#References

#Inside one schema

$ref takes a JSON Pointer fragment or an anchor:

{
    "$defs": {
        "positive": {"$anchor": "pos", "type": "integer", "minimum": 1}
    },
    "properties": {
        "count": {"$ref": "#/$defs/positive"},
        "limit": {"$ref": "#pos"}
    }
}

$id starts a new resource with its own base URI, and relative references inside it resolve against that base. A schema with no absolute $id gets DEFAULT_BASE, jsonschema:/root.json.

#Other documents

Nothing is fetched. A reference to another document resolves only if that document is in the Registry the schema was compiled with:

def reg as jsonschema.Registry init jsonschema.registry();
$reg = jsonschema.register($reg, "https://example.com/address.json", json.decode(fs.readString("address.json")));
def s as jsonschema.Schema init jsonschema.compileWith(json.decode(fs.readString("customer.json")), $reg);

register indexes the document at once, including any $id inside it, and the document's own absolute $id wins over the URI you give. Build the registry once and compile any number of schemas against it.

A reference that resolves to nothing throws an Error of kind jsonschema when evaluation reaches it, naming the reference.

#The meta-schemas

The draft 2020-12 meta-schema and its seven vocabulary meta-schemas are embedded. A schema that references one, such as {"$ref": "https://json-schema.org/draft/2020-12/schema"}, works without registering anything, which is how you validate a schema itself:

def meta as jsonschema.Schema init jsonschema.compile(json.decode('{"$ref": "https://json-schema.org/draft/2020-12/schema"}'));
jsonschema.isValid($meta, json.decode('{"type": 3}'));    # false

They are loaded only when something refers to them.

#$dynamicRef

Supported as draft 2020-12 defines it: a $dynamicRef to a plain-name fragment that first lands on a $dynamicAnchor resolves to the outermost resource in the dynamic scope with the same anchor. This is what lets a schema extend a recursive one, such as a tree whose nodes you constrain further.

#Vocabularies

When the root's $schema resolves to a meta-schema with a $vocabulary, two vocabularies are honoured:

• without the validation vocabulary, type, minimum, required and the other assertions are ignored;
• with the format-assertion vocabulary, format is asserted.

#Older drafts

Only 2020-12 is implemented. A schema written for an older draft is evaluated as 2020-12, which works for most of them: the one exception that matters, draft 7's dependencies, is honoured as well. definitions is not special; $ref to #/definitions/x still works because it is a plain JSON Pointer.