/**
   * Given an external view and a list of instance configs, computes the mapping of segment to servers. The mapping
   * will be cached if we use dynamic routing. By default, this will check ONLINE segments and active servers.
   *
   * @param externalView an external view
   * @param instanceConfigs a list of instance config
   * @return a mapping of segment to servers
   */
  protected Map<String, List<String>> computeSegmentToServersMapFromExternalView(ExternalView externalView,
      List<InstanceConfig> instanceConfigs) {
    Map<String, List<String>> segmentToServersMap = new HashMap<>();
    RoutingTableInstancePruner instancePruner = new RoutingTableInstancePruner(instanceConfigs);
    for (String segmentName : externalView.getPartitionSet()) {
      // List of servers that are active and are serving the segment
      List<String> servers = new ArrayList<>();
      for (Map.Entry<String, String> entry : externalView.getStateMap(segmentName).entrySet()) {
        String serverName = entry.getKey();
        if (entry.getValue().equals(<http://CommonConstants.Helix.StateModel.SegmentOnlineOfflineStateModel.ONLINE|CommonConstants.Helix.StateModel.SegmentOnlineOfflineStateModel.ONLINE>)
            && !instancePruner.isInactive(serverName)) {
          servers.add(serverName);
        }
      }
      if (!servers.isEmpty()) {
        segmentToServersMap.put(segmentName, servers);
      } else {
        handleNoServingHost(segmentName);
      }
    }
    return segmentToServersMap;
  }