与targetNamespace等效的json模式是什么？

Question

In any xml file i can say what namespace I refer to using xmlns-attributes to describe the namespace. It is well descibed here: What does "xmlns" in XML mean?

Then I can use a xml schema having a target namespace so that everyone knows that the schema describes that namespace. One question about that is found here: Why do we need targetNamespace?

Using json-schema we can define schemas for json documents. My mental model is that this is roughly equivalent to having a xsd file.

Now, how do I reference the schema in a json object? I can reference a schema using $schema attribute, but how do I declare the name of the schema i develop myself? I dont understand the equivalent of targetNamespace

Answer 1

Researching for writing the question I found the answer. The equivalent of targetNamespace is the $id attribute. The standard states...

The "$id" keyword defines a URI for the schema, and the base URI that other URI references within the schema are resolved against. A subschema's "$id" is resolved against the base URI of its parent schema. If no parent sets an explicit base with "$id", the base URI is that of the entire document, as determined per RFC 3986 section 5 [RFC3986].

... which is kind of the mirror image of the leading text for $schema ...

The "$schema" keyword is both used as a JSON Schema version identifier and the location of a resource which is itself a JSON Schema, which describes any schema written for this particular version. The value of this keyword MUST be a URI [RFC3986] (containing a scheme) and this URI MUST be normalized. The current schema MUST be valid against the meta-schema identified by this URI.

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After some more research I've concluded that the above - altough is correct - does not provide the full story. Some additions regarding having the $schema attribute in data:

a) you use $schema in a schema to define what schema should be used for defining your own custom schema. It is not stated in the spec that $schema in any kind of object should indicate validation for a schema.

b) You may define in your schema that $schema should be an indication about what schema to use for validation.

c) there are other ways to indicate the schema for data. One such example is using content-type in http headers. Another is to use link http headers.

d) vscode and visual studio both interpret $schema as a reference to a schema for use in validation

The issue has been discussed at the github repo for the spec.

https://github.com/json-schema/json-schema/issues/235

https://github.com/json-schema/json-schema/issues/220#issuecomment-209452992