



Network Working Group                                           R. Polli
Internet-Draft     Digital Transformation Department, Italian Government
Intended status: Informational                          12 December 2025
Expires: 15 June 2026


                     REST API Linked Data Keywords
                   draft-polli-restapi-ld-keywords-08

Abstract

   This document defines two keywords to provide semantic information in
   OpenAPI Specification and JSON Schema documents, and support
   contract-first semantic schema design.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-polli-restapi-ld-keywords/.

   information can be found at https://github.com/ioggstream/draft-
   polli-restapi-ld-keywords.

   Source for this draft and an issue tracker can be found at
   https://github.com/ioggstream/draft-polli-restapi-ld-keywords/issues.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 15 June 2026.







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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Goals and Design Choices  . . . . . . . . . . . . . . . .   3
     1.2.  Prosaic semantics . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  JSON Schema keywords  . . . . . . . . . . . . . . . . . . . .   6
     2.1.  The x-jsonld-type JSON Schema keyword . . . . . . . . . .   7
     2.2.  The x-jsonld-context JSON Schema keyword  . . . . . . . .   7
     2.3.  Interpreting schema instances . . . . . . . . . . . . . .   8
   3.  Interoperability Considerations . . . . . . . . . . . . . . .   8
     3.1.  Syntax is out of scope  . . . . . . . . . . . . . . . . .   9
     3.2.  Limited expressivity  . . . . . . . . . . . . . . . . . .   9
     3.3.  Disjoint with JSON-LD . . . . . . . . . . . . . . . . . .   9
     3.4.  Composability . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     4.1.  Integrity and Authenticity  . . . . . . . . . . . . . . .  11
     4.2.  Conflicts . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Schema with semantic information  . . . . . . . . . . . .  13
     A.2.  Schema with semantic and vocabulary information . . . . .  15
     A.3.  Cyclic schema . . . . . . . . . . . . . . . . . . . . . .  16
     A.4.  Composite instance context  . . . . . . . . . . . . . . .  18
     A.5.  Identifiers and IRI Expansion . . . . . . . . . . . . . .  21
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  21
   FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24





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1.  Introduction

   API providers usually specify semantic information in text or out-of-
   band documents; at best, this information is described in prose into
   specific sections of interface definition documents (see
   Section 1.2).

   This is because API providers do not always value machine-readable
   semantics, or because they have no knowledge of semantic technologies
   - that are perceived as unnecessarily complex.

   A full-semantic approach (e.g. writing RDF oriented APIs) has not
   become widespread because transferring and processing the semantics
   on every message significantly increases data transfer and
   computation requirements.

   Moreover the semantic landscape do not provide easy ways of defining
   / constraining the syntax of an object: tools like [SHACL] and [OWL]
   restrictions are considered computationally intensive to process and
   complex to use from web and mobile developers.

   This document provides a simple mechanism to attach semantic
   information to REST APIs that rely on different dialects of
   [JSONSCHEMA], thus supporting a contract-first schema design.

   For example, the OpenAPI Specifications (see [OAS]) allow to describe
   REST APIs interactions and capabilities using a machine-readable
   format based on [JSON] or [YAML].  OAS 3.0 is based on JSON Schema
   draft-4 while OAS 3.1 relies on the latest JSON Schema draft.

1.1.  Goals and Design Choices

   This document has the following goals:

   *  describe in a single specification document backed by [JSONSCHEMA]
      (e.g. an OpenAPI document) both the syntax and semantics of JSON
      objects.  This information can be either be provided editing the
      document by hand or via automated tools;

   *  easy for non-semantic experts and with reduced complexity;

   *  support for OAS 3.0 / JSON Schema Draft4;

   while it is not intended to:

   *  integrate the syntax defined using [JSONSCHEMA];

   *  infer semantic information where it is not provided;



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   *  convert [JSONSCHEMA] documents to RDF Schema (see [RDFS]) or XML
      Schema.

