@Override
    public void onDataNodeDelete(String instanceName) {
      if (listener != null) {
        listener.onDataNodeDelete(instanceName);
      }
      configsSeen.remove(instanceName);
    }
@param listener if non-null, this listener will be notified on any change.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/CompositeDataNodeConfigSource.java