The WoT Profile Specification defines a Profiling Mechanism and a WoT Core Profile, which enables out of the box interoperability among things and devices. 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.

The WoT Core Profile defines a set of constraints and rules, which compliant thing descriptions have to adopt to guarantee interoperability.

These rules are prescriptive, to ensure that compliant implementations satisfy the semantic guarantees implied by them. We call this set of rules a Profile.

The WoT Profile Specification as defined in this document serves two purposes:

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 name WoT Core Profile is still under discussion in the group and is used as a working title. It is subject to change after the profile specification has reached a certain level of maturity.

Motivation for a Profile

The W3C WoT Thing Architecture [[wot-architecture11]] and WoT Thing Description [[wot-thing-description11]] 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.

Introduction

The W3C WoT Architecture [[wot-architecture11]] and the WoT Thing Description [[wot-thing-description11]] 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.

Deployment Scenarios

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.

Summary of Use Cases and Requirements

A set of over 30 use cases for the Web of Things were contributed by stakeholders from multiple industries for various application domains. These have been published in the WoT Use Cases and Requirements document [[wot-usecases]].

Based on these use cases a set of requirements have been derived which drive the development of the W3C Web of Things specification family. Several of these domains require easy integration of devices from multiple vendors, in other words, out-of-the-box interoperability. However, the descriptive approach taken by the WoT specifications generally leads to a large variety of different protocols and data formats, which can work against out-of-the box interoperability.

For example, a WoT Thing Description (TD) can in theory include a description based on a networking protocol unknown to a device that wishes to connect to it. To ensure interoperability without additional customization (e.g. by writing software or performing complex setup or configuration steps), the range of such choices needs to be limited to a finite set so that a consumer of a Thing Description can be sure it will be able to interact with any possible Thing. A finite set of customization choices is also important for implementing devices with a fixed code base. Defining such constraints leads to the profile mechanism and the WoT Core profile.

In addition to multiple vertical use cases that will use HTTP(S) for their implementations, there are horizontal use cases that are addressed by this profile specification. The primary focus is to enable Multi-Vendor system integration with out of the box interoperability:

Multi-Vendor System Integration - Out of the box interoperability

Users of devices want to process data from all devices that conform to a class of devices in a uniform way. They need a guarantee that they are able to correctly interact with all affordances of the Thing that complies with this class of devices. Different interpretations of the same Thing Description, that lead to different behaviour, should not be possible. Users want to integrate devices in existing scenarios out of the box, i.e. with close to zero configuration tasks.

Why a 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.

WoT-Core-Profile-Picture
WoT Core Profile - A Subset of Affordances

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.

Out-of-the-box 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.

The following classification adopts the terminology as described in the "H2020 – CREATE-IoT Project - Recommendations for commonalities and interoperability profiles of IoT platforms" [[?H2020-CREATE-IoT]] report. The definitions below have been adapted to reflect the scope of the WoT profile.

Technical Interoperability

Technical Interoperability is usually associated with communication protocols and the infrastructure needed for those protocols to operate. This implies agreeing on a common protocol (e.g. HTTP / TCP/IP) and providing additional clarifications, where required.

Syntactic Interoperability

Syntactic Interoperability is usually associated with data formats and encodings along with techniques for compressing them. Examples for these formats and encodings in the WoT are JSON, XML, JSON-LD, UTF-8 payloads.

Semantic Interoperability

Semantic Interoperability is associated with a common understanding of the behavior of communication partners. In the profile context, it includes a common interpretation of (synchronous and asynchronous) action semantics, a common event model, how to set/get multiple properties, writable properties, a common error model and error messages.

Domain specific ontologies, e.g. semantic interop of automotive and medical devices exceed the scope of the profile.

Organisational Interoperability

Organisational Interoperability in the profile context implies that any consumer, which conforms with a given profile can interact with any Thing which conforms with the same profile, without additional customization.

Organisational Interoperability also requires commonly agreed approaches to security, trust and privacy, i.e. a consumer is provided access to Things only, when these common terms and conditions are applied.

Devices created by various engineers, vendors and SDOs that satisfy the requirements of the profile spec can be integrated with compliant consumers without additional customization. This works across infrastructures, regions and cultures.

Structure of this document

to be added.

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 .

Terminology

The fundamental WoT terminology such as Thing, Consumer, Thing Description (TD), WoT Thing Description, 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]].

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.

Additional Definitions

Profile
A set of prescriptive rules, to ensure that compliant implementations satisfy the semantic guarantees that are implied by them.
WoT Core Profile
The subset of the Thing Description defined by the present document.
Core Profile
Synonym for WoT Core Profile.
Core Data Model
A Data Model that conforms to the subset of the Thing Description specification as defined in section .

Profile Requirements

The following requirements were identified by the WoT profile task force and are addressed by the Profile 1.0 specification.

Interoperability

This is the most important objective of the profile. A TD Consumer satisfying the requirements of a profile should be able to process any TD also satisfying the profile and should be able to correctly interact with all affordances of the Thing such a TD describes.

This implies that a profile has three parts:
Note: The WoT HTTP profile does not prevent a TD to have forms for additional protocols, but these can be ignored by compliant consumers, and could be removed without affecting profile comformance and compatibility. This may be useful for consumers that support multiple profiles.

Limit and reduce complexity

Complexity addresses at least the follwing two things to simplify the development and reduce the implementation effort:

No Ambiguities, select single choice

Get rid of ambiguities, i.e. clarify specifications to define interpretation of a TD and behavior of the thing and consumer.

Examples are the choice of properties vs. actions, use of PUT or POST for HTTP, observe protocols.

Developer guidance

A profile should help define what needs to be implemented. This requirement also includes behavioral goals and recommendations about best practice for the implementation of Consumers and Things.

Multiple profiles (mechanism)

The mechanism used to indicate that a TD satisfies a profile should be general enough to indicate the TD satisfies the requirements for multiple profiles.

Composable profiles

It should be possible to combine multiple profiles both for production and consumption:

It should be possible to indicate that a consumer can ingest TDs that satisfy one or more profiles, even if each TDs individually only satisfies one profile. For example, a Smart Building may need to use both "constrained" devices and "unconstrained" devices. A gateway consuming TDs should be able to ingest TDs designed for both the constrained and unconstrained contexts.

A Thing that satisfies all the requirements for multiple TDs (for example, a device using protocols common to two different usage contexts) should be able to indicate that.