   Thus, the following design choices have been made:

   *  the semantic context of a JSON object will be described using
      [JSON-LD-11] and its keywords;

   *  property names are limited to characters that can be used in
      variable names (e.g. excluding : and .) to avoid interoperability
      issues with code-generation tools;

   *  privilege a deterministic behavior over automation and
      composability;

   *  interoperable with the mechanisms described in Section 6.1 of
      [JSON-LD-11] for conveying semantic context in REST APIs.

1.2.  Prosaic semantics

   [JSONSCHEMA] allows to define the structure of the exchanged data
   using specific keywords.  Properties' semantics can be expressed in
   prose via the description keyword.

   Person:
     description: A Person.
     type: object
     properties:
       givenName:
         description: The given name of a Person.
         type: string
       familyName:
         description: The family name, or surname, of a Person.
         type: string
     example:
       givenName: John
       familyName: Doe

    Figure 1: Example of JSON Schema model that provides semantic prose.

   [JSON-LD-11] defines a way to interpret a JSON object as JSON-LD: the
   example schema instance (a JSON document conformant to a given
   schema) provided in the above "Person" schema can be integrated with
   semantic information adding the @type and @context properties.







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   {
     "@context": {
       "@vocab": "https://w3.org/ns/person#"
     },
     "@type": "Person",
     "givenName": "John",
     "familyName": "Doe"
   }

      Figure 2: Example of a schema instance transformed in a JSON-LD
                                  object.

   This document shows how to integrate into a JSON Schema document
   information that can be used to add the @context and @type properties
   to the associated JSON Schema instances.

1.3.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

   The terms "content", "content negotiation", "resource", and "user
   agent" in this document are to be interpreted as in [HTTP].

   The terms "fragment" and "fragment identifier" in this document are
   to be interpreted as in [URI].

   The terms "node", "alias node", "anchor" and "named anchor" in this
   document are to be intepreded as in [YAML].

   The terms "schema" and "schema instance" in this document are to be
   intepreded as in [JSONSCHEMA] draft-4 and higher.

   The terms "JSON object", "JSON document", "member", "member name" in
   this document are to be intepreded as in [JSON].  The term "property"
   - when referred to a JSON document such as a schema instance - is a
   synonym of "member name", and the term "property value" is a synonym
   of "member value".

   The terms "@context", "@type", "@id", "@value" and "@language" are to
   be interpreted as JSON-LD keywords in [JSON-LD-11], whereas the term
   "context" is to be interpreted as a JSON-LD Context defined in the
   same document.



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   Since JSON-LD is a serialization format for RDF, the document can use
   JSON-LD and RDF interchangeably when it refers to the semantic
   interpretation of a resource.

   The JSON Schema keywords defined in Section 2 are collectively named
   "semantic keywords".

2.  JSON Schema keywords

   A schema (see [JSONSCHEMA]) MAY use the following JSON Schema
   keywords, collectively named "semantic keywords" to provide semantic
   information for all related schema instances.

   x-jsonld-type:  This keyword conveys an RDF type (see [RDF]) for the
      JSON schema instances described by the associate schema.  It is
      defined in Section 2.1.

   x-jsonld-context:  This keyword conveys a JSON-LD context for the
      JSON schema instances described by the associate schema.  It is
      defined in Section 2.2.

   This specification MAY be used to:

   *  populate the @type property along the schema instance objects;

   *  compose an "instance context" to populate the @context property at
      the root of the schema instance.

   The schema MUST be of type "object".  This is because [JSON-LD-11]
   does not define a way to provide semantic information on JSON values
   that are not JSON objects.

   The schema MUST NOT describe a JSON-LD (e.g. of application/ld+json
   media type) or conflicts will arise, such as which is the correct
   @context or @type (see Section 4.2).

   Both JSON Schema keywords defined in this document might contain URI
   references.  Those references MUST NOT be dereferenced automatically,
   since there is no guarantee that they point to actual locations.
   Moreover they could reference unsecured resources (e.g. using the
   "http://" URI scheme [HTTP]).

