title: REST API Linked Data Keywords abbrev: docname: draft-polli-restapi-ld-keywords-latest category: info
ipr: trust200902 area: General workgroup: keyword: Internet-Draft
stand_alone: yes pi: [toc, tocindent, sortrefs, symrefs, strict, compact, comments, inline, docmapping]
venue: home: https://github.com/ioggstream/draft-polli-restapi-ld-keywords repo: https://github.com/ioggstream/draft-polli-restapi-ld-keywords/issues
mail: [email protected]
ins: R. Polli
name: Roberto Polli
org: Digital Transformation Department, Italian Government
email: [email protected]
country: Italy
normative: YAML: title: YAML Ain't Markup Language Version 1.2 date: 2021-10-01 author: - ins: Oren Ben-Kiki - ins: Clark Evans - ins: Ingy dot Net - ins: Tina Müller - ins: Pantelis Antoniou - ins: Eemeli Aro - ins: Thomas Smith target: https://yaml.org/spec/1.2.2/ OAS: title: OpenAPI Specification 3.0.0 date: 2017-07-26 author: - ins: Darrel Miller - ins: Jeremy Whitlock - ins: Marsh Gardiner - ins: Mike Ralphson - ins: Ron Ratovsky - ins: Uri Sarid JSON-LD-11: title: JSON-LD 1.1 target: https://www.w3.org/TR/json-ld11/ authors: - ins: Gregg Kellogg - ins: Pierre-Antoine Champin - ins: Dave Longley JSONSCHEMA: title: JSON Schema target: https://json-schema.org/specification.html JSON: RFC8259 HTTP: RFC9110 URI: RFC3986 RDF: title: RDF Concepts and Abstract Syntax target: https://www.w3.org/TR/rdf11/ RDFS: title: RDF Schema 1.1 target: https://www.w3.org/TR/rdf-schema/ YAML-IANA: title: The application/yaml Media Type target: https://www.iana.org/assignments/media-types/application/yaml
informative: I-D.ietf-jsonpath-base: JSON-POINTER: RFC6901 JSONLD-11-API: target: https://www.w3.org/TR/json-ld11-api/ title: JSON-LD 1.1 Processing Algorithms and API JSON-SCHEMA-RDF: target: https://www.w3.org/2019/wot/json-schema/ title: JSON Schema in RDF SHACL: title: Shapes Constraint Language (SHACL) target: https://www.w3.org/TR/shacl/ date: 2017-07-20 OWL: title: OWL 2 Web Ontology Language Document Overview target: https://www.w3.org/TR/owl2-overview/ XS: title: XML Schema target: https://www.w3.org/2001/XMLSchema --- abstract
This document defines two keywords to provide semantic information in OpenAPI Specification and JSON Schema documents, and support contract-first semantic schema design.
--- middle
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 {{prosaic-semantics}}).
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.
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;
- 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.
[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{: title="Example of JSON Schema model that provides semantic prose." #ex-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.
{
"@context": {
"@vocab": "https://w3.org/ns/person#"
},
"@type": "Person",
"givenName": "John",
"familyName": "Doe"
}{: title="Example of a schema instance transformed in a JSON-LD object." #ex-json-ld}
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.
{::boilerplate bcp14+}
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.
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 {{keywords}} are collectively named "semantic 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 {{keywords-type}}.
x-jsonld-context: : This keyword conveys a JSON-LD context for the JSON schema instances described by the associate schema. It is defined in {{keywords-context}}.
This specification MAY be used to:
- populate the
@typeproperty along the schema instance objects; - compose an "instance context" to populate the
@contextproperty 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 {{sec-conflicts}}).
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]).
{{ex}} provides various examples of integrating semantic information in schema instances.
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;
it is thus related to the information provided via the x-jsonld-context keyword (see {{keywords-context}}).
It SHOULD NOT reference an RDF Datatype because it is not intended to provide syntax information, but only semantic information.
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.
Its value MUST be a valid JSON-LD Context (see Section 9.15 of JSON-LD-11 ).
When context composition (see {{int-composability}}) 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"{: title="Composing URL contexts requires dereferencing them." #ex-url-contexts}
This section describes an OPTIONAL workflow to interpret a schema instance as JSON-LD.
-
ensure that the initial schema instance does not contain any
@contextor@typeproperty. For further information see {{sec-conflicts}}; -
add the
@contextproperty with the value of x-jsonld-context. This will be the initial "instance context": the only one that will be mangled; -
add the
@typeproperty with the value of x-jsonld-type; -
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
@typeproperty 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 {{keywords}}) is an implementation detail.
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);
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"}
}
}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 section of [JSON-SCHEMA-RDF]).
This specification is not designed to restrict the syntax of a JSON value nor to support a conversion between JSON Schema and XMLSchema (see {{keywords-type}}).
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.
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.
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.
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{: title="A JSON-Schema describing a JSON-LD document." #ex-jsonld-schema}
Limited composability can be achieved applying the process described
in {{interpreting}}.
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:typearc 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. 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 {{keywords}}) at design time and use it to valorize x-jsonld-context (see {{ex-redundant-context}}).
Once a context is assembled, the RDF data can be generated using the algorithms described in [JSONLD-11-API] for example through a library.
from pyld import jsonld
...
jsonld_text = jsonld.expand(schema_instance, context)See the interoperability considerations for the media types and specifications used, including [YAML-IANA], [JSON], [OAS], [JSONSCHEMA] and [JSON-LD-11].
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.
If an OAS document includes the keywords defined in {{keywords}} 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.
None
--- back
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.
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"
{: title="A JSON Schema data model with semantic context and type." #ex-base}
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" .
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: "[email protected]"
country: "FRA"
custom_id: "12345"
{: title="A JSON Schema data model with semantic context and type." #ex-complete-example}
The resulting RDF graph is
@prefix schema: <https://schema.org/> .
@prefix country: <http://publications.europa.eu/resource/authority/country/> .
<mailto:[email protected]>
schema:familyName "Doe" ;
schema:givenName "John" ;
schema:addressCountry country:FRA .
{: title="An RDF graph with semantic context and type." #ex-rdf-from-json}
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"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 {{interpreting}} 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"
}
}
}
}
}{:title="An instance context containing redundant information" #ex-redundant-context}
In the following schema document, the "Citizen" schema references the "BirthPlace" schema.
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
{: title="A schema with object contexts." #ex-object-contexts}
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/"
}
}
}
}
}
}{:title="A @context that includes information from different schemas." #ex-composite-context}
That can be serialized as text/turtle as
@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>
.
{:title="The above entry in text/turtle" #ex-composite-context-turtle}
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.
{: numbered="false" removeinrfc="true"}
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? : This 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.
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? : Web 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: 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 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`.{: title="The above code is ambiguous, because the rdfs:range of CPV:taxCode is rdfs:Literal" #ex-invalid-x-jsonld-type}
{: numbered="false" removeinrfc="true"}
TBD