Web of Things (WoT) Hypermedia Controls Ontology

The concept of Representational State Transfer (REST) [rest] is at the core of most modern Web applications. The state of a Web resources, exposed via a collection of Web resources, can be browsed by clients by following links and acted upon by submitting forms to servers.

Links and forms have always be present in Web technologies, including HTML [html]. Web linking [rfc8288] is a fundamental principle of the Web architecture that can be leveraged to drive applications, e.g. when a server returns a link to a newly created resource as the result of a client's request. Conversely, forms, are request templates that servers can expose to clients for them to fill in with client-specific information and send back to servers. Forms are similar in spirit to operation desriptions as defined by the Open API Specification [openapis] or by the Hydra RDF vocabulary [hydra].

Links and forms are generically referred to as hypermedia controls. They are of increasing importance in the fields of the Web of Things and the Embedded Web, which respectively led to the standardization of the Thing Description (TD) model [wot-thing-description] and the Constrained RESTful Application Language (CoRAL) [coral]. The present hypermedia controls ontology is an attempt to formalize the concepts these two standards specify.

Please note that the Turtle version of the ontology can be always obtained by doing content negotiation as explained in Appendix D of the Thing Description Recommendation. You can include Accept: text/turtle in the request to obtain the Turtle version of this ontology.

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 fundamental WoT terminology such as Thing, Consumer, Thing Description (TD), Interaction Model, Interaction Affordance, Property, Action, Event, Protocol Binding, Servient, etc. is defined in Section 3 of the WoT Architecture specification [WOT-ARCHITECTURE].

The Thing Description terminology such as TD Information Model, TD Document, Term (Vocabulary Term), etc. is defined in Section 3 of the WoT Thing Description specification [WOT-THING-DESCRIPTION].

IRI: https://www.w3.org/2019/wot/hypermedia#AdditionalExpectedResponse

Communication metadata describing the expected response message for additional responses.

In the domain of	`hctl:hasAdditionalOutputSchema` `hctl:isSuccess`
In the range of	`hctl:additionalReturns`

IRI: https://www.w3.org/2019/wot/hypermedia#ExpectedResponse

Communication metadata describing the expected response message for the primary response.

In the range of hctl:returns

IRI: https://www.w3.org/2019/wot/hypermedia#Form

A form can be viewed as a statement of "To perform an operation type operation on form context, make a request method request to submission target" where the optional form fields may further describe the required request. In Thing Descriptions, the form context is the surrounding Object, such as Properties, Actions, and Events or the Thing itself for meta-interactions.

In the domain of	`hctl:additionalReturns` `hctl:forContentCoding` `hctl:forContentType` `hctl:forSubProtocol` `hctl:hasOperationType` `hctl:hasTarget` `hctl:returns` `td:hasSecurityConfiguration`
In the range of	`td:hasForm`

IRI: https://www.w3.org/2019/wot/hypermedia#Link

A link can be viewed as a statement of the form "link context has a relation type resource at link target", where the optional target attributes may further describe the resource.

In the domain of	`hctl:hasAnchor` `hctl:hasHreflang` `hctl:hasRelationType` `hctl:hasSizes` `hctl:hasTarget` `hctl:hintsAtMediaType`
In the range of	`td:hasLink`

IRI: https://www.w3.org/2019/wot/hypermedia#additionalReturns

This optional term can be used if additional expected responses are possible, e.g. for error reporting. Each additional response needs to be distinguished from others in some way (for example, by specifying a protocol-specific response code), and may also have its own data schema.

Domain includes	`hctl:Form`
Range includes	`hctl:AdditionalExpectedResponse`

IRI: https://www.w3.org/2019/wot/hypermedia#hasAdditionalOutputSchema

This optional term can be used to define a data schema for an additional response if it differs from the default output data schema. Rather than a DataSchema object, the name of a previous definition given in a schemaDefinitions map must be used.

Domain includes hctl:AdditionalExpectedResponse

IRI: https://www.w3.org/2019/wot/hypermedia#hasOperationType

Indicates the semantic intention of performing the operation(s) described by the form.

Domain includes hctl:Form

IRI: https://www.w3.org/2019/wot/hypermedia#returns

This optional term can be used if, e.g., the output communication metadata differ from input metadata (e.g., output contentType differ from the input contentType). The response name contains metadata that is only valid for the reponse messages.

Domain includes	`hctl:Form`
Range includes	`hctl:ExpectedResponse`

IRI: https://www.w3.org/2019/wot/hypermedia#forContentCoding

Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Examples of content coding include "gzip", "deflate", etc.