   Appendix A provides various examples of integrating semantic
   information in schema instances.







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2.1.  The x-jsonld-type JSON Schema keyword

   The x-jsonld-type value provides information on the RDF type of the
   associate schema instances.

   This value MUST be valid according to the JSON-LD @type keyword as
   described in Section 3.5 of JSON-LD-11 (https://www.w3.org/TR/json-
   ld11/#specifying-the-type); it is thus related to the information
   provided via the x-jsonld-context keyword (see Section 2.2).

   It SHOULD NOT reference an RDF Datatype (https://www.w3.org/TR/rdf11-
   concepts/#section-Datatypes) because it is not intended to provide
   syntax information, but only semantic information.

2.2.  The x-jsonld-context JSON Schema keyword

   The x-jsonld-context value provides the information required to
   interpret the associated schema instances as JSON-LD according to the
   specification in Section 6.1 of JSON-LD-11 (https://www.w3.org/TR/
   json-ld11/#interpreting-json-as-json-ld).

   Its value MUST be a valid JSON-LD Context (see Section 9.15 of JSON-
   LD-11 (https://www.w3.org/TR/json-ld11/#context-definitions) ).

   When context composition (see Section 3.4) is needed, the context
   SHOULD be provided in the form of a JSON object; in fact, if the x-
   jsonld-context is a URL string, that URL needs to be dereferenced and
   processed to generate the instance context.

   Place:
     type: object
     x-jsonld-context:
       "@vocab": "https://my.context/location.jsonld"
     properties:
       country: {type: string}

   Person:
     x-jsonld-context: https://my.context/person.jsonld
     type: object
     properties:
       birthplace:
         $ref: "#/Place"

       Figure 3: Composing URL contexts requires dereferencing them.







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2.3.  Interpreting schema instances

   This section describes an OPTIONAL workflow to interpret a schema
   instance as JSON-LD.

   1.  ensure that the initial schema instance does not contain any
       @context or @type property.  For further information see
       Section 4.2;

   2.  add the @context property with the value of x-jsonld-context.
       This will be the initial "instance context": the only one that
       will be mangled;

   3.  add the @type property with the value of x-jsonld-type;

   4.  iterate on each instance property like the following:

       *  identify the sub-schema associated to the property (e.g.
          resolving $refs) and check the presence of semantic keywords;

       *  for the x-jsonld-type, add the @type property to the sub-
          instance;

       *  for the x-jsonld-context, integrate its information in the
          instance context when they are not already present;

       *  iterate this process in case of nested entries.

   The specific algorithm for integrating the values of x-jsonld-context
   present in sub-schemas into the instance context (see Section 2) is
   an implementation detail.

3.  Interoperability Considerations

   See the interoperability considerations for the media types and
   specifications used, including [YAML-IANA], [JSON], [OAS],
   [JSONSCHEMA] and [JSON-LD-11].

   Annotating a schema with semantic keywords containing JSON-LD
   keywords (e.g. @context, @type and @language) may hinder its ability
   to be interpreted as a JSON-LD document (e.g. using the JSON-LD 1.1
   context for the JSON Schema vocabulary (https://www.w3.org/2019/wot/
   json-schema#json-ld11-ctx)); this can be mitigated extending that
   context and specifying that Linked Data keywords are JSON Literals.







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   { "@context": {
       "x-jsonld-context": { "@type": "@json"},
       "x-jsonld-type": { "@type": "@json"}
     }
   }

   This is generally not a problem, since a generic [JSONSCHEMA]
   document cannot be reliably interpreted as JSON-LD using a single
   context: this is because the same JSON member keys can have different
   meanings depending on their JSON Schema position (see the notes in
   the Interpreting JSON Schema as JSON-LD 1.1
   (https://www.w3.org/2019/wot/json-schema#interpreting-json-schema-as-
   json-ld-1-1) section of [JSON-SCHEMA-RDF]).

3.1.  Syntax is out of scope

   This specification is not designed to restrict the syntax of a JSON
   value nor to support a conversion between JSON Schema and XMLSchema
   (see Section 2.1).

