Copyright © 2020 W3C ® ( MIT , ERCIM , Keio , Beihang ). W3C liability , trademark and permissive document license rules apply.
The
present
document
WoT
Profile
Specification
defines
a
WoT
Core
Profile
that
,
which
enables
out
of
the
box
interoperability
among
things
and
devices.
It
Out
of
the
box
interoperability
implies,
that
devices
can
be
integrated
into
various
application
scenarios
without
deep
level
adaptations.
Typically
only
minor
configuration
operations
are
necessary
(such
as
entering
a
network
key,
or
IP
address)
to
use
the
device
in
a
certain
scenario.
These
actions
can
be
done
by
anyone
without
specific
training.
These
rules
are
prescriptive,
to
ensure
that
compliant
implementations
satisfy
the
semantic
guarantees,
that
are
guarantees
implied
by
them.
We
call
this
set
of
rules
a
Profile
.
The
WoT
Profile
Specification
as
defined
in
the
present
this
document
serves
two
purposes:
This
document
incudes
a
binding
of
the
core
data
model
to
HTTP(S)
and
selected
notification
protocols.
The
core
data
model
can
be
bound
to
other
protocols
-
it
is
expected
that
additional
profiles
for
thing
templates
and
bindings
to
other
protocols
(e.g.
MQTT,
CoAP)
will
be
defined
in
the
near
future.
A TD that is compliant to the core profile MUST adhere to both the constraints on the data model and the protocol binding.
Devices that constrain their use of the Thing Description to the WoT Core Profile can interoperate with each other out-of-the-box.
Note that the core profile is not exclusive. Device implementers are free to adopt other features of the thing description that go beyond the constraints of the core profile, however the interoperability guarantees of the core profile hold only for the WoT Core Profile subset.
The W3C WoT Thing Architecture [ wot-architecture ] and WoT Thing Description [ wot-thing-description ] define a powerful description mechanism and a format to describe myriads of very different devices, which may be connected over various protocols. The format is very flexible and open and puts very few normative requirements on devices that implement it.
However, this flexibility de-facto prevents interoperability, since, without additional rules , it allows implementers to make many choices that do not provide guarantees of common behavior between implementations.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. 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 First Public Working Draft. This document is intended to become a W3C Recommendation.
Comments regarding this document are welcome. Please send them to public-wot-wg@w3.org ( archives ).
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership.
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 1 August 2017 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 15 September 2020 W3C Process Document .
The W3C WoT Architecture [ wot-architecture ] and the WoT Thing Description [ wot-thing-description ] have been developed as a versatile format, that allows describing the interactions between multiple devices and protocols.
This flexibility permits an easy integration of new device types and protocols, however it risks interoperability, since there are no guarantees that two devices which are formally spec-compliant, will be able to communicate.
To increase adoption of the WoT specifications, interoperability between on premise devices, edge devices and the cloud is essential. Even if every manufacturer is implementing the current Thing Description specification in full flexibility, there is no interoperability guarantee; many choices are still left to the implementations and there are very few normative requirements that a device has to fulfill.
A Thing Description can be used in two fundamentally different deployment scenarios:
For green field deployments, where the implementations are being carried out and corresponding thing descriptions are being created, it is easier to achieve full interoperability by using a small, extensible Core Profile .
In the brown field area, due to the nature of existing deployments and protocols, a broad spectrum of variations and potentially high complexity of thing descriptions inhibits interoperability and will most likely lead to additional profiles of the WoT Thing Description and domain-specific thing consumer implementations.
The WoT Core Profile can be used by green field deployments and gives guidance to new implementers of the WoT specifications. It has already proved in brown-field scenarios in the PlugFests, where existing devices, that already existed as products, prototypes or demonstrators, were described with Thing Descriptions that are constrained to the Core Profile .
During the recent WoT PlugFests there were many de-facto agreements on the use of a small constrained subset of interaction patterns and protocol choices. These de-facto agreements select a common subset of the WoT Thing Description , based on proven interoperability among manufacturers.