Validatible TDs

Whether or not a TD satisfies the requirements of a given profile should be verifiable with automated tools. We can use the existing TD JSDON Schema as a basis and reuse the existing tooling (TD-playground)

Identification of profiles

There should be a mechanism to identify which profiles a TD satisfies. This mechanism should be intrinsic to a TD, i.e. be in-band.

Profile should define a finite set of features and capabilities to implement by the consumer

A profile should limit the number of options, for example the set of possible protocols, to a finite set, so that a consumer can consume any TD in a given profile with a finite and static code base.

Profiling Mechanism

In order to conform with a profile, a Web Thing MUST conform with all the normative statements in the profile's specification.

In order to denote that a given Web Thing conforms to one or more profiles, its Thing Description MUST include a profile member [[wot-thing-description11]]. The value of the profile member MUST be set to either a valid URI [[RFC3986]] identifying a single profile, or an array of valid URIs identifying multiple profiles. 

    {
      "@context": "https://www.w3.org/2019/wot/td/v1",
      "id": "urn:dev:ops:32473-WoTLamp-1234",
      "profile": "https://www.w3.org/2022/wot/profile/http-baseline/v1",
      "title": "My Lamp",
      "description": "A web connected lamp",
      ...
    }
    
    {
      "@context": "https://www.w3.org/2019/wot/td/v1",
      "id": "urn:dev:ops:32473-WoTLamp-1234",
      "profile": [
        "https://www.w3.org/2022/wot/profile/http-baseline/v1",
        "https://www.w3.org/2022/wot/profile/http-sse/v1"
      ],
      "title": "My Lamp",
      "description": "A web connected lamp",
      ...
    }

HTTP Profiles

The HTTP Profile specificiation defines several profiles that can be used for dedicated use cases.

Common Rules

The following sections are applicable for all profiles defined by this document.

Security Considerations

The security considerations of the WoT Architecture and WoT Thing Description MUST be adopted by compliant implementations.

Security Schemes

Below is a list of security schemes [[wot-thing-description]] which conformant Web Things MAY use:

Conformant Consumers MUST support all of these security schemes.

The list of security schemes to include in the HTTP Baseline Profile is still under discussion.

A Thing MAY implement multiple security schemes.

Security schemes MUST be applied using the top level security member of a Thing.

Security schemes MUST NOT be applied to individual Forms.

Privacy Considerations

The privacy considerations of the WoT Architecture and WoT Thing Description MUST be adopted by compliant implementations.

HTTP Baseline Profile

This section defines the HTTP Baseline Profile, which includes a Data Model and a Protocol Binding for reading and writing properties and invoking, querying and cancelling actions.

This profile may be used in conjunction with the HTTP SSE Profile or the HTTP Webhook Profile in order to provide operations for observing properties and listening for events.

Identifier

In order to denote that a given Web Thing conforms to the HTTP Baseline Profile, its Thing Description MUST have a profile member [[wot-thing-description]] with a value of https://www.w3.org/2022/wot/profile/http-baseline/v1.

Information Model

The information model of the HTTP Profile incorporates the data model defined by chapter 5 of the Thing Description specification. The normative rules defined by that model are the baseline for the definition of the HTTP Profile data model and are normative for the information model of the profiles defined in this document. A compliant implementation MUST additionally satisfy the requirements of this chapter.

General

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.

Common fields

One of the benefits of the WoT Thing Description over a typical IoT format is the possibility to automatically generate user interfaces that can be used by humans without reading out of band documentation.

Therefore it is strongly RECOMMENDED to use descriptive values for the fields title titles description desvriptions 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.

Date format

All date and time values MUST use the canonical dateTime representation format defined in section 3.2.7 of [xmlschema-2]. As described by this section the following constraints must be observed: All dateTime values MUST use UTC as the time zone and use the 'Z' identifier. A time value of 24:00 is not permitted; the value 00:00 MUST be used instead.

Length and Value Limits

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 MUST NOT be used.

Thing

The Core Data Model applies the following constraints and rules to the Thing class of section 5.3.1.1 of the WoT Thing Description specification.

Mandatory fields

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 Schema

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.

Data Schema Constraints

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 .

Property Affordance

The Core Data Model applies the following constraints and rules to the PropertyAffordance class of section 5.3.1.3 of the WoT Thing Description Specification.

Mandatory fields

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.

Additional Constraints

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 MUST NOT exceed a maximum depth of 5 levels of nested array or object elements. It is RECOMMENDED to keep the nesting of array or object elements below 4.

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, formats defined in section 7.3.1-7.3.6 of [[JSON-SCHEMA]] MUST 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.

Recommended Practice

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 the unit for string values with binary or hex data to achieve interoperability.

Action Affordances

The Core Data Model applies the following constraints and rules to the ActionAffordance class of section 5.3.1.4 of the WoT Thing Description Specification.

Mandatory fields

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 schemas describing the input to an action
output array of DataSchema schemas describing the output of an action
In the input field, all elements of the subclasses objectSchema and dataSchema MUST only contain simple types. In the output field, all elements of the subclasses objectSchema and dataSchema MUST only contain simple types.

Additional Constraints

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 indicated fields of the Interaction Affordances of a Thing Description conforming to the Core Data Model:

forms array of Forms In the forms field, the Array of Form of each action MUST contain only a single endpoint. format string If the field format is used, formats defined in section 7.3.1-7.3.6 of [[JSON-SCHEMA]] MUST be used. oneOf string The DataSchema field oneOf does not make sense for properties and MUST NOT be used. uriVariables Map of DataSchema The uriVariables field MUST NOT be used.

TODO:

- no optional parameters

- timeout

Recommended Practice

Event Affordance

The Core Data Model applies the following constraints and rules to the 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.

Mandatory fields

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

Additional Constraints

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.

Forms

A Thing MAY provide more than one event mechanism to enable a variety of consumers.

Mandatory fields

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

Additional Constraints

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 A security field at the form level MUST NOT be used.
scopes string or Array of string A scopes field MUST NOT be used.

Protocol Binding

This section defines a protocol binding which describes how a Consumer communicates with a Web Thing [[wot-architecture11]] using JSON [[JSON]] payloads over the HTTP [[HTTP11]] protocol.

A Consumer or Web Thing conforming to the HTTP Baseline Profile MUST implement this protocol binding.

