmultipleproperties",
"href": "https://mylamp.example.com/properties{?propertyNames,unitSystem}",
"contentType": "application/json",
"htv:methodName": "GET"
}]
}
For a readmultipleproperties
operation,
an example HTTP GET request to the URI
https://mylamp.example.com/properties?propertyNames=humidity,temperature&unitSystem=metric
would return the values humidity
and
temperature
Property Affordances, with the
metric
System of Measurement.
In the case of operation type
writeallproperties
, it is expected that the
Consumer provides all
writable (non readOnly
) properties and the
(new) assigned values (e.g., within payload). Similarly,
for the writemultipleproperties
operation
type, it is expected that the Consumer provides writable
(non readOnly
) properties. On the Thing side, Thing is expected to return
readable (non writeOnly
) properties in the
case of readmultipleproperties
and
readallproperties
operation types.
The data schemas of the WoT Thing Description defined
through the DataSchema
Class are based on a subset of
the JSON Schema terms [JSON-SCHEMA].
Thus, serializations of the TD data schemas can be fed
directly into JSON Schema validator implementations to
validate the data exchanged with Things.
Data schema serialization applies to
PropertyAffordance
instances, the values
assigned to input
and output
in
ActionAffordance
instances, the values
assigned to subscription
, data
,
and cancellation
in
EventAffordance
instances, and the value
assigned to uriVariables
in instances of
Subclasses of
InteractionAffordance
(when a form object uses a URI
Template).
All
name-value pairs of an instance of one of the Subclasses of
DataSchema
, where the name is a Vocabulary Term included
in the Signature of that Subclass or in the
Signature of
DataSchema
, MUST be
serialized as members of the JSON object that results from
serializing the DataSchema
Subclass's instance, with
the Vocabulary Term as
name.
The value assigned to
properties
in an instance of
ObjectSchema
MUST be
serialized as a JSON object.
The values assigned to
enum
, required
, and
oneOf
in an instance of
DataSchema
MUST be
serialized as a JSON array.
The value assigned to
items
in an instance of
ArraySchema
MUST be
serialized as a JSON object or a JSON array containing JSON
objects.
A TD snippet data schema members is given below. Note
that the surrounding object may be a data schema object
(e.g., for input
and output
) or a
Property object, which would contain additional
members.
The terms readOnly
and
writeOnly
can be used to signal which data
items are exchanged in read interactions (i.e., when
reading a Property) and which in write interactions (i.e.,
when writing a Property). This can be used as a workaround
when Properties of an unconventional Thing exhibit different data for
reading and writing, which can be the case when augmenting
an existing device or service with a Thing Description.
A TD snippet with the usage of readOnly
and
writeOnly
is given below:
...
"properties": {
"status": {
"description": "Read or write On/Off status.",
"type": "object",
"properties": {
"latestStatus": {
"type": "string",
"enum": ["on_value", "off_value"],
"readOnly": true
},
"newStatusValue": {
"type": "string",
"enum": ["on_value", "off_value"],
"writeOnly": true
}
},
forms: [...]
}
}
...
When the status
Property is read, the
status data is returned using a latestStatus
member in the payload. To update the status
Property, the new value must be provided through a
newStatusValue
member in the payload.
As an additional feature, a Thing Description instance
allows the usage of a unit
member within data
schemas. This can be used to associate a unit of measure to
a data item. Its string value can be selected freely.
However, it is recommended to select units defined in
well-known Vocabularies. See 7. TD Context Extensions for an
example.
The JSON-based serialization of Thing Descriptions is
identified by the media type application/td+json
or the CoAP Content-Format ID 432
(see 12.
IANA Considerations).
This section refers to tagging of assertions into different categories for the purposes of validation. This has not yet been done but will be done prior to CR transition.
In several contexts automatic validation of a JSON-based serialization of a Thing Description is useful. Formally, a valid TD satisfies all the assertions in this specification, but not all assertions can be validated given only the JSON serialization, for instance, the assertions listed under 8. Behavioral Assertions that relate a TD to the behavior of a Thing that it describes. Extensions are also problematic, in that even if formal metadata is given for validating an extension, dynamically fetching this metadata in a deployment might pose a privacy risk. In this section, therefore, we name and define various levels of validation appropriate for different contexts.
Minimal Validation is appropriate where validation needs to be self-contained (e.g. devices on isolated networks). It does not attempt to validate context extensions and vocabularies.
In practice, these assertions can be validated using a JSON Schema in combination with a few spot checks, for example to check that security schema names have matching definitions.
This level of validation includes all assertions implied by normative tables in this document.
Basic validation is appropriate in situations where network access is possible and does not pose a privacy risk, and for relatively unconstrained computing requirements. It is suitable for gateways, for example, but not for endpoints, since semantic processing is required. It can validate extensions.
In this case, context definition files and SHACL definitions can be used to validate additional assertions and check TDs for semantic consistency. In addition, if context definitions and SHACL constraints for extension vocabularies can be fetched, then these can be used to validate extensions.
This level of validation includes all those covered by 6.5.1 Minimal Validation.
Full validation confirms that all the assertions in this document are satisfied, including the assertions given in 8. Behavioral Assertions that confirm the TD is consistent with the Thing it describes. This level of validation is appropriate during development, before release, and possibly after installation. Validation during development would have to be on test Things. Actual installation of instances of such Things requires updating the TD with appropriate per-instance identifiers and URLs and so for maximum assurance, in-field validation would have to take place after installation.
This section is non-normative.
In addition to the standard Vocabulary definitions in 5. TD Information Model, the WoT Thing Description offers the possibility to add context knowledge from additional namespaces. This mechanism can be used to enrich the Thing Description instances with additional (e.g., domain-specific) semantics. It can also be used to import additional Protocol Bindings or new security schemes in the future.
For such TD Context
Extensions, the Thing Descriptions use the
@context
mechanism known from JSON-LD
[json-ld11].
When using TD Context
Extensions, the value of @context
of the
Class Thing
is an
Array with additional elements of type anyURI
identifying JSON-LD context files or Map containing namespace IRIs as
defined in 5.3.1.1 Thing
.
The serialization rules for complex types in 6.1 Mapping to JSON Types define the
serialization of an extended @context
name-value
pair. A snippet with TD Context Extensions is given
below:
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"eg": "http://example.org/iot#",
"cov": "http://www.example.org/coap-binding#"
},
"https://schema.org/"
],
...
}
TD
Context Extensions allow for the use of additional
Vocabulary Terms in
a Thing Description instance. If the included namespaces are
based on Class
definitions such as those provided by the RDF Schema or OWL,
they can be used to annotate any Class instance of a Thing
Description semantically by associating the instance to a
such an external Class
definition. This is done by assigning a Class name to the
@type
name-value pair or including Class name in its Array value for multiple
associations/annotations. Following the serialization rules
in 6.1 Mapping to JSON
Types, @type
is either serialized as a JSON
string or as a JSON array. @type
is the JSON-LD
keyword [json-ld11] used to set the type of a
node.
TD Context Extensions also allow the inclusion of additional name-value pairs and well-defined values within any Class instance of a Thing Description. These pairs and values are defined through the included Vocabulary Terms and are serialized as additional members in the corresponding JSON objects or values of existing members, respectively. Examples are additional version metadata for the Thing or units of measure for data items.
