HelixClusterChangeHandler getHelixClusterChangeHandler(String dcName) {
    if (clusterMapConfig.clusterMapUseAggregatedView) {
      return helixAggregatedViewClusterInfo.clusterChangeHandler;
    } else {
      return dcToDcInfo.get(dcName).clusterChangeHandler;
    }
  }
Exposed for testing
@param dcName data center name
@return {@link HelixClusterChangeHandler} that handles cluster changes for a given data center. If aggregated view
is enabled, same handler is used for all data centers.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java