private SortedSet<Address> getOnlineServers() {
    SortedSet<Address> onlineServers = new TreeSet<Address>();
    serverManager.getOnlineServers().keySet().stream().map(ServerName::getAddress)
      .forEach(onlineServers::add);
    return onlineServers;
  }
A dummy RSGroupInfoManager which only contains a default rs group.
hbase-server/src/main/java/org/apache/hadoop/hbase/rsgroup/DisabledRSGroupInfoManager.java