The examples provided throughout this section describe how a Consumer would communicate with a Web Thing which produces the following Thing Description: 

        {
          "@context": "https://www.w3.org/2019/wot/td/v1",
          "id": "https://mywebthingserver.com/things/lamp",
          "profile": "https://www.w3.org/2022/wot/profile/http-baseline/v1",
          "base": "https://mywebthingserver.com/things/lamp/",
          "title": "My Lamp",
          "description": "A web connected lamp",
          "securityDefinitions": {
            "oauth2": {
              "scheme": "oauth2",
              "flow": "code",
              "authorization": "https://mywebthingserver.com/oauth/authorize",
              "token": "https://mywebthingserver.com/oauth/token"
            }
          },
          "security": "oauth2",
          "properties": {
            "on": {
              "type": "boolean",
              "title": "On/Off",
              "description": "Whether the lamp is turned on",
              "forms": [{"href": "properties/on"}]
            },
            "level" : {
              "type": "integer",
              "title": "Brightness",
              "description": "The level of light from 0-100",
              "unit": "percent",
              "minimum" : 0,
              "maximum" : 100,
              "forms": [{"href": "properties/level"}]
            }
          },
          "actions": {
            "fade": {
              "title": "Fade",
              "description": "Fade the lamp to a given level",
              "input": {
                "type": "object",
                "properties": {
                  "level": {
                    "type": "integer",
                    "minimum": 0,
                    "maximum": 100,
                    "unit": "percent"
                  },
                  "duration": {
                    "type": "integer",
                    "minimum": 0,
                    "unit": "milliseconds"
                  }
                }
              },
              "forms": [{"href": "actions/fade"}]
            }
          },
          "forms": [
            {
              "op": ["readallproperties", "writemultipleproperties"],
              "href": "properties"
            },
            {
              "op": "queryallactions",
              "href": "actions"
            }
          ]
        }

Properties

readproperty

The URL of a Property resource to be used when reading the value of a property MUST be obtained from a Thing Description by locating a Form inside the corresponding PropertyAffordance for which:

  • After defaults have been applied, its op member contains the value readproperty.
  • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

The resolved value of the href member MUST then be used as the URL of the Property resource.

In order to read the value of a property, a Consumer MUST send an HTTP request to a Web Thing with:

  • Method set to GET
  • URL set to the URL of the Property resource
  • Accept header set to application/json 
          GET /things/lamp/properties/on HTTP/1.1
          Host: mythingserver.com
          Accept: application/json

If a Web Thing receives an HTTP request following the format above and the Consumer has permission to read the corresponding property, then upon successfully reading the value of the property it MUST send an HTTP response with:

  • Status code set to 200
  • Content-Type header set to application/json
  • A body with the value of the property serialized in JSON
    •  
              HTTP/1.1 200 OK
          Content-Type: application/json
          false
    writeproperty

    The URL of a Property resource to be used when writing the value of a property MUST be obtained from a Thing Description by locating a Form inside the corresponding PropertyAffordance for which:

    • After defaults have been applied, its op member contains the value writeproperty
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

    The resolved value of the href member MUST then be used as the URL of the Property resource.

    In order to write the value of a property, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to PUT
    • URL set to the URL of the Property resource
    • Content-Type header set to application/json
    • A body with a requested new value for the property serialized in JSON
     
              PUT /things/lamp/properties/on HTTP/1.1
          Host: mythingserver.com
          Content-Type: application/json
          true

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to write the corresponding property, then upon successfully writing the value of the property it MUST send an HTTP response with:

    • Status code set to 204
     
              HTTP/1.1 204 No Content
    readallproperties

    The URL of a Properties resource to be used when reading the value of all properties at once MUST be obtained from a Thing Description by locating a Form inside the top level forms member for which:

    • Its op member contains the value readallproperties
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

    The resolved value of the href member MUST then be used as the URL of the Properties resource.

    In order to read the value of all properties, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to GET
    • URL set to the URL of the Properties resource
    • Accept header set to application/json 
              GET /things/lamp/properties HTTP/1.1
          Host: mythingserver.com
          Accept: application/json

    If a Web Thing receives an HTTP request following the format above, then upon successfully reading the values of all the readable properties to which the Consumer has permission to access, it MUST send an HTTP response with:

    • Status code set to 200
    • Content-Type header set to application/json
    • A body with the values of all readable properties serialized in JSON, as an object keyed by property name
     
              HTTP/1.1 200 OK
          Content-Type: application/json
          {
            "on": false,
            "level": 100
          }
    writemultipleproperties

    The URL of a Properties resource to be used when writing the value of multiple properties at once MUST be obtained from a Thing Description by locating a Form inside the top level forms member for which:

  • Its op member contains the value writemultipleproperties
  • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

    • The resolved value of the href member MUST then be used as the URL of the Properties resource.

    In order to write the value of multiple properties at once, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to PUT
    • URL set to the URL of the Properties resource
    • Content-Type header set to application/json
    • A body with requested new values for the writable properties serialized in JSON, as an object keyed by property name
     
              PUT /things/lamp/properties HTTP/1.1
          Host: mythingserver.com
          Content-Type: application/json
          {
            "on": true,
            "level": 50
          }

    If a Web Thing receives an HTTP request following the format above, then upon successfully writing the values of the requested writable properties it MUST send an HTTP response with:

    • Status code set to 204
     
              HTTP/1.1 204 No Content

    The readmultipleproperties operation is currently excluded due to the complexities of the request payload format and because it doesn't add much functionality over readproperty and readallproperties. writeallproperties is currently excluded because it is just a special case of writemultipleproperties.

    Actions

    invokeaction

    The URL of an Action resource to be used when invoking an action MUST be obtained from a Thing Description by locating a Form inside the corresponding ActionAffordance for which:

    • After defaults have been applied, the value of its op member is invokeaction
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

    The resolved value of the href member MUST then be used as the URL of the Action resource.

    In order to invoke an action on a Web Thing, a Consumer MUST send an HTTP request to the Web Thing with:

    • Method set to POST
    • URL set to the URL of the Action resource
    • Accept header set to application/json
    • Content-Type header set to application/json
    • A body with an input to the action, if any, serialized in JSON
     
              POST /things/lamp/actions/fade HTTP/1.1
          Host: mythingserver.com
          Content-Type: application/json
          Accept: application/json
          {
            "level": 100,
            "duration": 5
          }

    If a Web Thing receives an HTTP request following the format above then it MUST respond with one of three response formats:

    1. Synchronous Action Response
    2. Asynchronous Action Response
    3. Error Response

    For long-running actions which are not expected to finish executing within the timeout period of an HTTP request (e.g. 30 to 120 seconds), it is RECOMMENDED that a Web Thing respond with an Asynchronous Action Response so that a Consumer may continue to monitor the status of an action request with a queryaction operation on a dynamically created ActionStatus resource, after the initial invokeaction response.

