'Returns the cache key to use for the given
BlockStructureModel or StubModel.'

@staticmethod
def _encode_root_cache_key(bs_model):

    if _is_storage_backing_enabled():
        return unicode(bs_model)
    else:
        return 'v{version}.root.key.{root_usage_key}'.format(version=unicode(BlockStructureBlockData.VERSION), root_usage_key=unicode(bs_model.data_usage_key))