The next subsections show some sample usage of different kind of ontologies in Thing Descriptions.
The sample TD snippet below provides additional metadata
terms from different external context files as provided in
@context
. The version information container is
extended by adding additional version information about the
used software (s:softwareVersion
). schema.org is used for providing
serial number and organisation information such as the
company name of the Thing. The SAREF ontology is used to
provide a semantic context of the Thing
(saref:TemperatureSensor
), and for the unit
assignment for the temperature property the
Ontology of Units of Measure (OM) is used.
Please note that these Vocabularies and ontologies are used as examples. Others can be used based on application domain and use case.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"saref": "https://w3id.org/saref#",
"om": "http://www.ontology-of-units-of-measure.org/resource/om-2/",
"schema": "http://schema.org"
}
],
"version": {
"instance": "1.2.1",
"schema:softwareVersion" : "1.0.1"
},
"schema:serialNumber": "4CE0460D0G",
"schema:manufacturer": {"name": "CompanyName"},
...
"@type": "saref:TemperatureSensor",
"properties": {
"temperature": {
"description": "Temperature value of the weather station",
"type": "number",
"minimum": -32.5,
"maximum": 55.2,
"unit": "om:degree_Celsius",
"forms": [...]
},
...
},
...
}
In many cases, TD Context Extensions may be used to annotate pieces of a data schema, to be able to semantically process the state information of the physical world object, which is represented by the data exchanged during an interaction (e.g., in the payload of a response). For example, a semantic description of this state information in RDF can be embedded in the TD Document and pieces of a data schema can be individually annotated as referring to specific parts of that RDF-modeled state of the physical world object.
The TD snippet below uses SAREF to describe the state of
a lamp. The external Vocabulary Term
ssn:forProperty
, taken from SSN, the Semantic
Sensor Network Ontology [VOCAB-SSN],
is being used to link the data schema of the
status
Property with the actual
on/off state of the physical world object.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"saref": "https://w3id.org/saref#",
"ssn": "http://www.w3.org/ns/ssn/"
}
],
"id": "urn:dev:ops:32473-WoTLamp-1234",
"@type": "saref:LightSwitch",
"saref:hasState": {
"@id": "urn:dev:ops:32473-WoTLamp-1234/state",
"@type": "saref:OnOffState"
},
...
"properties": {
"status": {
"ssn:forProperty": "urn:dev:ops:32473-WoTLamp-1234/state",
"type": "string",
"forms": [{"href": "https://mylamp.example.com/status"}]
},
"fullStatus": {
"ssn:forProperty": "urn:dev:ops:32473-WoTLamp-1234/state",
"type": "object",
"properties": {
"statusString": { "type": "string" },
"statusCode": { "type": "number" },
"statusDescription": { "type": "string" }
},
"forms": [{"href": "https://mylamp.example.com/status?full=true"}]
},
...
},
...
}
In Example 2, the state of the
Thing is given by the
status
affordance itself and possible state
changes are given by the toggle
affordance. In
other words, the state of the physical world object
directly provides the Interaction
Affordances of the Thing. This design is
satisfactory for simple cases. In more elaborate cases,
however, several affordances may be available for the same
physical state. In the example above, the
fullStatus
Property provides an
alternative, more verbose representation for the state of
the lamp.
This new subsection is in work in progress. Examples will be updated based on experience of the next PlugFests.
For many use cases like in building, agriculture, or smart city location based data is required. This information can be provided in the Thing Description in different ways and can be relied on different kind of location ontologies (e.g.,[w3c-basic-geo], schema.org) depending on purpose (e.g., indoor, outdoor). Also see [sdw-bp].
The TD snippet below uses lat
and
long
from the [w3c-basic-geo]
ontology to provide static latitude and longitude metadata
at Thing's top level.
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"geo": "http://www.w3.org/2003/01/geo/wgs84_pos#"
}
],
"@type": "Thing",
"geo:lat": "26.58",
"geo:long": "297.83",
...
"properties": {
...
}
In some use cases location based metadata have to be
provided at the interaction level, e.g., as provided as a
Property that returns
the latest longitude
, latitude
,
and elevation
values based on schema.org:
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"schema": "http://schema.org#"
}
],
...
"properties": {
"position": {
"type": "object",
"@type": "schema:GeoCoordinates",
"properties": {
"longitude": { "type": "number" },
"latitude": { "type": "number" },
"elevation": { "type": "number" }
},
"forms": [{"href": "https://robot.example.com/position"}]
},
...
},
...
}
In case a different name is desired for, e.g.,
longitude
, latitude
, and
elevation
in the data model, the
jsonld:context
can be used to link terms to
specific vocabulary from an ontology (also see
[JSON-SCHEMA-ONTOLOGY],
Section 3.3 Defining a JSON-LD context for data
instances):
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"schema": "http://schema.org#"
}
],
...
"properties": {
"position": {
"jsonld:context": {
"schema": "http://schema.org/"
"long": "schema:longitude",
"lat": "schema:latitude",
"height": "schema:elevation"
},
"type": "object",
"properties": {
"long": { "type": "number" },
"lat": { "type": "number" },
"height": { "type": "number" }
}
}
},
...
}
With the TD
Context Extensions in a Thing Description, the
communication metadata can be supplemented or new Protocol
Bindings added through additional Vocabulary Terms
serialized into JSON objects representing a Form
instance. (see also 8.3 Protocol
Bindings).
The following TD example uses a fictional CoAP Protocol
Binding, as no such Protocol Binding is
available at the time of writing this specification. This
TD
Context Extension assumes that there is a CoAP in RDF
vocabulary similar to HTTP Vocabulary in RDF
1.0 [HTTP-in-RDF10] that
is accessible via an example namespace
http://www.example.org/coap-binding#
. The
supplemented cov:methodName
member instructs the
Consumer which CoAP
method has to be applied (e.g., GET
for the CoAP
Method Code 0.01, POST
for the CoAP Method Code
0.02, or iPATCH
for CoAP Method Code 0.07).
Finally, new security schemes that are not included in
5.3.3 Security Vocabulary
Definitions can be imported using the TD Context Extension
mechanism. This example uses a fictional ACE security scheme
based on [RFC9200]
that is, for this example, defined by the namespace at
http://www.example.org/ace-security#
.
Additional security schemes
MUST be Subclasses of the Class SecurityScheme
.
{
@context: [
"https://www.w3.org/2022/wot/td/v1.1",
{
"cov": "http://www.example.org/coap-binding#",
"ace": "http://www.example.org/ace-security#"
}
],
...
"securityDefinitions": {
"ace_sc": {
"scheme": "ace:ACESecurityScheme",
...
"ace:as": "coaps://as.example.com/token",
"ace:audience": "coaps://rs.example.com",
"ace:scopes": ["limited", "special"],
"ace:cnonce": true
}
},
"security": ["ace_sc"],
"properties": {
"status": {
...
"forms": [{
"op": "readproperty",
"href": "coaps://rs.example.com/status",
"contentType": "application/cbor",
"cov:methodName": "GET",
"ace:scopes": ["limited"]
}]
}
},
"actions": {
"configure": {
...