3.2.  Limited expressivity

   Not all RDF resources can be expressed as JSON documents annotated
   with @context and @type: this specification is limited by the
   possibilities of Section 6.1 of JSON-LD-11 (https://www.w3.org/TR/
   json-ld11/#interpreting-json-as-json-ld).  On the other hand, since
   this approach delegates almost all the processing to of JSON-LD, as
   long as JSON-LD evolves it will cover further use cases.

3.3.  Disjoint with JSON-LD

   This specification is not designed to pre-process or mangle JSON-LD
   documents (e.g. to add a missing @type to a JSON-LD document), but
   only to support schemas that do not describe JSON-LD documents.

   Applications exchanging JSON-LD documents need to explicitly populate
   @type and @context, and use a proper media type since Linked Data
   processing and interpretation requires further checks.

   If these applications describe messages using [JSONSCHEMA] or [OAS],
   they need to process them with a JSON-LD processor and declare all
   required properties in the schema - like in the example below.









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   PersonLD:
     type: object
     required: [ "@context", "@type", "givenName", "familyName" ]
     properties:
       "@context":
         type: object
         enum:
         - "@vocab": "https://w3.org/ns/person#"
       "@type":
         type: string
         enum:
         - Person
       givenName:
         type: string
       familyName:
         type: string

           Figure 4: A JSON-Schema describing a JSON-LD document.

3.4.  Composability

   Limited composability can be achieved applying the process described
   in Section 2.3.  Automatic composability is not an explicit goal of
   this specification because of its complexity.  One of the issue is
   that the meaning of a JSON-LD keyword is affected by its position.
   For example, the @type keyword:

   *  in a node object, adds an rdf:type arc to the RDF graph (it also
      has a few other effects on processing, e.g. by enabling type-
      scoped contexts);

   *  in a value object, specifies the datatype of the produced literal;

   *  in the context, and more precisely in a term definition, specifies
      type coercion (https://www.w3.org/TR/json-ld11/#type-coercion).
      It only applies when the value of the term is a string.

   These issues can be tackled in future versions of this
   specifications.

   Moreover, well-designed schemas do not usually have more than 3 or 4
   nested levels.  This means that, when needed, it is possible to
   assemble and optimize an instance context (see Section 2) at design
   time and use it to valorize x-jsonld-context (see Figure 8).

   Once a context is assembled, the RDF data can be generated using the
   algorithms described in [JSONLD-11-API] for example through a
   library.



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   from pyld import jsonld
   ...
   jsonld_text = jsonld.expand(schema_instance, context)

4.  Security Considerations

   See the interoperability considerations for the media types and
   specifications used, including [YAML-IANA], [JSON], [OAS],
   [JSONSCHEMA] and [JSON-LD-11].

4.1.  Integrity and Authenticity

   Adding a semantic context to a JSON document alters its value and, in
   an implementation-dependent way, can lead to reordering of fields.
   This process can thus affect the processing of digitally signed
   content.

4.2.  Conflicts

   If an OAS document includes the keywords defined in Section 2 the
   provider explicitly states that the semantic of the schema instance:

   *  is defined at contract level;

   *  is the same for every message;

   *  and is not conveyed nor specific for each message.

   In this case, processing the semantic conveyed in a message might
   have security implications.

   An application that relies on this specification might want to define
   separate processing streams for JSON documents and RDF graphs, even
   when RDF graphs are serialized as JSON-LD documents.  For example, it
   might want to raise an error when an application/json resource
   contains unexpected properties impacting on the application logic
   like @type and @context.

5.  IANA Considerations

   None

6.  References

6.1.  Normative References






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   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [JSON-LD-11]
              "JSON-LD 1.1", n.d., <https://www.w3.org/TR/json-ld11/>.

   [JSONSCHEMA]
              "JSON Schema", n.d.,
              <https://json-schema.org/specification.html>.

