def detector_distance(event):
    """
    Gets the detector distance for an event retrieved from Karabo at XFEL.

    Karabo events don't currently contain detector distance information. This function
    takes it from the configuration file, specifically from the
    'fallback_detector_distance_in_mm' entry in the 'DataRetrievalLayer'
    parameter group.

    Arguments:

        event (:class:`~onda.utils.data_event.DataEvent`): an object storing the event
            data.

    Returns:

        float: the detector distance in mm.
    """
    return event.framework_info['detector_distance']