    For short-lived actions which are expected to finish executing within the timeout period of an HTTP request, a Web Thing MAY wait until the action has completed to send a Synchronous Action Response.

    If a Web Thing encounters an error in attempting to execute an action before responding to the invokeaction request, then it MUST send an Error Response.

    Conforming Consumers MUST support all three types of response to the initial invokeaction request. After the initial request, support for subsequent operations on an ActionStatus resource is OPTIONAL.

    ActionStatus object

    The status of an action invocation request is represented by an ActionStatus object which includes the following members:

    Member Description Assignment Type
    status The status of the action request. mandatory string (one of pending, running, completed or failed)
    output The output data, if any, of a completed action which MUST conform with the output data schema of the corresponding ActionAffordance. optional any type
    error An error message, if any, associated with a failed action which MUST use the JSON serialization of the Problem Details format [[RFC7807]] (only needed in response to a queryaction operation). optional object
    href The [[URL]] of an ActionStatus resource which can be polled with a queryaction operation to get the latest status of the action, the URI scheme [[RFC3986]] of which MUST resolve to http or https (only needed for an Asynchronous Action Response). optional string
    timeRequested A timestamp indicating the time at which the Thing received the request to execute the action, in ISO format [[ISO8601-1]] (see Date format for additional constraints). optional string
    timeEnded A timestamp indicating the time at which the Thing successfully completed executing the action, or failed to execute the action, in ISO format [[ISO8601-1]] (see Date format for additional constraints). optional string

    It is possible that a Thing's clock may not be set to the correct time. If timings are important then a Consumer may therefore choose to treat the timeEnded member of an ActionStatus object as being relative to the timeRequested member, but not necessarily as relative to its own internal clock, or the clocks of other Things.

    Synchronous Action Response

    If providing a Synchronous Action Response, a Web Thing MUST send an HTTP response with:

    • Status code set to 200
    • Content-Type header set to application/json
    • A body containing an ActionStatus object serialized in JSON
     
                HTTP/1.1 200 OK
            Content-Type: application/json
            {
              "status": "completed",
              "timeRequested": "2021-11-10T11:43:20.135Z",
              "timeEnded": "2021-11-10T11:43:25.135Z"
            }
    Asynchronous Action Response

    If providing an Asynchronous Action Response, a Web Thing MUST send an HTTP response containing the URL of an ActionStatus resource, the URI scheme [[RFC3986]] of which MUST resolve to http or https. The response MUST have:

    • Status code set to 201
    • Content-Type header set to application/json
    • Location header set to the URL of the ActionStatus resource
    • A body containing an ActionStatus object serialized in JSON, with its href member set to the URL of the ActionStatus resource 
                HTTP/1.1 201 CREATED
            Content-Type: application/json
            Location: /things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655
            {
              "status": "pending",
              "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
              "timeRequested": "2021-11-10T11:43:19.135Z"
            }
    queryaction

    A queryaction operation is used to query the current state of an ongoing action request.

    A Web Thing which provides Asynchronous Action Responses to an invokeaction operation on an Action MUST also support queryaction operations on that same Action. A Web Thing which only provides Synchronous Action Responses to an invokeaction operation on an Action SHOULD NOT support queryaction operations on that same Action.

    The URL of an ActionStatus resource to be used in a queryaction operation MUST be obtained from the Location header of an Asynchronous Action Response, or the href member of the ActionStatus object in its body.

    In order to query the status of an action request, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to GET
    • URL set to the URL of the ActionStatus resource
    • Accept header set to application/json 
              GET /things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655
          Host: mythingserver.com
          Accept: application/json

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to query the corresponding ActionStatus resource, then upon successfully reading the status of the action request it MUST send an HTTP response with:

    • Status code set to 200
    • Content-Type header set to application/json
    • A body containing an ActionStatus object representing the current status of the action request, serialized in JSON
     
                HTTP/1.1 200 OK
            Content-Type: application/json
            {
              "status": "running",
              "timeRequested": "2021-11-10T11:43:19.135Z"
            }

    If the queried action failed to execute, then the status member of the ActionStatus object MUST be set to "failed". If the queried action failed to execute, then the error member MAY provide additional error information conforming to the Problem Details format [[RFC7807]]. 

                HTTP/1.1 200 OK
            Content-Type: application/json
            {
              "status": "failed",
              "error": {
                "type": "https://mythingserver.com/docs/errors/invalid-level",
                "title": "Invalid value for level provided",
                "invalid-params": [
                  {
                    "name": "level",
                    "reason": "Must be a valid number between 0 and 100"
                  }
                ]
              },
              "timeRequested": "2021-11-10T11:43:19.135Z",
              "timeEnded": "2021-11-10T11:43:20.513Z"
            }
    cancelaction

    A cancelaction operation is used to cancel an ongoing Action request.

    A Web Thing which provides Asynchronous Action Responses to an invokeaction operation on an Action MAY also support cancelaction operations on that same Action. A Web Thing which only provides Synchronous Action Responses to an invokeaction operation on an Action SHOULD NOT support cancelaction operations on that same Action.

    class="rfc2119-assertion" id="http-baseline-profile-protocol-binding-cancelaction-2"> The URL of an ActionStatus resource to be used in a cancelaction operation MUST be obtained from the Location header of an Asynchronous Action Response, or the href member of the ActionStatus object in its body.

    In order to cancel an action request, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to DELETE
    • URL set to the URL of the ActionStatus resource 
                DELETE /things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655 HTTP/1.1
            Host: mythingserver.com

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to cancel the corresponding Action request, then upon successfully cancelling Action it MUST send an HTTP response with:

    • Status code set to 204
     
                HTTP/1.1 204 No Content
    queryallactions

    The URL of an Actions resource to be used when querying the status of all ongoing action requests MUST be obtained from a Thing Description by locating a Form inside the top level forms member for which:

    • Its op member contains the value queryallactions
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https

    The resolved value of the href member MUST then be used as the URL of the Actions resource.

