static String toTaskDirString(final TaskId taskId) {
        return taskId.subtopology() + "_" + taskId.partition();
    }
@return The string representation of the subtopology and partition metadata, ie the task id string without
        the named topology, which defines the innermost task directory name of this task's state
streams/src/main/java/org/apache/kafka/streams/processor/internals/StateManagerUtil.java