   [OAS]      Darrel Miller, Jeremy Whitlock, Marsh Gardiner, Mike
              Ralphson, Ron Ratovsky, and Uri Sarid, "OpenAPI
              Specification 3.0.0", 26 July 2017.

   [RDF]      "RDF Concepts and Abstract Syntax", n.d.,
              <https://www.w3.org/TR/rdf11-concepts/>.

   [RDFS]     "RDF Schema 1.1", n.d.,
              <https://www.w3.org/TR/rdf-schema/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [YAML]     Oren Ben-Kiki, Clark Evans, Ingy dot Net, Tina Müller,
              Pantelis Antoniou, Eemeli Aro, and Thomas Smith, "YAML
              Ain't Markup Language Version 1.2", 1 October 2021,
              <https://yaml.org/spec/1.2.2/>.






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   [YAML-IANA]
              "The application/yaml Media Type", n.d.,
              <https://www.iana.org/assignments/media-types/application/
              yaml>.

6.2.  Informative References

   [I-D.ietf-jsonpath-base]
              Gössner, S., Normington, G., and C. Bormann, "JSONPath:
              Query expressions for JSON", Work in Progress, Internet-
              Draft, draft-ietf-jsonpath-base-21, 24 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              jsonpath-base-21>.

   [JSON-POINTER]
              Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/rfc/rfc6901>.

   [JSON-SCHEMA-RDF]
              "JSON Schema in RDF", n.d.,
              <https://www.w3.org/2019/wot/json-schema/>.

   [JSONLD-11-API]
              "JSON-LD 1.1 Processing Algorithms and API", n.d.,
              <https://www.w3.org/TR/json-ld11-api/>.

   [OWL]      "OWL 2 Web Ontology Language Document Overview", n.d.,
              <https://www.w3.org/TR/owl2-overview/>.

   [SHACL]    "Shapes Constraint Language (SHACL)", 20 July 2017,
              <https://www.w3.org/TR/shacl/>.

   [XS]       "XML Schema", n.d., <https://www.w3.org/2001/XMLSchema>.

Appendix A.  Examples

A.1.  Schema with semantic information

   The following example shows a Person JSON Schema with semantic
   information provided by the x-jsonld-type and x-jsonld-context.  Type
   information is provided as a URI reference.








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   Person:
     "x-jsonld-type": "https://schema.org/Person"
     "x-jsonld-context":
        "@vocab": "https://schema.org/"
        custom_id: null  # detach this property from the @vocab
        country:
          "@id": addressCountry
     type: object
     required:
     - given_name
     - family_name
     properties:
       familyName: { type: string, maxLength: 255  }
       givenName:  { type: string, maxLength: 255  }
       country:    { type: string, maxLength: 3, minLength: 3 }
       custom_id:  { type: string, maxLength: 255  }
     example:
       familyName: "Doe"
       givenName: "John"
       country: "FRA"
       custom_id: "12345"

     Figure 5: A JSON Schema data model with semantic context and type.

   The example object is assembled as a JSON-LD object as follows.

   {
     "@context": {
       "@vocab": "https://schema.org/",
       "custom_id": null
       "country": {
          "@id": "addressCountry"
       }
     },
     "@type": "https://schema.org/Person",
     "familyName": "Doe",
     "givenName": "John",
     "country": "FRA",
     "custom_id": "12345"
   }

   The above JSON-LD can be represented as text/turtle as follows.









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   @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
   @prefix schema: <https://schema.org/>

   _:b0 rdf:type schema:Person    ;
        schema:addressCountry  "FRA"  ;
        schema:familyName  "Doe"  ;
        schema:givenName   "John" .

A.2.  Schema with semantic and vocabulary information

   The following example shows a "Person" schema with semantic
   information provided by the x-jsonld-type and x-jsonld-context.