    In order to query the status of all ongoing action requests, a Consumer MUST send an HTTP request to a Web Thing with:

    • Method set to GET
    • URL set to the URL of the Actions resource
    • Accept header set to application/json 
              GET /things/lamp/actions HTTP/1.1
          Host: mythingserver.com
          Accept: application/json

    If a Web Thing receives an HTTP request following the format above, then upon successfully retreiving the status of all ongoing action requests to which the Consumer has permission to access, it MUST send an HTTP response with:

    • Status code set to 200
    • Content-Type header set to application/json
    • A body containing an object, keyed by Action name, with the value of each object member being an array of ActionStatus objects representing the action requests, serialized in JSON.

    Each array in the result object MUST be sorted in reverse chronological order such that the most recent action request appears first. 

              HTTP/1.1 200 OK
          Content-Type: application/json
          {
            "fade": [
              {
                "status": "completed",
                "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-426655",
                "timeRequested": "2021-11-10T11:43:19.135Z",
                "timeEnded": "2021-11-10T11:43:20.513Z"
              },
              {
                "status": "failed",
                "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a456-558329",
                "timeRequested": "2021-11-10T11:42:15.133Z",
                "timeEnded": "2021-11-10T11:42:22.524Z"
              },
              {
                "status": "running",
                "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a457-434656",
                "timeRequested": "2021-11-10T11:41:53.351Z"
              },
              {
                "status": "pending",
                "href": "/things/lamp/actions/fade/123e4567-e89b-12d3-a457-ea9519",
                "timeRequested": "2021-11-10T11:39:53.651Z"
              }
            ]
          }
    When an Action request is cancelled with a cancelaction operation, its ActionStatus object is deleted and need not be retained. For all other Action requests it is assumed that a Web Thing will store the ActionStatus object so that its status may later be queried with a queryaction or queryallactions operation. It is not expected that ActionStatus objects should be retained indefinitely, they may be stored in volatile memory and/or periodically pruned. The length of time for which to retain ActionStatus objects is expected to be implementation-specific and may depend on application-specific requirements or resource constraints.

    Error Responses

    If any of the operations defined above are unsuccessful then the Web Thing MUST send an HTTP response with an HTTP error code which describes the reason for the failure.

    It is RECOMMENDED that error responses use one of the following HTTP error codes:

    • 400 Bad Request
    • 401 Unauthorized
    • 403 Forbidden
    • 404 Not Found
    • 500 Internal Server Error

    A Web Thing MUST NOT issue any 3xx status codes. A Consumer MAY treat all 3xx codes as errors that do not change the status or behavior of the consumer.

    Web Things MAY respond with other valid HTTP error codes (e.g. 418 I'm a teapot). Consumers MAY interpret other valid HTTP error codes as a generic 4xx or 5xx error with no special defined behaviour.

    TODO: If we define the finite set of error responses as above then we should also define what a Consumer should do if it receives a 3xx redirect type response.

    If an HTTP error response contains a body, the content of that body MUST conform with with the Problem Details format [[RFC7807]].

    Security

    This assertion has two keywords and needs to be broken into two statements. The question is how to do it without repeating the list.

    Below is a list of security schemes [[wot-thing-description]] which conformant Web Things MAY use and conformant Consumers MUST support:

    The list of security schemes to include in the HTTP Baseline Profile is still under discussion.

    A Thing MAY implement multiple security schemes.

    Security schemes MUST be applied using the top level security member of a Thing.

    Security schemes MUST NOT be applied to individual Forms.

    Please see WoT Security Best Practices for implementation advice.

    HTTP SSE Profile

    This section defines the HTTP SSE Profile, including a Protocol Binding for observing properties and listening for events using Server-Sent Events [[EVENTSOURCE]].

    This profile may be used in conjunction with the HTTP Baseline Profile in order to provide operations to read and write properties and invoke, query and cancel actions.

    Identifier

    In order to denote that a given Web Thing conforms to the HTTP SSE Profile, its Thing Description MUST have a profile member [[wot-thing-description]] with a value of https://www.w3.org/2022/wot/profile/http-sse/v1.

    Protocol Binding

    This section defines a protocol binding which describes how a Consumer communicates with a Web Thing [[wot-architecture11]] using Server-Sent Events [[EVENTSOURCE]].

    A Consumer or Web Thing conforming to the HTTP SSE Profile MUST implement this protocol binding.

    The examples provided throughout this section describe how a Consumer would communicate with a Web Thing which produces the following Thing Description: 

            {
          "@context": "https://www.w3.org/2019/wot/td/v1",
          "id": "https://mywebthingserver.com/things/lamp",
          "profile": [
            "https://www.w3.org/2022/wot/profile/http-baseline/v1",
            "https://www.w3.org/2022/wot/profile/http-sse/v1",
          ],
          "base": "https://mywebthingserver.com/things/lamp/",
          "title": "My Lamp",
          "description": "A web connected lamp",
          "securityDefinitions": {
            "oauth2": {
              "scheme": "oauth2",
              "flow": "code",
              "authorization": "https://mywebthingserver.com/oauth/authorize",
              "token": "https://mywebthingserver.com/oauth/token"
            }
          },
          "security": "oauth2",
          "properties": {
            "on": {
              "type": "boolean",
              "title": "On/Off",
              "description": "Whether the lamp is turned on",
              "forms": [{"href": "properties/on"}]
            },
            "level" : {
              "type": "integer",
              "title": "Brightness",
              "description": "The level of light from 0-100",
              "unit": "percent",
              "minimum" : 0,
              "maximum" : 100,
              "forms": [{"href": "properties/level"}]
            }
          },
          "actions": {
            "fade": {
              "title": "Fade",
              "description": "Fade the lamp to a given level",
              "input": {
                "type": "object",
                "properties": {
                  "level": {
                    "type": "integer",
                    "minimum": 0,
                    "maximum": 100,
                    "unit": "percent"
                  },
                  "duration": {
                    "type": "integer",
                    "minimum": 0,
                    "unit": "milliseconds"
                  }
                }
              },
              "forms": [{"href": "actions/fade"}]
            }
          },
          "events": {
            "overheated": {
              "title": "Overheated",
              "data": {
                "type": "number",
                "unit": "degree celsius"
              },
              "description": "The lamp has exceeded its safe operating temperature",
              "forms": [{
                "href": "events/overheated",
                "subprotocol": "sse"
              }]
            }
          },
          "forms": [
            {
              "op": ["readallproperties", "writemultipleproperties"],
              "href": "properties"
            },
            {
              "op": "queryallactions",
              "href": "actions"
            },
            {
              "op": ["subscribeallevents", "unsubscribeallevents"],
              "href": "events",
              "subprotocol": "sse"
            }
          ]
        }

    Properties

    observeproperty

    The URL of a Property resource to be used when observing the value of a property MUST be obtained from a Thing Description by locating a Form inside the corresponding PropertyAffordance for which:

    • Its op member contains the value observeproperty
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of sse

    The resolved value of the href member MUST then be used as the URL of the Property resource.

