The Json Schema allows schemas to be defined inline or retrieved from a reference.

To indicate a reference the $ref keyword is used with the value being the reference to the schema using the JSON Pointer convention.

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "type": "object",
    "additionalProperties": false,
    "properties": {
        "ProductCode": {
            "$ref": "#/definitions/ProductCodeType"
        }
    },
    "definitions": {
        "ProductCodeType": {
            "type": "string",
            "pattern": "[A-Z][0-9]*"
        }
    }
}

The resolution of a reference is carried out as follows.

• The base context for of the schema containing the $ref keyword is determined.
• This $ref value is combined with this base context to produce a qualified reference.
• The qualified traversed to find the schema that is referred to.

So working through our example, let's assume our schema is at http://example/test.json.

• We need to calculate the base context for the schema containing $ref ("ProductCode"). This is easies to do working back towards the root. In simple terms we do this by looking at the source and getting all the quoted names in the path, so working "ProductCode, "properties", the root schema takes its context from the URL "http://example/test.json". These are all combined to produce our base context which is http://example/test.json#/properties/ProductCode.
• We combine this base context (http://example/test.json#/properties/ProductCode) and our reference value (#/definitions/ProductCodeType) which gives us http://example/test.json#/definitions/ProductCodeType
• We then traverse the schema source to find the schema that is referenced.