{
    "$id": "https://example.com/schemas/customer",
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "first_name": { "type": "string" },
      "last_name": { "type": "string" },
      "shipping_address": { "$ref": "#/$defs/address" },
      "billing_address": { "$ref": "#/$defs/address" }
    },
    "required": ["first_name", "last_name", "shipping_address", "billing_address"],

    "$defs": {
      "address": {
        "$id": "/schemas/address",
        "$schema": "http://json-schema.org/draft-07/schema#",

        "type": "object",
        "properties": {
          "street_address": { "type": "string" },
          "city": { "type": "string" },
          "state": { "$ref": "#/$defs/definitions/state" }
        },
        "required": ["street_address", "city", "state"]
      },
      "definitions": {
        "state": { "enum": ["CA", "NY", "... etc ..."] }
      }
    }
  }