    In order to observe a property, a Consumer MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to open a connection with the Web Thing at the URL of the Property resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to GET
    • URL set to the URL of the Property resource
    • Accept header set to text/event-stream
    • Connection header set to keep-alive 
                GET /things/lamp/properties/level HTTP/1.1
            Host: mythingserver.com
            Accept: text/event-stream
            Connection: keep-alive

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be initiated using the EventSource constructor. 

                  const levelSource = new EventSource('/things/lamp/properties/level');

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to observe the corresponding property, then it MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to maintain an open connection with the Consumer and push a property value to the Consumer each time the value of the specified property changes.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to text/event-stream 
                HTTP/1.1 200 OK
            Content-Type: text/event-stream

    Whenever the value of the specified property changes while the Web Thing has an open connection with a Consumer, the Web Thing MUST send a property value to the Consumer using the event stream format in the Server-Sent Events [[EVENTSOURCE]] specification. For each message sent, the Web Thing MUST set the event field to the name of the PropertyAffordance and populate the data field with the property value, serialized in JSON and following the data schema specified in the PropertyAffordance. The id field SHOULD be set to a unique identifier for the property change, for use when re-establishing a dropped connection (see below). It is RECOMMENDED that the identifier is a timestamp representing the time at which the property changed, in the "date-time" format specified by [[RFC3339]]. 

                event: level\n
            data: 42\n
            id: 2021-11-17T15:33:20.827Z\n\n

    If the connection between the Consumer and Web Thing drops (except as a result of the unobserve operation defined below), the Consumer MUST re-establish the connection following the steps outlined in the Server-Sent Events specification [[EVENTSOURCE]]. Once the connection is re-established the Web Thing SHOULD, if possible, send any missed property changes which occured since the last change specified by the Consumer in a Last-Event-ID header.

    unobserveproperty

    In order to stop observing a property, a Consumer MUST terminate the corresponding Server-Sent Events connection with the Web Thing as specified in the Server-Sent Events specification [[EVENTSOURCE]].

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be terminated using the close() method on an EventSource [[EVENTSOURCE]] object. 

                  levelSource.close();

    observeallproperties

    The URL of a properties resource to be used when observing changes to all properties of a Web Thing MUST be obtained from a Thing Description by locating a Form inside the top level forms member of a Thing Description for which:

    • Its op member contains the value observeallproperties
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of sse

    The resolved value of the href member MUST then be used as the URL of the properties resource.

    In order to observe changes to all properties of a Web Thing, a Consumer MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to open a connection with the Web Thing at the URL of the properties resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to GET
    • URL set to the URL of the properties resource
    • Accept header set to text/event-stream
    • Connection header set to keep-alive 
                GET /things/lamp/properties HTTP/1.1
            Host: mythingserver.com
            Accept: text/event-stream
            Connection: keep-alive

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be initiated using the EventSource constructor. 

                  const lampPropertiesSource = new EventSource('/things/lamp/properties');

    If a Web Thing receives an HTTP request following the format above then it MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to maintain an open connection with the Consumer and push new property values to the Consumer for all properties for which it has permission to observe.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to text/event-stream 
                HTTP/1.1 200 OK
            Content-Type: text/event-stream

    Whenever a property changes while the Web Thing has an open connection with a Consumer, the Web Thing MUST send the new property value to the Consumer using the event stream format in the Server-Sent Events [[EVENTSOURCE]] specification. For each message sent, the Web Thing MUST set the event field to the name of the PropertyAffordance and populate the data field with the new property value. The property data MUST follow the data schema specified in the PropertyAffordance and MUST be serialized in JSON. The id field SHOULD be set to a unique identifier for the event, for use when re-establishing a dropped connection (see below). It is RECOMMENDED that the identifier is a timestamp representing the time at which the property changed, in the "date-time" format specified by [[RFC3339]]. 

                event: level\n
            data: 42\n
            id: 2021-11-17T15:33:20.827Z\n\n

    If the connection between the Consumer and Web Thing drops (except as a result of the unobserveallproperties operation defined below), the Consumer MUST re-establish the connection following the steps outlined in the Server-Sent Events specification [[EVENTSOURCE]]. Once the connection is re-established the Web Thing SHOULD, if possible, send any missed property changes which occured since the last change specified by the Consumer in a Last-Event-ID header.

    unobserveallproperties

    In order to unobserve all properties, a Consumer MUST terminate the corresponding Server-Sent Events connection with the properties endpoint of the Web Thing, following the steps specified in the Server-Sent Events specification [[EVENTSOURCE]].

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be terminated using the close() method on an EventSource [[EVENTSOURCE]] object. 

                  lampPropertiesSource.close();

    Events

    The HTTP SSE Profile uses Server-Sent Events [[EVENTSOURCE]] as a mechanism for Consumers to subscribe to events emitted by a Web Thing.

    Consumers are not required to implement the EventSource JavaScript API from the Server-Sent Events specification in order to conform with this profile. Any programming language may be used to consume an event stream.

    subscribeevent

    The URL of an Event resource to be used when subscribing to an event MUST be obtained from a Thing Description by locating a Form inside the corresponding EventAffordance for which:

    • After defaults have been applied, its op member contains the value subscribeevent
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of sse

    The resolved value of the href member MUST then be used as the URL of the Event resource.