The aim of this specification is to formalize these agreements by defining a WoT Core Profile based on the choices that were made by the implementers of PlugFest devices.
The WoT Core Profile contains additional normative requirements that MUST be satisfied by devices to be compliant to the profile.
Adoption of the WoT Core Profile will significantly limit the implementation burden of device and cloud implementors.
The WoT Core Profile was defined with the following main goals:
It makes choices on the required metadata fields as well as the supported interactions and protocol endpoints. It introduces some constraints on data schemas for properties and actions which are required for resource constrained devices in real-world deployments. The format does not forbid the use of additional elements of the WoT Thing Description for vendor specific extensions, however this will impact interoperability.
Devices, which implement the Core Profile , are out-of-the-box interoperable with other Core Profile compliant devices. Furthermore, the Core Profile simplifies device validation and compliance testing since a corresponding conformance test suite can be defined.
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 , NOT RECOMMENDED , RECOMMENDED , and SHOULD 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 device or consumer implementation complies with this specification if it follows the normative statements in the present document.
A JSON Schema [ JSON-SCHEMA ] to validate the compliance of a Thing Description with the core profile is provided in Appendix § A. JSON Schema of the Core Profile .
This specification uses the same terminology as the WoT Architecture and Thing Description specifications.
For convenience of the reader, we use the terms keyword and field for the linguistic notion vocabulary term as defined in the Thing Description Specification.
We use the terms device and thing in an interchangeable manner.
This section describes a generic mechanism to define a profile of the WoT Thing Description in a unambiguous way.
The W3C WoT Thing Description specification defines a formal language, i.e. a set of vocabulary terms (keywords), a set of classes that are built from these keywords, and a set of additional rules, that define constraints on permitted values and keyword presence (mandatory / optional) dependent on the context where the keyword is used. In addition the WoT Thing Description specification defines relationships and corresponding cardinalities between these classes.
The WoT Thing Description specification already has some constraints, but there is a wide variety of variations that are left to the interpretation or the discretion of an implementer. The rationale for the Core Profile is not to forbid complex things, rather to enable statements like:
A profile is a set of constraints and rules, which provide additional semantic guarantees that are applied to the WoT Thing Description specification. These constraints define a subset of valid WoT Thing Descriptions by defining additional rules on various aspects of the WoT Thing Description specification.
Constraints on | Rationale | Example |
---|---|---|
vocabulary of Thing Description classes | guaranteed set of metadata fields | Make specific vocabulary terms mandatory, remove others |
class relationships | unambiguous structure | limited cardinality, e.g. only one form per operation per interaction affordance. |
values of vocabulary terms | simplified processing | Limit the length of characters per string. Always use arrays, where the spec permits a string or an array of strings. |
data schemas | simplified processing | No arbitrary nested objects or arrays of arrays |
security | reduced implementation effort | Only a restricted set of security mechanisms |
protocol binding | guaranteed protocol semantics | limited protocol(s) and protocol features, Example: predefined mapping of http verbs (GET/PUT) to operation verbs, similar constraints for other protocols. |
These constraints and rules fall into two categories:
These two categories are orthogonal to each other, i.e. a data model that conforms to a profile can be mapped to different protocols. The protocol binding for each protocol may contain additional (protocol-specific) constraints.
A profile is not exclusive, i.e. a thing may conform to multiple profiles. Profiles can build on top of each other or overlap, extended profiles can be built on top of the core profile.
This specification does not put any requirements on the scope and contents of other profiles.
In the present document, we define a Core Profile by defining a Core Data Model and a set of Protocol Binding Rules for selected protocols.
The core data model incorporates the data model defined by chapter 5 of the Thing Description specification. The normative rules defined by that data model are the baseline for the definition of the core data model and are normative for the core data model. A core profile compliant implementation MUST additionally satisfy the requirements of this chapter.
The following rules are applicable to multiple classes of the WoT Thing Description Specification, as they provide clearer semantics, improved readability and simplified processing on resource constrained devices.