"forms": [{
"op": "invokeaction",
"href": "coaps://rs.example.com/configure",
"contentType": "application/cbor",
"cov:methodName": "POST",
"ace:scopes": ["special"]
}]
}
},
...
}
Note that all security schemes defined in 5.3.3 Security Vocabulary Definitions are already part of the TD context and need not to be included through a TD Context Extension.
The following assertions relate to the behavior of components of a WoT system, as opposed to the representation or information model of the TD. However, note that TDs are descriptive, and may in particular be used to describe pre-existing network interfaces. In these cases, assertions cannot be made that constrain the behavior of such pre-existing interfaces. Instead, the assertions are to be interpreted as constraints on the TD to accurately represent such interfaces.
To enable secure interoperation, security configurations need to accurately reflect the requirements of the Thing:
The data schemas provided in the TD should accurately represent the data payloads returned and accepted by the described Thing in the interactions specified in the TD. In general, Consumers should follow the data schemas strictly, not generating anything not given in the WoT Thing Description, but should accept additional data from the Thing not given explicitly in the WoT Thing Description. In general, Things are described by WoT Thing Descriptions, but Consumers are constrained to follow WoT Thing Descriptions when interacting with Things.
ObjectSchema
and
ArraySchema
(when items
is an
Array of DataSchema
) where there can be
additional properties or items in the data returned. This
behaves as if "additionalProperties":true
or
"additionalItems":true
as defined in
[JSON-SCHEMA].ObjectSchema
and ArraySchema
(when items
is an Array of
DataSchema
) where there can be additional
properties or items in the data returned. This behaves as
if "additionalProperties":true
or
"additionalItems":true
as defined in
[JSON-SCHEMA].A Protocol Binding is the
mapping from an Interaction
Affordance to concrete messages of a specific protocol
such as HTTP [RFC7231],
CoAP [RFC7252],
or MQTT [MQTT].
Protocol Bindings of
Interaction
Affordances are serialized as forms
as
defined in 6.3.9
forms
.
Every form in a WoT Thing Description needs to have a
submission target, given by the href
member, as
indicated in Form. The
URI scheme [RFC3986]
of this submission target indicates what Protocol
Binding the Thing
implements [WOT-ARCHITECTURE].
For instance, if the target starts with http
or
https
, a Consumer can then infer the
Thing implements the Protocol
Binding based on HTTP and it should expect HTTP-specific
terms in the form instance (see next section, 8.3.1 Protocol Binding based on HTTP).
href
member as indicated by the Binding
Templates specification at
Creating a New Protocol Binding.Optimally, the protocols used are listed as a scheme in the IANA registry [IANA-URI-SCHEMES]). This guarantees a unique Protocol Binding assignment. In case the desired protocol is not yet registered with IANA, it is recommended to follow the scheme value of the protocol specifications, if available. In principle, to avoid ambiguity in the identification of the protocol via the scheme, the Protocol Binding document will provide a recommended scheme value to enable unique protocol identification in the context of WoT.
Per default the Thing Description supports the Protocol
Binding based on HTTP by including the HTTP RDF
vocabulary definitions from HTTP Vocabulary in RDF
1.0 [HTTP-in-RDF10].
This vocabulary can be directly used within TD instances by
the usage of the prefix htv
, which points to
http://www.w3.org/2011/http#
. Further details
of Protocol Binding
based on HTTP can be found in [WOT-BINDING-TEMPLATES].
To interact with a Thing that implements the
Protocol Binding
based on HTTP, a Consumer needs to know what
HTTP method to use when submitting a form. In the general
case, a Thing Description can explicitly include a term
indicating the method, i.e., htv:methodName
.
For the sake of conciseness, the Protocol
Binding based on HTTP defines Default Values
for the operation types listed below, which also aims at
convergence of the methods expected by Things (e.g., GET to read, PUT
to write). When no method is indicated in a
form representing an Protocol Binding
based on HTTP, a Default Value MUST be assumed as shown in the following
table.
Vocabulary term | Default value | Context |
---|---|---|
htv:methodName |
GET |
Form with operation type
readproperty ,
readallproperties ,
readmultipleproperties |
htv:methodName |
PUT |
Form with operation type
writeproperty ,
writeallproperties ,
writemultipleproperties |
htv:methodName |
POST |
Form with operation type
invokeaction |
For example, the Example 1 in 1. Introduction does not contain operation types and HTTP methods in the forms. The following Default Values should be assumed for the forms in the Example 1:
In the case of a forms
entry that has
multiple op
values the usage of the
htv:methodName
is not permitted. A TD Processor
will extend the multiple op
values to separate
forms
entries and associates a single
operation with the default assumption. The address
information (e.g. href
) and other metadata are
taken over in the extended version.
The number of Protocol Bindings a Thing can implement is not restricted. Other Protocol Bindings (e.g., for CoAP, MQTT, or OPC UA) are intended to be standardized in separate documents such as a protocol Vocabulary similar to HTTP Vocabulary in RDF 1.0 [HTTP-in-RDF10] or specifications including Default Value definitions. Such protocols can be simply integrated into the TD by the usage of the TD Context Extension mechanism (see 7. TD Context Extensions).
Please refer to [WOT-BINDING-TEMPLATES] for information on how to describe IoT platforms and ecosystems.
In general the security measures taken to protect a WoT system will depend on the threats and attackers that system may face and the value of the assets that need to be protected. A detailed discussion of security (and privacy) considerations for the Web of Things, including a threat model that can be adapted to various circumstances, is presented in the informative document [WOT-SECURITY-GUIDELINES]. Many WoT Things are similar to and use the same technologies as web services. In addition to the specific security considerations below, the security risks and mitigations discussed in guides such as the OWASP Top 10 [OWASP-Top-10] for web services should be evaluated, and if applicable, addressed. This section discusses only security risks and possible mitigations directly relevant to the WoT Thing Description.
A WoT Thing Description can describe both secure and insecure network interfaces. When a Thing Description is retro-fitted to an existing network interface, no change in the security status of the network interface is to be expected.
The use of a WoT Thing Description introduces the security risks given in the following sections. After each risk, we suggest some possible mitigations.
Intercepting and tampering with TDs can be used to launch man-in-the-middle attacks, for example by rewriting URLs in TDs to redirect accesses to a malicious intermediary that can capture or manipulate data.
Intercepting and tampering with context definition files can be used to facilitate attacks by modifying the interpretation of vocabulary. Context extensions (see 7. TD Context Extensions) that are loaded from the Web over non-secure connections, such as HTTP, run the risk of being altered by an attacker, and may modify the TD Information Model in ways that could compromise security.
As recommended in 10.1 Context Fetching, on constrained implementations context definition files should be pre-installed and managed using a secure software update process and the context URLs only used to identify known contexts, not to fetch them. This consideration therefore applies only when fetching context definition files dynamically is otherwise unavoidable, for example in a directory service supporting general semantic processing.
In some scenarios, it may be desirable to limit the scope and duration of access to a set of Things by some users. For example, if A is visiting B's house, B may want to provide A with temporary and limited access to the garage door opener and car charger so A can use them. The scope however may be limited so that A cannot access certain administrative functions of these Things (for example, to change how long the garage door can remain open, or to change the charging rate). In addition, the access should expire after A is expected to have left, e.g. after one week.