Domain includes	`hctl:Form`
Range includes	`schema:Text`

IRI: https://www.w3.org/2019/wot/hypermedia#forContentType

Assign a content type based on a media type [IANA-MEDIA-TYPES] (e.g., 'text/plain') and potential parameters (e.g., 'charset=utf-8') for the media type.

Domain includes	`hctl:Form`
Range includes	`schema:Text`

IRI: https://www.w3.org/2019/wot/hypermedia#forSubProtocol

Indicates the exact mechanism by which an interaction will be accomplished for a given protocol when there are multiple options. For example, for HTTP and Events, it indicates which of several available mechanisms should be used for asynchronous notifications such as long polling, websub (also see https://www.w3.org/TR/websub/), or server sent events (also see https://www.w3.org/TR/eventsource/). Please note that there is no restriction on the sub-protocol selection and other mechanisms can also be announced by this subprotocol term.

Domain includes	`hctl:Form`
Range includes	`schema:Text`

IRI: https://www.w3.org/2019/wot/hypermedia#hasAnchor

By default, the context, or anchor, of a link conveyed in the Link header field is the URL of the representation it is associated with, as defined in RFC7231, Section 3.1.4.1, and is serialized as a URI.

Domain includes hctl:Link

IRI: https://www.w3.org/2019/wot/hypermedia#hasHreflang

The hreflang attribute specifies the language of a linked document. The value of this must be a valid language tag [BCP47].

Domain includes hctl:Link

IRI: https://www.w3.org/2019/wot/hypermedia#hasRelationType

A link relation type identifies the semantics of a link.

Domain includes hctl:Link

IRI: https://www.w3.org/2019/wot/hypermedia#hasSizes

Target attribute that specifies one or more sizes for the referenced icon. Only applicable for relation type 'icon'. The value pattern follows {Height}x{Width} (e.g., "16x16", "16x16 32x32")

Domain includes hctl:Link

IRI: https://www.w3.org/2019/wot/hypermedia#hasTarget

target IRI of a link or submission target of a form.

Domain includes hctl:Form
hctl:Link

IRI: https://www.w3.org/2019/wot/hypermedia#hintsAtMediaType

Target attribute providing a hint indicating what the media type [IANA-MEDIA-TYPES] of the result of dereferencing the link should be.

Domain includes	`hctl:Link`
Range includes	`schema:Text`

IRI: https://www.w3.org/2019/wot/hypermedia#isSuccess

Signals if the additional response should not be considered an error.

Domain includes	`hctl:AdditionalExpectedResponse`
Range includes	`schema:Boolean`

No AnnotationProperty found in the ontology.

The following table gives alignment between the hypermedia controls ontology and Hydra, an alternative vocabulary for hypermedia-driven Web APIs. Each row represents a close match between two classes. These alignments are not to be understood as formal semantic equivalences but rather as hints to Hydra users.

`hctl:Link`	`hydra:Link`
`hctl:Form`	`hydra:Operation`

Same terms as specified by JSON hyper-schema and the TD model.

Example 1: JSON-LD context for hypermedia controls

{
    "@context": {
        "@vocab": "https://www.w3.org/2019/wot/hypermedia#",
        "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
        "href": { "@id": "hasTarget" },
        "rel": { "@id": "hasRelationType", "@type": "@vocab" },
        "anchor": { "@id": "hasAnchor", "@type": "@id" },
        "type": { "@id": "hintsAtMediaType" },
        "op": { "@id": "rdf:type"; "@type": "@vocab" },
        "contentType": { "@id": "forContentType" },
        "contentCoding": { "@id": "forContentCoding" },
        "subprotocol": { "@id": "forSubProtocol" },
        "response": { "@id": "returns" },
        "readproperty": { "@id": "ReadPropertyOperation" },
        "writeproperty": { "@id": "WritePropertyOperation" },
        "observeproperty": { "@id": "ObservePropertyOperation" },
        "unobserveproperty": { "@id": "UnobservePropertyOperation" },
        "readmultipleproperties": { "@id": "ReadMultiplePropertiesOperation" },
        "writemultipleproperties": { "@id": "WriteMultiplePropertiesOperation" },
        "readallproperties": { "@id": "ReadAllPropertiesOperation" },
        "writeallproperties": { "@id": "WriteAllPropertiesOperation" },
        "invokeaction": { "@id": "InvokeActionOperation" },
        "subscribeevent": { "@id": "SubscribeEventOperation" },
        "unsubscribeevent": { "@id": "UnsubscribeEventOperation" }
    }
}

OCF example (equivalent to an OpenAPI definition). HTTP in RDF vocabulary.