One of the primary benefits of the WoT Thing Description over a typical IoT format is the additional documentation for a human reader.
Therefore,
the
fields
title
and
description
are
MANDATORY
for
Things,
Property
Affordances,
Action
Affordances,
Event
Affordances
and
Data
Schemas.
It is possible to have empty values for these fields, if, for specific purposes it is not desired to provide documentation, however this is NOT RECOMMENDED and the conscious decision is obvious from the TD.
The
length
of
id
,
description
and
descriptions
values
is
limited
to
512
characters.
The
length
of
title
and
titles
values
is
limited
to
64
characters.
Where
a
type
permits
using
an
array
of
string
or
a
string
,
an
array
of
string
MUST
be
used.
TODO: decide if multiple types and contexts are required.
In this case the following section could be added:
The
only
exception
to
this
rule
are
@context
and
@type
annotations,
where
both
string
or
array
of
string
MAY
be
used.
Where
a
type
permits
using
an
array
of
DataSchema
or
a
DataSchema
,
an
array
of
DataSchema
MUST
be
used.
All
elements
of
an
enum
MUST
be
either
string
or
number
.
Different
types
in
a
single
enum
are
NOT
PERMITTED.
To provide minimum interoperability, the following metadata fields of a Thing MUST be contained in a compliant Thing Description:
keyword | type | remarks |
---|---|---|
title | string | human readable documentation |
id | urn_type | a globally unique urn of the thing |
description | string | human readable documentation |
created | date | human readable documentation |
modified | date | human readable documentation |
support | urn_type | human readable documentation |
security | array of string | simplified handling |
version | VersionInfo | clear versioning, easy to compare different TDs |
It is RECOMMENDED to use the value "" for strings, where the value cannot be determined.
If a Thing Description is used solely within a company, the email address of the developer SHOULD be used in the support field, if the Thing Description is provided externally, a support email address SHOULD be used.
It will be evaluated whether the profile also recommends some new TD terms that may be introduced in TD 1.1. Currently the following terms are discussed: serialNumber, hardwareRevision, softwareRevision, loc_latitude, loc_longitude loc_altitude, loc_height, and loc_depth. If these, or some of them, are defined in the TD 1.1 model, they may be recommended here in one of the next draft updates.
Data Schemas are used for the values of Properties, Action input and output parameters and Event message payloads. The value of a Data Schema can be a simple type (boolean, integer, number, string) or an instance of a structured type (array and object).
The Core Data Model applies the following constraints and rules to the
DataSchema
class
of
section
5.3.2.1
of
the
WoT
Thing
Description
Specification.
This
section
defines
a
subset
of
the
class
DataSchema
that
can
be
processed
on
resource-constrained
devices.
The
Core
Data
Model
restricts
the
use
of
arrays
and
objects
to
the
top
level
of
Data
Schemas,
i.e.
only
a
one-level
hierarchy
is
permitted.
The
members
of
a
top
level
object
or
array
MUST
NOT
be
array
or
object
types.
This may appear as a severe limitation, however it is motivated by integrating with multiple cloud services. Many enterprise services and applications are based on (relational) databases, where individual property values are stored. Of course databases can also store objects (e.g. encoded as a JSON string), however this will prevent processing by other enterprise applications.
If a property conceptually has a deeper structure, such as grid of lamps with RGB colors, the structure can be represented in the keyword of the property, i.e. lamp1_color_r, lamp1_color_g and lamp1_color_b. A similar mapping can be done for arrays and hierarchical objects. This constraint leads to simpler Thing Descriptions that can be handled by very limited devices.
The following fields MUST be contained in a DataSchema:
keyword | type | constraints |
---|---|---|
description | human readable description | |
type | string | one of boolean, integer, number, string, array or object |
The
values
object
,
array
MAY
only
be
used
at
the
top
level
of
a
Data
Schema.
The
type
value
MUST
NOT
be
null
.
