@Override
    protected StorageID id0() {
        String id = IDManager.ServiceID.buildServiceHierarchyRelationId(
            new IDManager.ServiceID.ServiceHierarchyRelationDefine(
                serviceId, serviceLayer, relatedServiceId, relatedServiceLayer));

        return new StorageID().appendMutant(new String[] {
            SERVICE_ID,
            SERVICE_LAYER,
            RELATED_SERVICE_ID,
            RELATED_SERVICE_LAYER
        }, id);
    }
The service layer of the lower service.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/hierarchy/service/ServiceHierarchyRelationTraffic.java