def spec_is_valid(json):
    schema = {
        "type": "object",
        "properties": {
            "schema":           { "$ref": "#/pScheduler/Cardinal" },
            "duration":         { "$ref": "#/pScheduler/Duration" },
            "host":             { "$ref": "#/pScheduler/Host" },
            "host-node":        { "$ref": "#/pScheduler/URLHostPort" },
            "interval":         { "$ref": "#/pScheduler/Duration" },
            "parting-comment":  { "$ref": "#/pScheduler/String" },
            "starting-comment": { "$ref": "#/pScheduler/String" },
            },
        "required": [
            "duration"
            ]
        }

    return json_validate(json, schema, max_schema=MAX_SCHEMA)