PropertyAffordance
class
of
section
5.3.1.3
of
the
WoT
Thing
Description
Specification.
The
following
property
fields
MUST
be
contained
in
the
properties
element
of
a
Profile
compliant
TD
:
keyword | type | constraints |
---|---|---|
title | string | unique name among all properties |
description | string | human readable description |
type | string |
one
of
boolean
,
string
,
number
,
integer
,
object
or
array
.
The
type
value
null
MUST
NOT
be
used.
|
The
Thing
Description
permits
arbitrary
object
depths
for
properties.
Parsing
of
a
deeply
nested
structure
is
not
possible
on
resource
constrained
devices.
Therefore
each
property
MAY
contain
array
or
object
elements
only
at
top
level
.
The following additional constraints MUST be applied to the Property Affordances of a Thing Description conforming to the Core Profile :
keyword | type | constraint |
---|---|---|
const | anyType | MUST NOT be used |
enum | array of simple type | Values of enums MAY only be simple types. Handling of any type is too complex to implement on resource constrained devices |
forms | array of Forms |
The
Array
of
Form
of
each
property
MUST
contain
only
a
single
endpoint
for
each
operation
readproperty
,
writeproperty
,
observeproperty
,
unobserveproperty
.
|
format | string |
If
the
field
format
is
used,
only
formats
defined
in
section
7.3.1-7.3.6
of
[
JSON-SCHEMA
]
MAY
be
used.
|
oneOf | string |
The
DataSchema
field
oneOf
does
not
make
sense
for
properties
and
MUST
NOT
be
used.
|
uriVariables | Map of DataSchema |
uriVariables
MUST
NOT
be
used.
|
It
is
highly
RECOMMENDED
to
always
specify
a
unit
,
if
a
value
has
a
metric.
Authors
of
Thing
Descriptions
should
be
aware,
that
units
that
are
common
in
their
geographic
region
are
not
globally
applicable
and
may
lead
to
misinterpretation
with
drastic
consequences.
The
field
unit
could
be
used
for
non-decimal
numeric
types
as
well,
e.g.
a
string
value
with
binary
or
hex
data
(
0xCAFEBABE
,
0b01000010
),
where
the
unit
is
hex
or
bin
,
to
indicate
how
the
value
should
be
interpreted.
It
is
strongly
RECOMMENDED
to
use
the
values
hex
,
oct
or
bin
in
this
case
to
achieve
interoperability.
ActionAffordance
class
of
section
5.3.1.4
of
the
WoT
Thing
Description
Specification.
The following fields MUST be contained in an action element of an Core Profile compliant TD:
keyword | type | constraints |
---|---|---|
title | string | unique name among all actions |
input | array of DataSchema | all elements of the subclasses objectSchema and dataSchema MUST only contain simple types. |
output | array of DataSchema | all elements of the subclasses objectSchema and dataSchema MUST only contain simple types. |
The
elements
of
the
DataSchema
subclasses
ArraySchema
and
ObjectSchema
for
the
fields
input
and
output
are
restricted
to
simple
types
in
a
Thing
Description
conforming
to
the
Core
Data
Model
.
Without
this
limitation
a
higher
implementation
burden
would
be
put
on
resource
constrained
devices
(arbitrary
cascaded
arrays
and
multi-level
objects)
which
cannot
be
satisfied
by
all
consuming
devices.
The following additional constraints MUST be applied to the Interaction Affordances of a Thing Description conforming to the Core Data Model :
keyword | type | constraint |
---|---|---|
forms | array of Forms |
The
Array
of
Form
of
each
action
MUST
contain
only
a
single
endpoint.
|
format | string |
If
the
field
format
is
used,
only
formats
defined
in
section
7.3.1-7.3.6
of
[
JSON-SCHEMA
]
MAY
be
used.
|
oneOf | string |
The
DataSchema
field
oneOf
does
not
make
sense
for
properties
and
MUST
NOT
be
used.
|
uriVariables | Map of DataSchema |
uriVariables
MUST
NOT
be
used.
|
TODO:
- no optional parameters
- timeout
EventAffordance
class
of
section
5.3.1.5
of
the
WoT
Thing
Description
Specification.
