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Interpreting JSON Schema as JSON-LD 1.1

The following JSON-LD context [[json-ld11]] can be added to JSON schema definitions to obtain an RDF representation of these definitions.

The two following examples show the equivalence between the JSON and the Turtle representations of schema definitions. These examples were taken from the collection of schemas managed by the Open Connectivity Foundation (OCF).

The value defined for the const and enum terms is always interpreted as a raw JSON value, even if it is an array or an object. In RDF, it becomes a typed literal with datatype rdf:JSON.

In its latest version, the JSON schema items term can be interpreted in two different ways: if the value is a single schema definition, all items in the array must then be proper instance of that schema ; if the value is an array of schema definitions, an item in the instance array must be an instance of the schema at the same index in the schema array. In the latter case, the schema array should be interpreted as an RDF list rather as a set of statement (to preserve order). It is however not possible to express both cases with a single JSON-LD context. The context given in only reflects the former case.

JSON schema allows schemas to be defined in a "collection" object under definitions. This feature is purely syntactical, hence not reflected in the present RDF vocabulary for JSON schema. Interpreting schemas under definitions as JSON-LD requires prior normalization, by substituting references in the main schema with their definition. This normalization procedure may be directly supported by JSON-LD processors in the future.

The JSON Schema RDF vocabulary does not include terms for title and description. Instead, the example context maps them to widely used Dublin Core terms with equivalent semantics.

Referencing and linking

JSON schema defines the $id and $ref keywords to increase reusability but also to define schemas in a recursive fashion. This feature is natively supported by RDF: if schema definitions are named resources (identified by an IRI) instead of blank nodes, they can be referenced from anywhere on the Web.

The id keyword (an alias for @id) can be used for that purpose, as shown in the following example, taken from an OCF standard.

If the same schema definition (with the same IRI) is used twice as the value of one of the properties of an object schema, it may result in name collisions. Indeed, after transformation to RDF, this schema will have two statements with :propertyValue that do not link back to the object from which they respectively originate.

Besides IRI references between schemas, JSON hyper-schema [[json-hyper-schema]] allows arbitrary links to be embedded in a schema definition. An alternative to JSON hyper-schema is to use plain RDF statements or the RDF vocabulary for hypermedia controls. Examples can be found in the documentation of that vocabulary.

Defining a JSON-LD context for data instances

JSON schema definitions may specify JSON structures that are themselves interpretable as RDF (or JSON-LD). To be able to do so, instances of a certain schema must be given a JSON-LD context to map JSON terms to RDF IRIs. This can be done within a schema definition using the JSON-LD vocabulary in RDF, and more precisely with the property jsonld:context.

The example above allows schema-aware agents on the Web to interpret any instance of the schema as JSON-LD by appending the context given as annotation to the instance JSON, as shown below.

Note the difference with using @context in the schema definition: it is used to interpret instances as JSON-LD, while @context would alter the way the schema itself is interpreted.

The JSON-LD vocabulary in RDF includes terms to represent any JSON-LD context in RDF. As a result, contexts can be directly embedded in JSON schema definitions as well. The example below is a variant of in which only a subset of schema.org's default context is embedded in the schema.

Embedding schema definitions in data instances

Although schema definitions are often put in separate documents, it may be convenient to embed them in instance data itself, as in the following example, which uses the Smart Appliance Reference vocabulary (SAREF) [[smartm2m]]. This usage of the JSON schema vocabulary is similar to that of schema:PropertyValue and schema:PropertyValueSpecification.

This way, it is possible to convey a JSON tree representation that does not strictly follow the RDF structure of some entity. In the following example, the temperature property expressed in SAREF terms embeds a variant of the schema of .