   Person:
     "x-jsonld-type": "https://schema.org/Person"
     "x-jsonld-context":
        "@vocab": "https://schema.org/"
        "@base": "https://example.org/people/"
        email: "@id"
        custom_id: null  # detach this property from the @vocab
        country:
          "@id": addressCountry
          "@type": "@vocab"
          "@context":
             "@vocab": "http://publications.europa.eu/resource/authority/country/"

     type: object
     required:
     - email
     - given_name
     - family_name
     properties:
       email: { type: string, maxLength: 255  }
       familyName: { type: string, maxLength: 255  }
       givenName:  { type: string, maxLength: 255  }
       country:    { type: string, maxLength: 3, minLength: 3 }
       custom_id:  { type: string, maxLength: 255  }
     example:
       familyName: "Doe"
       givenName: "John"
       email: "jon@doe.example"
       country: "FRA"
       custom_id: "12345"

     Figure 6: A JSON Schema data model with semantic context and type.

   The resulting RDF graph is




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   @prefix schema: <https://schema.org/> .
   @prefix country: <http://publications.europa.eu/resource/authority/country/> .

   <https://example.org/people/jon@doe.example>
     a schema:Person           ;
     schema:familyName "Doe"          ;
     schema:givenName "John"          ;
     schema:addressCountry country:FRA .

           Figure 7: An RDF graph with semantic context and type.

   Note that in the above example:

   *  the email property is mapped to @id, thus making the instance IRI
      dependent on its value.  Since it's not an absolute IRI, the @base
      keyword is used to provide a base IRI i.e. https://example.org/
      people/ + jon@doe.example;

   *  the country property is associated with the country vocabulary
      using the @type: @vocab keyword.

A.3.  Cyclic schema

   The following schema contains a cyclic reference.  Type information
   is resolved using the @vocab keyword specified in the x-jsonld-
   context.

   Person:
     description: Simple cyclic example.
     x-jsonld-type: Person
     x-jsonld-context:
       "email": "@id"
       "@vocab": "https://w3.org/ns/person#"
       children:
         "@container": "@set"
     type: object
     properties:
       email: { type: string }
       children:
         type: array
         items:
           $ref: '#/Person'
     example:
       email: "mailto:a@example"
       children:
       - email: "mailto:dough@example"
       - email: "mailto:son@example"




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   The example schema instance contained in the above schema results in
   the following JSON-LD document.

   {
     "email": "mailto:a@example",
     "children": [
       {
         "email": "mailto:dough@example",
         "@type": "Person"
       },
       {
         "email": "mailto:son@example",
         "@type": "Person"
       }
     ],
     "@type": "Person",
     "@context": {
       "email": "@id",
       "@vocab": "https://w3.org/ns/person#",
       "children": {
         "@container": "@set"
       }
     }
   }

   Applying the workflow described in Section 2.3 just recursively
   copying the x-jsonld-context, the instance context could have been
   more complex.

   {
     ...
     "@context": {
       "email": "@id",
       "@vocab": "https://w3.org/ns/person#",
       "children": {
         "@container": "@set",
         "@context": {
           "email": "@id",
           "@vocab": "https://w3.org/ns/person#",
           "children": {
             "@container": "@set"
           }
         }
       }
     }
   }

       Figure 8: An instance context containing redundant information



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A.4.  Composite instance context

   In the following schema document, the "Citizen" schema references the
   "BirthPlace" schema.















































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   Citizen:
     x-jsonld-type: Person
     x-jsonld-context:
       "email": "@id"
       "@vocab": "https://w3.org/ns/person#"
     type: object
     properties:
       email: { type: string }
       birthplace:
         $ref: "#/BirthPlace"
     example:
       email: "mailto:a@example"
       givenName: Roberto
       familyName: Polli
       birthplace:
         province: LT
         country: ITA
   BirthPlace:
     x-jsonld-type: https://w3id.org/italia/onto/CLV/Feature
     x-jsonld-context:
       "@vocab": "https://w3id.org/italia/onto/CLV/"
       country:
         "@id": "hasCountry"
         "@type": "@vocab"
         "@context":
           "@vocab": "http://publications.europa.eu/resource/authority/country/"
       province:
         "@id": "hasProvince"
         "@type": "@vocab"
         "@context":
           "@vocab": "https://w3id.org/italia/data/identifiers/provinces-identifiers/vehicle-code/"
     type: object
     required:
       - province
       - country
     properties:
       province:
         description: The province where the person was born.
         type: string
       country:
         description: The iso alpha-3 code of the country where the person was born.
         type: string
     example:
       province: RM
       country: ITA

                  Figure 9: A schema with object contexts.