A Thing may provide more than one event mechanism to enable a variety of consumers.
TODO:
The events section needs to be signifcantly extended and define addtional constraints to ensure OOTBI. LongPoll, WebSockets and WebHooks can be considered as initial candidates for supported protocols for the event mechanism to identify appropriate data model constraints.
The individual protocol constraints need to be defined in a respective protocol binding chapter after they have been identified/evaluated in plugfests.
The following fields MUST be present in an event element of a Core TD :
keyword | type | constraints |
---|---|---|
title | string | unique name among all events |
description | string | human readable description |
data | set of DataSchema instances in a JSON object | only the DataSchema subclasses booleanSchema, IntegerSchema, NumberSchema, StringSchema are permitted |
The following additional constraints MUST be applied to the Event Affordances of a WoT Thing Description conforming to the profile:
keyword | type | constraint |
---|---|---|
forms | array of Forms |
The
Array
of
Form
of
each
event
MUST
contain
only
a
single
endpoint.
|
uriVariables | Map of DataSchema |
uriVariables
MUST
NOT
be
used.
|
A Thing may provide more than one event mechanism to enable a variety of consumers.
The following fields MUST be present in a form element of a Core TD :
keyword | type | constraints |
---|---|---|
title | string | unique name among all events |
description | string | human readable description |
data | set of DataSchema instances in a JSON object | only the DataSchema subclasses booleanSchema, IntegerSchema, NumberSchema, StringSchema are permitted |
The following additional constraints MUST be applied to the Form elements of a WoT Thing Description conforming to the Core profile :
keyword | type | constraint |
---|---|---|
security | string or Array of string |
security
at
form
level
MUST
NOT
be
used.
|
scopes | string or Array of string |
scopes
MUST
NOT
be
used.
|
The "type" relationship as defined in chapter 6 of [ RFC6903 ] is reserved for indicating an instance relationship between a thing and a thing template. The Core Data Model does not put additional constraints or requirements on links. The interpretation of a link is out of scope.
The Core Data Model defines a subset of the security schemes that MAY be implemented on resource constrained devices. A security scheme MUST be defined at the thing level. The security scheme is applied to the thing as a whole, a thing may adopt multiple security schemes.
The set of security schemes supported in the Core Data Model is based on the PlugFest results. To ensure interoperability, a TD consumer, which compliant with the Core Data Model MUST support all of the following security schemes:
When using the "no security" or "Basic Auth" security schemes it is strongly recommended to use transport layer encryption.
This
section
describes
how
the
Core
Data
Model
can
be
is
bound
to
different
protocols.
In
addition
to
a
set
of
mapping
rules,
it
defines
additional
behavior,
e.g.
timeouts,
error
behavior,
action
semantics,
etc.
All communication is using JSON payloads over HTTP(s). The content type header MUST be set to "application/json".
The HTTP verbs GET and PUT are mapped on reading and writing a property - all other protocol verbs return an error "405 Method Not Allowed".
Note: Since HTTP does not provide a pub/sub mechanism, the observe interaction is not supported directly. The event mechanism can be used instead to send notifications on property changes.
Multiple properties can be set/get by accessing the Properties endpoint.
Actions can be synchronous and asynchronous. The current TD specification does not distinguish these two cases and does not describe a detailed mechanism.
The HTTP verb POST is mapped to invoking an action on the actions endpoint - all other protocol verbs return an error "405 Method Not Allowed".
The supported protocols for events are WebHooks, WebSockets, SSE and Long polling.
The default representation is JSON. Semantic annotations based on JSON-LD MAY be present but are not required to perform all interactions with the thing instance.
A Thing Description can be syntactically validated with the JSON Schema [ JSON-SCHEMA ] for compliance with the core profile.
Todo: Define a JSON-SCHEMA.