Upgrade informative ACE-OAuth citation to normative RFC9200 citation if it passes.
An attacker with access to a set of TDs, for example those returned by WoT Discovery, may be able to use this information to identify vulnerable devices and plan attacks on them.
auto
security scheme MAY be used if vulnerability scanning is a
concern.Many strings given in TDs, in particular the values
carried in title
/titles
and
description
/descriptions
, are meant
to be human-readable. An application may take such strings
and use them to generate a user interface, for example, a web
dashboard listing a set of available Things with their titles
and descriptions. If such an interface is naively generated
using string substitution, for example inserting the values
of these strings into marked places in a HTML template to
create final HTML, any HTML markup in the original string
will be interpreted in the context of the browser displaying
the dashboard. It is possible for an attacker to embed
scripts in HTML in various ways and have these scripts
executed upon user interaction or even automatically (e.g.
upon page load, or upon an error, which can be done
intentionally). Since the string will be generated by the TD
producer and the dashboard will be generated by a different
origin, this is a form of cross-site-scripting (XSS)
attack.
See RFC 8259,
section 12: JSON should not be parsed as JavaScript using
eval()
. A WoT Thing Description is intended to
be a pure data exchange format for Thing metadata, not for holding
executable content. An (invalid) TD may, however, contain
JavaScript code that, when executed, could have side effects
compromising the security of a system.
eval()
function to be parsed.There are additional code injection risks
discussed in [WOT-DISCOVERY].
Other strings in TDs, such as the values given for
title
and description
, should be
sanitized before being used in templates for SQL, HTML, or
other executable contexts. This risk, however, is
specifically about the Javascript injection risk when
parsing JSON.
JSON-LD processing usually includes the replacement of short terms with longer IRIs [RFC3987]. For this reason, WoT Thing Descriptions may expand considerably when processed using a JSON-LD 1.1 processor and, in the worst case, the resulting data might consume all of the recipient's resources or cause an exploitable buffer overflow.
Privacy risks will depend on the association of Things with identifiable people and both the direct information and the inferred information available from such an association. A detailed discussion of privacy (and security) considerations for the Web of Things, including a threat model that can be adapted to various circumstances, is presented in the informative document [WOT-SECURITY-GUIDELINES]. This section discusses only privacy risks and possible mitigations directly relevant to the WoT Thing Description.
The use of a WoT Thing Description introduces the privacy risks given in the following sections. After each risk, we suggest some possible mitigations.
WoT Thing Descriptions can be evaluated with a JSON-LD 1.1 processor [json-ld11], which typically follows links to remote contexts (i.e., TD context extensions, see 7. TD Context Extensions) automatically, resulting in the transfer of files without the explicit request of the Consumer for each one. If remote contexts are served by third parties, it may allow them to gather usage patterns or similar information leading to disclosure of private information, or information that can be used to infer private information. In the case of the WoT, an attacker can also observe the network traffic produced by such fetches and can use the metadata of the fetch, such as the destination IP address, to infer information about the device, especially if domain-specific vocabularies are used. This is a risk even if the connection is encrypted, and is related to DNS privacy leaks. See also 9.2 Context Interception and Tampering, which is a related security risk which can also be avoided with the following mitigations.
A Thing Description containing an identifier
(id
) may describe a Thing that is associated
with an identifiable person. Such identifiers pose various
risks including tracking. However, if the identifier is also
immutable, then the tracking risk is amplified, since a
device may be sold or given to another person and the known
ID used to track that person.
id
of a Thing when necessary.
Specifically, the id
of a Thing should not be fixed in
hardware. This does, however, conflict with the Linked
Data ideal that identifiers are fixed URIs. In many
circumstances it will be acceptable to only allow updates
to identifiers if a Thing is reinitialized. In
this case as a software entity the old Thing ceases to exist and a
new Thing
is created. This can be sufficient to break a tracking
chain when, for example, a device is sold to a new owner.
Alternatively, if more frequent changes are desired
during the operational phase of a device, a mechanism can
be put into place to notify only authorized users of the
change in identifier when a change is made. Note however
that some classes of devices, e.g., medical devices, may
require immutable IDs by law in some jurisdictions. In
this case extra attention should be paid to secure access
to files, such as Thing Descriptions, containing such
immutable identifiers. It may also be desirable to not
share the "true" immutable identifier in such a case in
the TD whenever possible.
As noted above, the id
member in a TD can
pose a privacy risk. However, even if the id
is
updated as described to mitigate its tracking risk, it may
still be possible to associate a TD with a particular
physical device, and from there to an identifiable person,
through fingerprinting.
Even if a specific device instance cannot be identified through fingerprinting, it may be possible to infer the type of a device from the information in the TD, such as the set of interactions, and use this type to infer private information about an identifiable person, such as a medical condition.
id
can be
omitted. If the Consumer does not need certain interactions
for its use case, they can be omitted. If the Consumer is
not authorized to use certain interactions, they can
likewise be omitted.The value of the id
field of a TD might
become available to entities that do not have access to the
full TD. If the value of the id
contains
embedded metadata, such as the type of the device or the
owner, this could be used to infer personal information.
id
of
a TD SHOULD NOT contain metadata
describing the Thing or from the TD itself.
Any temporary ID generated to
manage TDs, for example an ID for a database or directory
service, SHOULD NOT contain
metadata describing the Thing or from the TD
itself. Using random UUIDs as recommended in
10.5 Globally
Unique Identifiers also mitigates this risk.
Globally unique identifiers pose a privacy risk if a centralized authority is needed to create and distribute them, since then a third party has knowledge of the identifiers.
id
field in TDs is intentionally not
required to be globally unique. There are several
cryptographic mechanisms (e.g. random UUIDs) available to
generate suitable IDs in a distributed fashion that do not
require a central registry. These mechanisms typically have
a very low probability of generating duplicate identifiers,
and this needs to be taken into account in the system
design; for example, by detecting duplicates and
regenerating IDs when necessary. The scope of IDs also does
not need to be global: it is acceptable to use identifiers
that only distinguish Things in a certain context, such as
within a home or factory. TD identifiers SHOULD be generated using a distributed
mechanism such as UUIDs that provides a high probability of
uniqueness. TD identifiers SHOULD NOT be generated using a centralized
authority.In many locales, in order to protect the privacy of users, there are legal requirements for the handling of personally identifiable information, that is, information that can be associated with a particular person. Such information can of course be generated by IoT devices directly. However, the existence and metadata of IoT devices (the kind of data stored in a Thing Description) can also contain or be used to infer personally identifiable information. This information can be as simple as the fact that a certain person owns a certain type of device, which can lead to additional inferences about that person.
The following section has its origin in [wot-thing-description], Annex C. Here Thing Description Template is renamed to Thing Model, but keeps the same intention. For this version of the specification, Thing Model and its model features (e.g., extensions, referencing, obligations, placeholder) are formal introduced. For Thing Model, an own content type is under discussion. Please note this section is in work in progress.
