public List<LayerLevel> listLayerLevels() {
        return getLayerLevels().entrySet()
                               .stream()
                               .map(entry -> new LayerLevel(entry.getKey(), entry.getValue()))
                               .collect(Collectors.toList());
    }
@return return the related service hierarchy recursively, e.g. A-B-C, query A will return A-B, B-C
If the relation could be conjectured will be removed, e.g. A-B-C-D and A-D, query A will return A-B, B-C, C-D because A-D could be conjectured.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/HierarchyQueryService.java