@Override
    public void onDataNodeConfigChange(Iterable<DataNodeConfig> configs) {
      if (listener != null) {
        listener.onDataNodeConfigChange(configs);
      }
      for (DataNodeConfig config : configs) {
        configsSeen.put(config.getInstanceName(), config);
      }
    }
@param listener if non-null, this listener will be notified on any change.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/CompositeDataNodeConfigSource.java