The figure below illustrates the relation of the Thing Model and Thing Description. A Thing Model mainly describes interaction affordances such as the Properties, Actions, and Events and common metadata. This kind of template should be valid and followed for all instantiated Thing Descriptions that are relied on this Thing Model. This paradigm can be compared with abstract class or interface definition (~Thing Model) in object-oriented programming to create objects (~Thing Descriptions).
The Thing Model is a logical description of the interface and possible interaction with Thing's Properties, Actions, and Events, however it does not contain Thing instance-specific information, such as concrete protocol usage (e.g., IP address), or even a serial number and GPS location. However, Thing Models allows to include, e.g., security schemes if they apply to the entire class of instances the model describes. They might have URLs (e.g., like token servers) that might need to be omitted or parameterized (with templates) although in a lot of cases these might also be given.
Thing Model can be serialized in the same JSON-based format as a Thing Description which also allows JSON-LD processing. Note that a Thing Model cannot be validated in the same way as Thing Description instances due to some missing mandatory terms. You can use the JSON Schema in the GitHub repository to validate TM instances that are serialized as JSON.
The link for the TM needs to be updated to a permanent one before publication.
A Thing
Model is recognized by the top level @type
.
Thing Model definitions
MUST use the keyword
@type
at top level and a value of type string or
array that equals or respectively contains
tm:ThingModel
. Additionally,
in order to identify it as a JSON-LD document, Thing Model definitions
MUST use the keyword
@context
at top level with same rules as a Thing
Description. The prefix tm
is defined
within Thing Descriptions'
context and points to the Thing Model namespace as
defined in 4. Namespaces. It is intended that
vocabulary from the tm
context only be used in
Thing
Model definitions and are removed or replaced when
Thing Descriptions are
generated (also see 11.4 Derivation of
Thing Description Instances).
A Thing Model MAY NOT contain instance specific Protocol Binding and security information such as endpoint addresses. Consequently, Thing Model definitions will also be valid if there are no JSON members like forms, base, securityDefinitions, and security. Thing Models are also valid even if these JSON members are used (e.g., as template), however, the nested mandatory members like href are omitted.
Example 3 shows a valid sample lamp Thing Model without any protocol and security information.
In the context of Thing Model definitions specific features are introduced that can be used for Thing modelling.
When
the Thing
Model definitions change over time, this SHOULD be reflected in the version
container. The string-based term model
is used within the version
container to
provide such versioning information, like [SEMVER]. The
following snippet shows the usage of model
in
a Thing Model instance.
{
...
"@type" : "tm:ThingModel",
"title": "Lamp Thing Model",
"description": "Lamp Thing Description Model",
"version" : {"model" : "1.0.0" },
...
}
Due to the definition of Thing Model the term
instance
can be omitted within the
version
container.
When Thing Models are updated and have a new version, this may affect other Thing Models that use the extension and import features (see Section 11.3.2 Extension and Import). In some cases it is also useful to reflect a new version in the file name and/or in a corresponding URL to identify the version.
A Thing
Model can extend an existing Thing Model by using the
tm:extends
mechanism announced in the
links
definition: When a Thing Model
extends another Thing Model, at least one
links
entry with
"rel":"tm:extends"
that targets a Thing Model that is be
extended MUST be used. The
Thing
Model will inherit all definitions from the extended
Thing
Model. There is the opportunity to extend the existing
definition with further metadata by providing further JSON
name-value pairs from the existing TD information model
(5. TD Information
Model) or using the context extension concept (7. TD Context Extensions). A Thing Model can also
overwrite existing definitions such as
title(s)
and maximum
etc.. For
this there exist two limitations: A
Thing
Model SHOULD NOT overwrite the
JSON names defined within the properties
,
actions
, and/or events
Map of the extended Thing Model.
Definitions SHOULD
NOT be overwritten in such a way that possible
instance values are no longer valid compared to the origin
extended definitions. Those assertions preserve the
semantics throughout of the extended Thing Model. E.g., it
is not allowed that a "minimum":2
from a
extended Thing
Model can be overwritten with "minimum":0
.
Meanwhile, overwriting with "minimum":5
would
work since all instances values will always fulfill the
restrictions of the extended Thing Model (also see
Figure Figure
6 for further
explanation).
Lets assume we have a basic model description as provided in the following example:
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Basic On/Off Thing Model",
"properties": {
"onOff": {
"type": "boolean"
}
}
}
Now a new device class model called 'Smart Lamp Control'
that will be used as template for creating TD instances is
designed. This model will reuse the existing definition of
the 'Basic On/Off Thing Model' and extend it with a
dim
property:
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Smart Lamp Control with Dimming",
"links" : [{
"rel": "tm:extends",
"href": "http://example.com/BasicOnOffTM",
"type": "application/td+json"
}],
"properties" : {
"dim" : {
"title": "Dimming level"
"type": "integer",
"minimum": 0,
"maximum": 100
}
}
}
Please note that the title
is overwritten
and will be used when TD instances are created (also see in
the next subsection 11.4 Derivation of
Thing Description Instances).
The tm:extends
feature only permits
inheriting all definitions of one Thing Model. In many
use cases, however, it is desired only to import pieces of
definitions of one or more existing Thing Models.
For
importing pieces of definitions of one or more existing
Thing
Models, the tm:ref
term is introduced that
provides the location of an existing (sub-)definition that
SHOULD be reused.
The
tm:ref
value MUST
follow the pattern
#
character, and followed bytm:ref
is used, the referenced
pre-definition and its dependencies (e.g., by context
extension) MUST be assumed at the
new defined definition.
Portions of the tm:ref
value
might contain non-ASCII characters that require URL
("percent") encoding before use. Before applying escapes
to a tm:ref
value, implementations should
check that the value is not already encoded.
The following example shows a new TM definition that
imports the existing definition of the property
onOff
from Example
53 into the new property definition
switch
.
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Smart Lamp Control",
"properties" : {
"switch" : {
"tm:ref" :"http://example.com/BasicOnOffTM.tm.jsonld#/properties/onOff"
}
}
}
At the place the "tm:ref" is defined, additional
name-value pairs can be added. It is also permitted to
override name-value pairs from the referenced definition.
If the intention is to
override an existing JSON name-value pair definition from
tm:ref
, the same JSON name MUST be used at the same level of the
tm:ref
declaration that provides a new
value. The process to overwrite
MUST follow the JSON Merge Patch
algorithm as defined in [RFC7396] where the content of the
referenced definition is patched with the new provided JSON
name-value pairs.
It is noted that the values can also be based on a JSON
object
or array
, or simply be a
null
value. null
would result to
a removal of existing JSON name-value pair in the
target.
Similar to
tm:extends
and to keep the semantic meaning,
definitions SHOULD NOT be
overwritten in such a way that possible instance values are
no longer valid compared to the origin referenced
definition.
The following example shows a new TM definition that
overwrites (maximum
), enhances
(unit
), and removes (title
)
existing definitions from Example 54.
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Smart Lamp Control",
"properties" : {
"dimming" : {
"tm:ref" :"http://example.com/SmartLampControlwithDimming.tm.jsonld#/properties/dim",
"title": null,
"maximum": 80,
"unit" : "%"
}
}
}
Based on the JSON Merge Patch algorithm the
{"title": null,"maximum": 80,"unit" : "%"}
would act as a patch for the referenced origin content
{"title": "Dimming level", "type": "integer",
"minimum": 0, "maximum": 100}
.
