@Override
  public List<ServerName> getOnlineServersList() {
    ServerManager sm = services.getServerManager();
    return sm != null ? sm.getOnlineServersList() : Collections.emptyList();
  }
Use to add balancer decision history to ring-buffer
hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/MasterClusterInfoProvider.java