REST API Linked Data Keywords
draft-polli-restapi-ld-keywords-03
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Roberto Polli | ||
| Last updated | 2024-01-08 | ||
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draft-polli-restapi-ld-keywords-03
Network Working Group R. Polli
Internet-Draft Digital Transformation Department, Italian Government
Intended status: Informational 8 January 2024
Expires: 11 July 2024
REST API Linked Data Keywords
draft-polli-restapi-ld-keywords-03
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 July 2024.
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Copyright Notice
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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/
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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 . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . 12
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
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 21
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
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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.
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 ones.
2.2. The x-jsonld-context JSON Schema keyword
The x-jsonld-context value provides the information required to
interpret the associate 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 an URL string, to generate the instance context
that URL needs to be dereferenced and processed.
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.
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;
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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], [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.
{ "@context": {
"x-jsonld-context: { "@type": "@json"},
"x-jsonld-type: { "@type": "@json"}
}
}
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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 describes messages using [JSONSCHEMA] or [OAS],
they needs 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
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 their position.
For example, @type:
* 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 7).
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], [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/>.
[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/>.
6.2. Informative References
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[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.
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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
"@language": en
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 4: 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
},
"@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.
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
@prefix schema: <https://schema.org/>
_:b0 rdf:type schema:Person ;
schema:country "FRA" ;
schema:familyName "Doe" ;
schema:givenName "John" .
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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/"
email: "@id"
custom_id: null # detach this property from the @vocab
country:
"@id": addressCountry
"@type": "@id"
"@context":
"@base": "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 5: A JSON Schema data model with semantic context and type.
The resulting RDF graph is
@prefix schema: <https://schema.org/> .
@prefix country: <http://publications.europa.eu/resource/authority/country/> .
<mailto:jon@doe.example>
schema:familyName "Doe" ;
schema:givenName "John" ;
schema:addressCountry country:FRA .
Figure 6: An RDF graph with semantic context and type.
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A.3. Cyclic schema
The following schema contains a cyclic reference.
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"
The example schema instance contained in the above schema results in
the following JSON-LD document.
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{
"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 7: 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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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": "@id"
"@context":
"@base": "http://publications.europa.eu/resource/authority/country/"
province:
"@id": "hasProvince"
"@type": "@id"
"@context":
"@base": "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
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
Figure 8: 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": "RM",
"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": "@id",
"@context": {
"@base": "http://publications.europa.eu/resource/authority/country/"
}
},
"province": {
"@id": "hasProvince",
"@type": "@id",
"@context": {
"@base": "https://w3id.org/italia/data/identifiers/provinces-identifiers/vehicle-code/"
}
}
}
}
}
}
Figure 9: 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/RM>
.
Figure 10: The above entry in text/turtle
Appendix B. Acknowledgements
Thanks to Giorgia Lodi, Matteo Fortini and Saverio Pulizzi for being
the initial contributors of this work.
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.
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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? This 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. ~~~ yaml 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" ~~~
For this reason, composability is limited to the object level.
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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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