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   The example schema instance contained in the above schema results in
   the following JSON-LD document.  The instance context contains
   information from both "Citizen" and "BirthPlace" semantic keywords.

   {
     "email": "mailto:a@example",
     "givenName": "Roberto",
     "familyName": "Polli",
     "birthplace": {
       "province": "LT",
       "country": "ITA",
       "@type": "https://w3id.org/italia/onto/CLV/Feature"
     },
     "@type": "Person",
     "@context": {
       "email": "@id",
       "@vocab": "https://w3.org/ns/person#",
       "birthplace": {
         "@context": {
           "@vocab": "https://w3id.org/italia/onto/CLV/",
           "city": "hasCity",
           "country": {
             "@id": "hasCountry",
             "@type": "@vocab",
             "@context": {
               "@vocab": "http://publications.europa.eu/resource/authority/country/"
             }
           },
           "province": {
             "@id": "hasProvince",
             "@type": "@vocab",
             "@context": {
               "@vocab": "https://w3id.org/italia/data/identifiers/provinces-identifiers/vehicle-code/"
             }
           }
         }
       }
     }
   }

       Figure 10: A @context that includes information from different
                                  schemas.

   That can be serialized as text/turtle as







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   @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
   @prefix eu: <https://w3.org/ns/person#> .
   @prefix itl: <https://w3id.org/italia/onto/CLV/> .

   <mailto:a@example>
     rdf:type eu:Person ;
     eu:birthplace _:b0 ;
     eu:familyName "Polli" ;
     eu:givenName  "Roberto"
   .
   _:b0 rdf:type itl:Feature ;
     itl:hasCountry <http://publications.europa.eu/resource/authority/country/ITA> ;
     itl:hasProvince <https://w3id.org/italia/data/identifiers/provinces-identifiers/vehicle-code/LT>
   .

                 Figure 11: The above entry in text/turtle

A.5.  Identifiers and IRI Expansion

   IRI expansion expects string identifiers, so an @id that should be
   expanded in conjunction with a @base can only be assigned to string
   properties.

   Person:
     type: object
     x-jsonld-type: "Person"
     x-jsonld-context:
       "@vocab": "https://w3id.org/italia/onto/CPV/"
       "@base": "https://example.org/people/"
       taxCode: "@id"  # taxCode is a string property.
     required:
     - taxCode
     properties:
       # Since taxCode is an identifier to be expanded
       #   with @base, it must be a string.
       taxCode:
         type: string
     example:
       taxCode: "RSSMRA85M01H501U"

    Figure 12: A schema that uses IRI expansion with a string property.

Appendix B.  Acknowledgements

   Thanks to Giorgia Lodi, Matteo Fortini and Saverio Pulizzi for being
   the initial contributors of this work.





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   In addition to the people above, this document owes a lot to the
   extensive discussion inside and outside the workgroup.  The following
   contributors have helped improve this specification by opening pull
   requests, reporting bugs, asking smart questions, drafting or
   reviewing text, and evaluating open issues:

   Pierre-Antoine Champin, and Vladimir Alexiev.

FAQ

   This section is to be removed before publishing as an RFC.

   Q: Why this document?  There's currently no standard way to provide
      machine-readable semantic information in [OAS] / [JSONSCHEMA] to
      be used at contract time.

   Q: Does this document support the exchange of JSON-LD resources?  Thi
      s document is focused on annotating schemas that are used at
      contract/design time, so that application can exchange compact
      JSON object without dereferencing nor interpreting external
      resources at runtime.