The tm:extends
and the import mechanism
based on tm:ref
can also be used at the same
time in a TM definition. The following example extends the
TM from Example 53 and imports the
status
and dim
definitions from
Example
3 and Example 54 respectively.
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Smart Lamp Control",
"links" : [{
"rel": "extends",
"href": "http://example.com/BasicOnOffTM",
"type": "application/td+json"
}],
"properties" : {
"status" : {
"tm:ref" :"http://example.com/LampTM.tm.jsonld#/properties/status"
},
"dimming" : {
"tm:ref" :"http://example.com/LampWithDimmingTM.tm.jsonld#/properties/dim"
}
}
}
The tm:extends
and the import mechanism
based on tm:ref
explicitly supports transitive
extension (a hierarchy of extensions). For example,
assuming there are 3 TMs: "A" which defines a
tm:extends
of the TM "B" which itself defines
a tm:extends
of the TM "C". Consequently, the
"A" TM extends all definitions of both "B" and "C".
Recursive extensions leading
to an infinite loop MUST NOT be
defined.
The following figure summarizes the allowable override
behaviour of the extension and imports TM functions
presented in this section. Three Thing Models use the
tm:ref
or tm:extends
feature to
reuse TM definitions of the Smart Lamp Control Thing Model. The first
Thing
Model imports and overwrites the maximum
value to 120
within the dimmer
property. However, this results in possible instance values
(at runtime) that may not be in the range of the original
dim
definition between 0
and
100
of the dim definition of the Smart Lamp
Control Thing Model. Thus, such a Thing Model definition
is not allowed. The second model overwrites the property
type
value by number
. Again, this
will potentially result in numeric dim
values
that are not accepted by the definition of the origin
dim
type definition (integer) of the Smart
Lamp Control Thing
Model. The last model is defined in a correct way. The
new ranges of dim
produce potential instance
values that are also fulfilled by the original
dim
definition.
In some applications, it is beneficial to reuse existing
Thing
Model definitions and compose them into a new IoT
system. An example would be that a new Smart Ventilator is
designed to consist of two sub/child Thing Model definitions
such as a Ventilation Thing Model that provides
on/off
and adjustRpm
capabilities, and an LED Thing Model that provides
dimmable
and RGB
capabilities.
Such composition can be introduced by the usage of the
links
container. If it is
desired to provide information that a Thing Model consists of
one or more (sub-)Thing Models, the
links
entries MUST
use the "rel":"tm:submodel"
that targets to
the (sub-) Thing
Models. Optionally an
instanceName
MAY be
provided to associate an individual name to the composed
(sub-) Thing
Model. This is useful when multiple similar
Thing
Model definitions are composed and needs to be
distinguished.
Different strategies can be followed to generate
Thing Descriptions
from composed Thing Model definitions.
The default recommendation is to generate from each parent
and sub/child Thing Model a
corresponding Thing Descriptions
(also see 11.4 Derivation of Thing
Description Instances). The composition relation can be
reflected by the collection
and
item
relation types in the links container of
the Thing Descriptions.
An example based on Smart Ventilation is given here:
A single TD can also be generated which contains the
interaction definitions of the top level/parent Thing Model and all
interaction definitions of all sub/child Thing Models.
To avoid name collisions of the
sub/child interaction names it is recommended to rename the
JSON name to the instanceName
followed with
'_'
and the interaction name of the sub/child
Thing
Model. The following example shows a generated
(self-contained) Thing Description of
the Smart Ventilator Thing Model.
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"title": "Smart Ventilator",
"securityDefinitions": {
"basic_sc": {
"scheme": "basic",
"in": "header"
}
},
"security": "basic_sc",
"links": [
{
"rel": "type",
"href": "./SmartVentilator.tm.jsonld",
"type": "application/tm+json"
}
],
"properties": {
"status": {
"type": "string",
"enum": [
"on_value",
"off_value",
"error_value"
],
"forms": [
{
"href": "http://127.0.13.232:4563/status"
}
]
},
"ventilation_switch": {
"type": "boolean",
"description": "True=On; False=Off",
"forms": [
{
"href": "http://127.0.13.212:4563/switch"
}
]
},
"ventilation_adjustRpm": {
"type": "number",
"minimum": 200,
"maximum": 1200,
"forms": [
{
"href": "http://127.0.13.212:4563/adjustRpm"
}
]
},
"led_R": {
"type": "number",
"description": "Red color",
"forms": [
{
"href": "http://127.0.13.211:4563/R"
}
]
},
"led_G": {
"type": "number",
"description": "Green color",
"forms": [
{
"href": "http://127.0.13.211:4563/G"
}
]
},
"led_B": {
"type": "number",
"description": "Blue color",
"forms": [
{
"href": "http://127.0.13.211:4563/B"
}
]
}
},
"actions": {
"led_fadeIn": {
"title": "fadeIn",
"input": {
"type": "number",
"description": "fadeIn in ms"
},
"forms": [
{
"href": "http://127.0.13.211:4563/fadeIn"
}
]
},
"led_fadeOut": {
"title": "fadeOut",
"input": {
"type": "number",
"description": "fadeOut in ms"
},
"forms": [
{
"href": "http://127.0.13.211:4563/fadeOut"
}
]
}
}
}
In some cases it is desirable to enforce which
interaction affordances are mandatory and have to be
implemented in a Thing Description
instance or can be always expected by the Thing Model.
To
guarantee the implementation of particular kinds of
interaction models, Thing Model definitions MUST use the JSON member name
tm:required
. tm:required
MUST be a JSON array at the top
level. The value of
tm:required
MUST
provide JSON Pointer [RFC6901]
references to the required interaction model
definitions. The JSON Pointers of
tm:required
MUST
resolve to an entire interaction affordance Map
definition.
The following sample shows the usage of
tm:required
for the Property interaction
status
and Action interaction
toggle
.
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"title": "Lamp Thing Model",
"description": "Lamp Thing Description Model",
"tm:required": [
"#/properties/status",
"#/actions/toggle"
],
"properties": {
"status": {
"description": "current status of the lamp (on|off)",
"type": "string",
"readOnly": true
}
},
"actions": {
"toggle": {
"description": "Turn the lamp on or off"
}
},
"events": {
"overheating": {
"description": "Lamp reaches a critical temperature (overheating)",
"data": {"type": "string"}
}
}
}
Since the Event
overheating
is not mandatory it may not be
available in a Thing Description
instance.
A Thing
Model can specify which terms should be used in a TD
instance, but their values are unspecific and are first
known during TD instantiation. In a case
where TD instance terms, but not their values, are known in
advance, the placeholder labeling MAY be used in a Thing Model.
The placeholder labeling
MUST be substituted with a
concrete value (e.g., as JSON number, JSON string, JSON
object, etc) when TD instance is created from the Thing
Model. The string-based pattern of the
placeholder MUST follow a valid
pattern based on the regular expression {{2}[
-~]+}{2}
(e.g.,
{{
PLACEHOLDER_IDENTIFIER
}}
).