Example 2: Links and forms for an OCF temperature sensor

{
    "@context": [
        ...,
        {
            "rdfs": "http://www.w3.org/2000/01/rdf-schema#",
            "htv": "http://www.w3.org/2011/http#"
        }
    ],
    "@graph": [
        {
            "href": "https://openconnectivityfoundation.github.io/IoTDataModels/TemperatureResURI.swagger.json",
            "rel": "rdfs:seeAlso"
        },
        {
            "href": "/TemperatureResURI",
            "op": "readproperty",
            "htv:methodName": "GET",
            "response": [
                { "htv:statusCodeNumber": 200 },
                { "htv:statusCodeNumber": 403 }
            ]
        },
        {
            "href": "/TemperatureResURI",
            "op": "writeproperty",
            "htv:methodName": "POST",
            "response": [
                { "htv:statusCodeNumber": 200 },
                { "htv:statusCodeNumber": 403 }
            ]
        }
    ]
}

Example 3: Reified link

{
    "@context": { "about": "http://schema.org/about" },
    "@id": "https://en.wikipedia.org/wiki/Web_of_Things",
    "about": "http://www.wikidata.org/entity/Q2814098"
}

{
    "@context": [
        ...,
        { "about": "http://schema.org/about" }
    ],
    "links": [
        {
            "anchor": "https://en.wikipedia.org/wiki/Web_of_Things",
            "rel": "about",
            "href": "http://www.wikidata.org/entity/Q2814098"
        }
    ]
}

Either as RDF statements or as reified statements. Primarily for WoT compatibility but also e.g. to include validity metadata.

[BCP47]: Tags for Identifying Languages. A. Phillips, Ed.; M. Davis, Ed.. IETF. September 2009. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc5646
[coral]: The Constrained RESTful Application Language (CoRAL). Klaus Hartke. IETF. March 2019. Internet-Draft. URL: https://tools.ietf.org/html/draft-hartke-t2trg-coral
[html]: HTML Standard. Anne van Kesteren; Domenic Denicola; Ian Hickson; Philip Jägenstedt; Simon Pieters. WHATWG. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[hydra]: Hydra Core Vocabulary. Markus Lanthaler. Hydra W3C Community Group. URL: https://www.hydra-cg.com/spec/latest/core/
[IANA-MEDIA-TYPES]: Media Types. IANA. URL: https://www.iana.org/assignments/media-types/
[openapis]: OpenAPI Specification. Darrell Miller; Jeremy Whitlock; Marsh Gardiner; Mike Ralphson; Ron Ratovsky; Uri Sarid; Tony Tam; Jason Harmon. OpenAPI Initiative. URL: https://www.openapis.org/
[rest]: REST: Architectural Styles and the Design of Network-based Software Architectures. University of California, Irvine. 2000. PhD thesis.
[rfc8288]: Web Linking. M. Nottingham. IETF. October 2017. Proposed Standard. URL: https://httpwg.org/specs/rfc8288.html
[WOT-ARCHITECTURE]: Web of Things (WoT) Architecture. Matthias Kovatsch; Ryuichi Matsukura; Michael Lagally; Toru Kawaguchi; Kunihiko Toumura; Kazuo Kajimoto. W3C. 9 April 2020. W3C Recommendation. URL: https://www.w3.org/TR/wot-architecture/
[wot-thing-description]: Web of Things (WoT) Thing Description. Sebastian Käbisch; Takuki Kamiya; Michael McCool; Victor Charpenay; Matthias Kovatsch. W3C. 9 April 2020. W3C Recommendation. URL: https://www.w3.org/TR/wot-thing-description/

Web of Things (WoT) Hypermedia Controls Ontology

Abstract

Status of This Document

1. Introduction

2. Conformance

3. Terminology

4. Axiomatization

4.1 Classes

4.1.1 AdditionalExpectedResponse

4.1.2 ExpectedResponse

4.1.3 Form

4.1.4 Link

4.2 Object Properties

4.2.1 additionalReturns

4.2.2 hasAdditionalOutputSchema

4.2.3 hasOperationType

4.2.4 returns

4.3 Datatype Properties

4.3.1 forContentCoding

4.3.2 forContentType

4.3.3 forSubProtocol

4.3.4 hasAnchor

4.3.5 hasHreflang

4.3.6 hasRelationType

4.3.7 hasSizes

4.3.8 hasTarget

4.3.9 hintsAtMediaType

4.3.10 isSuccess

4.4 Annotation Properties

5. Alignments

5.1 Hydra

6. Usage Examples

6.1 JSON Representation of Links & Forms

6.2 Reified Links

A. References

A.1 Normative references