    In order to subscribe to an event, a Consumer MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to open a connection with the Web Thing at the URL of the Event resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to GET
    • URL set to the URL of the Event resource
    • Accept header set to text/event-stream
    • Connection header set to keep-alive 
              GET /things/lamp/events/overheated HTTP/1.1
          Host: mythingserver.com
          Accept: text/event-stream
          Connection: keep-alive

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be initiated using the EventSource constructor. 

                  const overheatedEventSource = new EventSource('/things/lamp/events/overheated');

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to subscribe to the corresponding event, then it MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to maintain an open connection with the Consumer and push event data to the Consumer as events of the specified type are emitted.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to text/event-stream 
              HTTP/1.1 200 OK
          Content-Type: text/event-stream

    Whenever an event of the specified type occurs while the Web Thing has an open connection with a Consumer, the Web Thing MUST send event data to the Consumer using the event stream format in the Server-Sent Events [[EVENTSOURCE]] specification. For each message sent, the Web Thing MUST set the event field to the name of the EventAffordance and populate the data field with event data, if any. The event data MUST follow the data schema specified in the EventAffordance and be serialized in JSON. The id field SHOULD be set to a unique identifier for the event, for use when re-establishing a dropped connection (see below). 

                event: overheated\n
            data: 90\n
            id: 12345\n\n

    If the connection between the Consumer and Web Thing drops (except as a result of the unsubscribe operation defined below), the Consumer MUST re-establish the connection following the steps outlined in the Server-Sent Events specification [[EVENTSOURCE]]. Once the connection is re-established the Web Thing SHOULD, if possible, send any missed events which occured since the last event specified by the Consumer in a Last-Event-ID header.

    unsubscribeevent

    In order to unsubscribe from an event, a Consumer MUST terminate the corresponding Server-Sent Events connection with the Web Thing as specified in the Server-Sent Events specification [[EVENTSOURCE]].

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be terminated using the close() method on an EventSource [[EVENTSOURCE]] object. 

                  overheatedEventSource.close();
    subscribeallevents

    The URL of an events resource to be used when subscribing to all events emitted by a Web Thing MUST be obtained from a Thing Description by locating a Form inside the top level forms member of a Thing Description for which:

    • Its op member contains the value subscribeallevents
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of sse

    The resolved value of the href member MUST then be used as the URL of the events resource.

    In order to subscribe to all events emitted by a Web Thing, a Consumer MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to open a connection with the Web Thing at the URL of the events resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to GET
    • URL set to the URL of the events resource
    • Accept header set to text/event-stream
    • Connection header set to keep-alive 
                GET /things/lamp/events HTTP/1.1
            Host: mythingserver.com
            Accept: text/event-stream
            Connection: keep-alive

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be initiated using the EventSource constructor. 

                  const lampEventsSource = new EventSource('/things/lamp/events');

    If a Web Thing receives an HTTP request following the format above then it MUST follow the Server-Sent Events [[EVENTSOURCE]] specification to maintain an open connection with the Consumer and push event data to the Consumer for all event types for which it has permission to subscribe.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to text/event-stream 
                HTTP/1.1 200 OK
            Content-Type: text/event-stream

    Whenever an event occurs while the Web Thing has an open connection with a Consumer, the Web Thing MUST send event data to the Consumer using the event stream format in the Server-Sent Events [[EVENTSOURCE]] specification. For each message sent, the Web Thing MUST set the event field to the name of the EventAffordance and populate the data field with event data, if any. The event data MUST follow the data schema specified in the EventAffordance and be serialized in JSON. The id field SHOULD be set to a unique identifier for the event, for use when re-establishing a dropped connection (see below). 

                event: overheated\n
            data: 90\n
            id: 12345\n\n

    If the connection between the Consumer and Web Thing drops (except as a result of the unsubscribeallevents operation defined below), the Consumer MUST re-establish the connection following the steps outlined in the Server-Sent Events specification [[EVENTSOURCE]]. Once the connection is re-established the Web Thing SHOULD, if possible, send any missed events which occured since the last event specified by the Consumer in a Last-Event-ID header.

    unsubscribeallevents

    In order to unsubscribe from all events, a Consumer MUST terminate the corresponding Server-Sent Events connection with the events endpoint of the Web Thing, following the steps specified in the Server-Sent Events specification [[EVENTSOURCE]].

    For Consumers implemented in JavaScript [[ECMASCRIPT]] and executed in a runtime which exposes the EventSource interface, a Server-Sent Events connection can be terminated using the close() method on an EventSource [[EVENTSOURCE]] object. 

                  lampEventsSource.close();

    HTTP Webhook Profile

    This section defines the HTTP Webhook Profile, including a Protocol Binding for observing properties and listening for events using WebHooks.

    The HTTP Webhook profile MAY be used in conjunction with the HTTP Baseline Profile in order to provide operations to read and write properties and invoke, query and cancel actions.

    The HTTP Webhook profile MAY be used as an alternative event mechanism to the HTTP SEE Profile.

    Introduction

    Webhooks are a mechanism for Consumers to subscribe to events emitted by a Web Thing.

    The mechanism is scalable and supports Consumers that receive events from multiple Things. Consumers implement a Webhook listener that is handling the event streams generated by Things.

    Describe how Webhooks are used in the WoT context. Explain listeners, data payloads, dataResponse payloads. Create normative language on multiple subscriptions to the same event by the same consumer.

    Depending on the use case, a single listener for multiple things and multiple event types MAY be used.

    Webhook Example

    The following sequence diagram illustrates the flow of messages between a Consumer and a Web Thing when subscribing to, unsubscribing from and receiving events.

    TODO: The example needs more work on the data and dataResponse schemas.

    Webhook example - sequence diagram
    HTTP Webhook Profile - Sequence of operations and notifications

    The following example contains a snippet from a TD that illustrates how a Webhook event can be described.

                ...
    
            {
                "events": {
                    "fireAlarm": {
                        "title": "Fire Alarm"
                        "description": "Critical Condition - Fire!",
                        "subscription": {
                            "type": "object",
                            "properties": {
                              "callbackURL": {
                                  "type": "string",
                                  "format": "uri",
                                  "description": "Callback URL provided by subscriber for Webhook notifications.",
                                  "writeOnly": true
                              },
                              ...
                            }
                        "data": {
                            "type": "boolean",
                            "description": "true, if the alert button has been pushed, false, if the button was armed again."
                        },
                        "dataResponse": {
                            "type": "string",
                            "description": "sprinkler status"
                        },
            
                        "cancellation": {
                          "type": "object",
                          "properties": {
                              "subscriptionID": {
                                  "type": "integer",
                                  "description": "subscription ID to cancel",
                                  "writeOnly": true
                              }
                          }
                      },
                      "forms": [
                            {
                                "op": "subscribeevent",
                                "href": "http://192.168.0.124:8080/events/fireAlarm/subscribe",
                                "contentType": "application/json",
                                "htv:methodName": "POST"
                            },
                            {
                                "op": "unsubscribeevent",
                                "href": "http://192.168.0.124:8080/events/fireAlarm/unsubscribe",
                                "htv:methodName": "POST"
                            }
                        ]
                    },
                    "batteryLow": {
                        ...
                    }
                }
            }

    Identifier

    In order to denote that a given Web Thing conforms to the HTTP Webhook Profile, its Thing Description MUST have a profile member [[wot-thing-description]] with a value of https://www.w3.org/2022/wot/profile/http-webhook/v1.