The characters between {{
and }}
are used as identifier name of the placeholder. The
identifier name can be used to identify the placeholder for
the substitution process. A placeholder
MUST be applied within the value
of the JSON name-value pair. If a non
string-based value of a JSON name-value pair has a
placeholder, the value MUST be
(temporarily) typed as string. After replacing the
placeholder, e.g. when creating a Thing Description
instance, the original type is applied with the
corresponding replaced value.
The following Thing Model example defines different placeholders. The placeholder map is used to apply the replacement and to transform the intended value type.
Thing Models can be used as templates to generate a Thing Description based on the restrictions defined in Sections 5. TD Information Model and 6. TD Representation Format. During this process missing data such as communication and security metadata have to be complemented to create valid Thing Description instances. A Thing Model MUST be defined in such a way that there are no inconsistencies that would result in a Thing Description not being able to meet the requirements as described in Section 5. TD Information Model and 6. TD Representation Format. A TM-to-TD generator to derive a Thing Description instance from a Thing Model transforms it to a Partial TD using the following steps:
links
element entry with
"rel":"tm:extends"
MUST be removed from the current Partial TD
tm:ThingModel
value of the top-level
@type
MUST be removed
in the Partial
TD instance.
tm:required
feature is used based on Section
11.3.4 Required, the
required interactions MUST be
taken over to the Partial TD instance.
securityDefinitions
and
security
and/or 6.3.9 forms
.
It is recommended that the id
value of a
Thing
Model provides a placeholder such as
"id":"urn:example:
{{
RANDOM_ID_PATTERN
}}
"
for the TD generation process. Please avoid including
metadata in the id
pattern.
Thing Description
instances that follow a Thing Model can carry the
information regarding which type of Thing Model is derived.
In this context, the linking concept can be used with
"rel" : "type"
(also see Section 5.3.4.1
Link
), as shown in the following
example:
Please note that a TD can only be an instance of one TM at
a time. That means for Thing Descriptions: The
links
array can use the entry with "rel" :
"type"
a maximum of once. If it is desired to
reflect all relationships to other Things in a Thing
Description, the composition mechanism in TMs can be
considered (see Section 11.3.3
Composition).
The following Thing Model extends the
model as shown in Example 54 and overwrites the
maximum
value of the dim
property
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "tm:ThingModel",
"links" : [{
"rel": "tm:extends",
"href": "http://example.com/SmartControlLampTM",
"type": "application/td+json"
}],
"properties" : {
"dim" : {
"maximum": 200
}
}
}
The expected Thing Description that is derived from this Thing Model would be (with HTTP Binding and basic security applied):
{
"@context": ["https://www.w3.org/2022/wot/td/v1.1"],
"@type" : "Thing",
"title": "Smart Lamp Control",
"securityDefinitions": {
"basic_sc": {"scheme": "basic", "in": "header"}
},
"security": "basic_sc",
"links" : [{
"rel": "type",
"href": "url/to/SmartLampControlModifiedDimTM",
"type": "application/td+json"
}
],
"properties" : {
"onOff : {
"type: "boolean",
"forms": [{"href": "https://smartlamp.example.com/onoff"}]
},
"dim" : {
"type": "integer",
"minimum": 0,
"maximum": 200,
"forms": [{"href": "https://smartlamp.example.com/dim"}]
}
}
}
The cited section allows UTF-16 and UTF-32. We probably should state that only UTF-8 is allowed (8-bit compatible, RFC2045).
Rules for processing both conforming and non-conforming content are defined in this specification.
This has been updated to point to the TD 1.1 specification. Should we note that versions (and TDs vs. TMs) can be distinguished using information interior to the content?
We may want to explictly allow use of JSON
Pointer as a fragment identifier [RFC6901].
There is in fact an assertion requiring them in TMs for
tm:required
. These are internal
references, however, so we might not have to
allow them for external references. By default the json
media type does not support a fragment identifier but
td+json could.
We have been using .td.jsonld and .tm.jsonld in testing. Should we also define these formally? If we are using the same media type for TMs, should we still have a distinct filename extension? If so, should we (also?) define .jsontm for consistency? Note: the IANA Considerations for HTML allows multiple suffixes, html and htm.
Should the contact be updated?
Rules for processing both conforming and non-conforming content are defined in this specification.
IANA assigns compact CoAP Content-Format IDs for media types in the CoAP Content-Formats subregistry within the Constrained RESTful Environments (CoRE) Parameters registry [RFC7252]. The Content-Format ID for WoT Thing Description is 432 and for the WoT Thing Model is - (tbd).
This section is non-normative.
Feature list of the Thing:
{
"@context": [
"https://www.w3.org/2022/wot/td/v1.1",
{
"cov": "http://www.example.org/coap-binding#"
}
],
"id": "urn:dev:ops:32473-WoTLamp-1234",
"title": "MyLampThing",
"description": "MyLampThing uses JSON serialization",
"securityDefinitions": {"psk_sc":{"scheme": "psk"}},
"security": ["psk_sc"],
"properties": {
"status": {
"description": "Shows the current status of the lamp",
"type": "string",
"forms": [{
"op": "readproperty",
"href": "coaps://mylamp.example.com/status",
"cov:methodName": "GET"
}]
}
},
"actions": {
"toggle": {
"description": "Turn on or off the lamp",
"forms": [{
"href": "coaps://mylamp.example.com/toggle",
"cov:methodName": "POST"
}]
}
},
"events": {
"overheating": {
"description": "Lamp reaches a critical temperature (overheating)",
"data": {"type": "string"},
"forms": [{
"href": "coaps://mylamp.example.com/oh",
"cov:methodName": "GET",
"subprotocol": "cov:observe"
}]
}
}
}
Feature list of the Thing:
/illuminance
by the MQTT broker
running behind the address 192.168.1.187:1883.{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"title": "MyIlluminanceSensor",
"id": "urn:dev:ops:32473-WoTIlluminanceSensor-1234",
"securityDefinitions": {"nosec_sc": {"scheme": "nosec"}},
"security": ["nosec_sc"],
"events": {
"illuminance": {
"data":{"type": "integer"},
"forms": [
{
"href": "mqtt://192.168.1.187:1883/illuminance",
"contentType": "text/plain",
"op": "subscribeevent"
}
]
}
}
}
Feature list of the Thing:
temperature
which periodically
pushes the latest temperature value to the Consumer using a Webhook
mechanism, where the Thing sends POST requests to a
callback URI provided by the Consumer. To describe this,
the subscription
member defines a write-only
parameter callbackURL
, which must be
submitted through the subscribeevent
form.
The read-only parameter subscriptionID
is
returned by the subscription. The WebhookThing
will then periodically POST to this callback URI with a
payload defined by data
. To unsubscribe, the
Consumer has to submit the
unsubscribeevent
form with the
subscriptionID
as described in
cancellation
. Alternatively,
uriVariables
approache can be used that
informs the Consumer to include the
subscriptionID
string into the URI that have
to be called with the delete method (see tab 'With
uriVariables'). In such setup, the
cancellation
container can be obmitted. In
general, this example can be further automated by using a
TD
Context Extension to include proper semantic
annotations.