      While you can use the provided semantic information to generate
      JSON-LD objects, it is not the primary goal of this specification:
      context information are not expected to be dereferenced at runtime
      (see security considerations in JSON-LD) and the semantics of
      exchanged messages is expected to be constrained inside the
      application.

   Q: Why don't use existing [JSONSCHEMA] keywords like externalDocs
   ?  We already tried, but this was actually squatting a keyword
      designed for human readable documents (https://github.com/OAI/
      OpenAPI-Specification/blob/main/
      versions/3.1.0.md#externalDocumentationObject).

   Q: Why using x- keywords?  OpenAPI 3.0 considers invalid unregistered
      keywords that don't start with x-, and we want a solution that is
      valid for all OAS versions >= 3.0.

   Q: Why not using a full-semantic approach?  This approach allows API
      providers to attach metadata to their specification without
      modifying their actual services nor their implementation, since
      custom keywords are ignored by OpenAPI toolings like Gateways and
      code generators.

   Q: Why not defining a mechanism to attach semantic information to

   non-object schemas (e.g. JSON Strings) like other implementations?  T



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      his is actually problematic.  Look at this example that reuses the
      TaxCode schema and semantic in different properties.

   Q: Why don't use SHACL or OWL restrictions instead of JSON Schema?  W
      eb and mobile developers consider JSON Schema is easier to use
      than SHACL.  Moreover, OWL restrictions are about semantics, and
      are not designed to restrict the syntax.

   Q: Why don't design for composability first?  JSON-LD is a complex
      specification.  Consider the following schemas, where Contract
      references TaxCode.

       TaxCode:
         type: string
         $linkedData:
           "@id": "https://w3id.org/italia/onto/CPV/taxCode"
           "term": "taxCode"
       Contract:
         ...
         properties:
           employer_tax_code:
             # Beware! TaxCode.$linkedData.term == 'taxCode'
             $ref: "#/components/schemas/TaxCode"
           employee_tax_code:
             # Here we are reusing not only the schema,
             #   but even the same term.
             $ref: "#/components/schemas/TaxCode"

   The result will be that only one of the properties will be correctly
   annotated.  For this reason, composability is limited to the object
   level.

   Q: Why not use keywords such as x-refersTo, x-kindOf, etc.?  When we
      started enriching OAS documents with x-refersTo and similar
      keywords, we realized that composing a JSON-LD @context from
      multiple keywords was increasingly complex when considering nested
      objects and cyclic references, especially when we needed to adapt
      existing OAS documents to ontologies defined by third parties.
      Then, we realized that JSON-LD provided most of the needed
      features to describe semantics without the need of re-assembling a
      @context from multiple keywords.  Moreover, as long as JSON-LD is
      evolving, our approach does not require the addition of new
      keywords.

   Q: Can the value of x-jsonld-type be an rdf:Property?  Would this
   allow to reuse the same schema in different objects without
   modifying the @context?  Under normal circumstances, i.e. when




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      designing public or financial service APIs, you don't want x-
      jsonld-type to be an rdf:Property.  The value of x-jsonld-type
      usually maps to a owl:Class, not an owl:DataTypeProperty; for
      example a sensible value for x-jsonld-type would be rdfs:Literal
      (that is, the rdfs:range of CPV:taxCode), but this would be mostly
      a syntactic information, which instead is provided by JSON Schema.

       TaxCode:
         type: string
         x-jsonld-type: "https://w3id.org/italia/onto/CPV/taxCode"
         description: |-
           This example is ambiguous, because:

           1. it treats a CPV:taxCode as an owl:Class,
              while it's an owl:DataTypeProperty;
           2. the `rdfs:range` for CPV:taxCode is `rdfs:Literal`.

     Figure 13: The above code is ambiguous, because the rdfs:range of
                        CPV:taxCode is rdfs:Literal

Change Log

   This section is to be removed before publishing as an RFC.

   TBD

Author's Address

   Roberto Polli
   Digital Transformation Department, Italian Government
   Italy
   Email: robipolli@gmail.com



















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