    Note that the profile member is an array that may contain multiple profile entries, which indicates that a Web Thing conforms to all of the profiles in that array.

    Message Format

    Event notification messages MUST comply with the following data schema.

    TODO: Describe data and dataResponse schemas.

    For each notification message, the Web Thing MUST set the event field to the name of the EventAffordance.

    The event data MUST follow the data schema specified in the EventAffordance and MUST be serialized in JSON.

    The id field MUST be set to a unique identifier for the event.

    Protocol Binding

    This section defines a protocol binding which describes how a Consumer and a Web Thing communicate using Webhook Events.

    A Consumer or Web Thing conforming to the HTTP Webhook Profile MUST implement this protocol binding.

    Operations

    subscribeevent

    The URL of an Event resource to be used when subscribing to an event MUST be obtained from a Thing Description by locating a Form inside the corresponding EventAffordance for which:

    • After defaults have been applied, its op member contains the value subscribeevent
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of "webhook"

    The resolved value of the href member MUST then be used as the URL of the Event resource.

    In order to subscribe to an event, a Consumer MUST provide the listener URL in the data payload of the subscribe operation of the Event resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to POST
    • URL set to the URL of the Event resource
    • Accept header set to application/json
    • A body with a subscription payload, serialized in JSON.

      • TODO: Define subscription payload. 

                  POST /things/lamp/events/overheated HTTP/1.1
              Host: mythingserver.com
              Accept: application/json
              
              {
                "properties": {
                  ...
                  listener: uri,
                  ...
                }
              }

    If a Web Thing receives an HTTP request following the format above and the Consumer has permission to subscribe to the corresponding event, then it MUST send event messages to the Consumer as events of the specified type are emitted.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to application/json 
                  HTTP/1.1 200 OK
              Content-Type: application/json
    
              {
                  subscriptionId: 1234-4544-1211
              }

    Whenever an event of the specified type occurs, the Web Thing MUST send event data to the Consumer using the event payload format defined in section ["#sec-http-webhook-profile-protocol-binding-events-notification"].

    unsubscribeevent

    In order to unsubscribe to an event, a Consumer MUST provide the subscriptionID in the data payload of the unsubscribe operation of the Event resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to POST
    • URL set to the URL of the Event resource
    • Accept header set to application/json
    • Request Payload contains a JSON object with a valid subscriptionId.

      • TODO: define subscriptionId format - urn?

    subscribeallevents

    The URL of an Events resource to be used when subscribing to all events emitted by a Web Thing MUST be obtained from a Thing Description by locating a Form inside the top level forms member of a Thing Description for which:

    • Its op member contains the value subscribeallevents
    • After being resolved against a base URL where applicable, the URI scheme [[RFC3986]] of the value of its href member is http or https
    • Its subprotocol member has a value of "sse"

    The resolved value of the href member MUST then be used as the URL of the events resource.

    In order to subscribe to all events emitted by a Web Thing, a Consumer MUST send an HTTP request to the Web Thing with:

    • Method set to POST
    • URL set to the URL of the events resource
    • Accept header set to text/event-stream
    • Request Payload contains a JSON object with a subscription payload. 
                    POST /things/lamp/events/overheated HTTP/1.1
                Host: mythingserver.com
                Accept: application/json
                
                {
                  "properties": {
                    ...
                    listener: uri,
                    ...
                  }
                }

    If a Web Thing receives an HTTP request following the format above, then it MUST send event messages to the Consumer for all event types for which it has permission to subscribe.

    This involves the Web Thing initially sending an HTTP response to the Consumer with:

    • Status code set to 200
    • Content-Type header set to application/json
    • Response payload contains a JSON object with a subscriptionId.
                    HTTP/1.1 200 OK
                Content-Type: text/event-stream
    
                {
                  subscriptionId: 1234-4544-1211
                }

    Whenever an event occurs, the Web Thing MUST send event data to the Consumer using the event payload format defined in section Message Format.

    If the connection between the Web Thing and the Consumer drops, the Web Thing MUST re-establish the connection.

    Once the connection is re-established the Web Thing SHOULD, if possible, send any missed events which occured since the last succesful event notification.

    unsubscribeallevents

    In order to unsubscribe from all events, a Consumer MUST invoke the unsubscribe operation of the Event resource.

    This involves the Consumer sending an HTTP request to the Web Thing with:

    • Method set to POST
    • URL set to the URL of the Event resource
    • Accept header set to application/json
    • Request Payload contains a JSON object with a valid subscriptionId.

    TODO: define subscription id format - urn?

    Event Connections

    A HTTP(s) connection to the event listener of a Consumer MUST be initiated and managed by the Web Thing.

    A Consumer MUST terminate the subscription with the events endpoint of the Web Thing.

    If a Consumer becomes unavailable and the Web Thing cannot successfully transmit event messages to the consumer, it SHOULD attempt several retries at increasing intervals.

    After the maximum number of retries was reached, the Web Thing MAY terminate the subscription without having received an unsubscribe or unsubscribeall operation.

    If the connection between the Web Thing and the Consumer drops, the Web Thing MUST attempt to re-establish the connection.

    Once the connection is re-established, the Web Thing SHOULD, if possible, send any missed events which occured since the last succesful event notification.

    The Web Thing MAY reuse an existing connection to a listener for subsequent message traffic, or it MAY establish a new connection for each message.

    When an event message has been received, the Consumer MUST respond with a "HTTP 200 OK" message.

    An (optional) JSON payload MAY be provided to return a response back to the Web Thing via the same communication channel.

    JSON Schema for HTTP Profiles

    A Thing Description can be syntactically validated with the JSON Schema [[?JSON-SCHEMA]] for compliance with the HTTP profiles.

    TODO: Define a JSON-SCHEMA.

    Open Issues