Instead of a periodically POST call of the Thing the consumer may provide a response data with information when the next POST should be provided by the Thing. The WoT WG is currently working on a definition that covers this scenario for the next update of the TD document (W3C TD 1.1 CR).
This section is non-normative.
A JSON Schema [JSON-SCHEMA] document for syntactically validating Thing Description instances serialized in JSON based format is available in the GitHub repository. This JSON Schema does not require the terms with Default Values to be present. Thus, the terms with Default Values are optional. (see also 5.4 Default Value Definitions)
The Thing Description defined by this document
allows for adding external vocabularies by using
@context
mechanism known from JSON-LD
[json-ld11],
and the terms in those external vocabularies can be used in
addition to the terms defined in 5. TD Information Model. For this reason,
the below JSON schema is intentionally non-strict in that
regard. You can replace the value of
additionalProperties
schema property
true
with false
in different
scopes/levels in order to perform a stricter validation in
case no external vocabularies are used.
The $id
field in the JSON Schemas
need to be updated to a static URL before publication, as
well as the actual link pointing to the schema.
This section is non-normative.
The present specification introduces the TD Information Model as a set of
constraints over different Vocabularies, i.e. sets of
Vocabulary Terms. This
section briefly explains how a machine-readable definition of
these constraints can be integrated into client applications,
by making use of the mandatory @context
of a TD
document.
Accessing the TD Information Model from a TD document is done in two steps. First, clients must retrieve a mapping from JSON strings to IRIs. This mapping is defined as a JSON-LD context, as explained later. Second, clients can access the constraints defined on these IRIs by dereferencing them. Constraints are defined as logical axioms in the RDF format, readily interpretable by client programs.
All Vocabulary
Terms referenced in 5. TD Information
Model are serialized as (compact) JSON strings in a TD
document. However, each of these terms is unambiguously
identified by a full IRI, as per the first Linked Data
principle [LINKED-DATA]. The
mappings from JSON keys to IRIs is what the
@context
value of a TD points to. For instance,
the file at
https://www.w3.org/2022/wot/td/v1.1
includes the following mappings (among others):
properties |
→ |
https://www.w3.org/2019/wot/td#hasPropertyAffordance |
object |
→ |
https://www.w3.org/2019/wot/json-schema#ObjectSchema |
basic |
→ |
https://www.w3.org/2019/wot/security#BasicSecurityScheme |
href |
→ |
https://www.w3.org/2019/wot/hypermedia#hasTarget |
... |
This JSON file follows the JSON-LD 1.1 syntax
[JSON-LD11].
Numerous JSON-LD libraries can automatically process the
@context
of a TD and expand all the JSON strings
it includes.
Once every Vocabulary Term of a TD is expanded to a IRI, the second step consists in dereferencing this IRI to get fragments of the TD Information Model that refer to that Vocabulary Term. For instance, dereferencing the IRI
https://www.w3.org/2019/wot/json-schema#ObjectSchema
results in an RDF document stating that the term
ObjectSchema
is a Class and more precisely, a
sub-class of DataSchema
. Such logical axioms are
represented in RDF using formalisms of various complexity:
here, sub-class relations are expressed as RDF Schema axioms
[RDF-SCHEMA]. Moreover, these axioms
may be serialized in various formats. Here, they are serialized
in the Turtle format [TURTLE]:
<https://www.w3.org/2019/wot/json-schema#ObjectSchema>
a rdfs:Class .
<https://www.w3.org/2019/wot/json-schema#ObjectSchema>
rdfs:subClassOf <https://www.w3.org/2019/wot/json-schema#DataSchema> .
By default, if a user agent does not perform any content
negotiation, a human-readable HTML documentation is returned
instead of the RDF document. To negotiate content, clients must
include the HTTP header Accept: text/turtle
in
their request.
Thing
the rules for
@context
were clarified by stating that TD
1.1 consumers must accept TD 1.0 TDs.
ActionAffordance
the term
synchronous
was added.
NullSchema
it was clarified that
null
does not mean the absence of a value.
AutoSecurityScheme
with the corresponding
"scheme": "auto"
was added to indicate that
the security parameters are going to be negotiated by the
underlying protocols at runtime.
Link
the vocabulary term
hreflang
was added that to specify the
language of a linked document.
op
keywords.
tm:extends
and the import mechanism based on
tm:ref
supports transitive extension.
application/tm+json
Media Type
Registration was added to provide Thing Model
registration information.
titles
and
descriptions
members that appear in
sub-sections 5.3.1.1
Thing
, 5.3.1.2
InteractionAffordance
, 5.3.2.1 DataSchema
and
5.3.3.1 SecurityScheme
were clarified.
Thing
:
op
member of a Form
were
expanded.forms
term was clarified.@context
has changed in Thing Description 1.1.@context
for Thing Description 1.1
to be able to be consumed by TD 1.0 consumers
are specified.PropertyAffordance
:
ActionAffordance
:
op
member of a Form
were
expanded.DataSchema
:
unit
term
was clarified.contentEncoding
and
contentMediaType
were moved to section
5.3.2.7
StringSchema
.
BasicSecurityScheme
, the
assignment of the in
member was
clarified.
DigestSecurityScheme
, the
assignment of the in
and
qop
member was clarified.
APIKeySecurityScheme
, the
assignment of the in
member was
clarified.
BearerSecurityScheme
, the
assignment of the alg
,
format
and in
member was
clarified.
Link
, a new value
tm:submodel
was added to the table
describing the values used for relation types.
Form
:
op
term was
clarified.Values subscribeallevents
,
unsubscribeallevents
,
queryallactions
,
queryaction
and
cancelaction
were added to the type
definition of op
term.op
term.
PropertyAffordance
class's
observable
member and
AdditionalExpectedResponse
class's
contentType
member.
@context
has changed in Thing Description
1.1.
securityDefinitions
and
security
:
flow
that appears
in Example
18 now uses client
as a
value.
uriVariables
, more examples were
added.
href
semantics.tm:ref
was clarified.Changes from First Public Working Draft 24 November 2020 are described in the Second Public Working Draft.
The editors would like to special thank Matthias Kovatsch (co-editor of TD 1.0), Michael Koster, Michael Lagally, Kazuyuki Ashimura, Ege Korkan, Daniel Peintner, Toru Kawaguchi, María Poveda, Dave Raggett, Kunihiko Toumura, Takeshi Yamada, Ben Francis, Manu Sporny, Klaus Hartke, Addison Phillips, Jose M. Cantera, Tomoaki Mizushima, Soumya Kanti Datta and Benjamin Klotz for providing contributions, guidance and expertise.
Also, many thanks to the W3C staff and all other current and former active Participants of the W3C Web of Things Interest Group (WoT IG) and Working Group (WoT WG) for their support, technical input and suggestions that led to improvements to this document.
Finally, special thanks to Joerg Heuer for leading the WoT IG for 2 years from its inception and guiding the group to come up with the concept of WoT building blocks including the Thing Description.
Temporary ReSpec fix regarding non-listed references: [RFC6068], [RFC3966], [html], [RFC6750], [RFC7519], [RFC7797], [RFC8392], [RFC7516], [LDML], [SEMVER], [RFC7617], [RFC7616]
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