private Set<Address> getOnlineServers() {
    return masterServices.getServerManager().getOnlineServers().keySet().stream()
      .map(ServerName::getAddress).collect(Collectors.toSet());
  }
Returns Set of online Servers named for their hostname and port (not ServerName).
hbase-server/src/main/java/org/apache/hadoop/hbase/rsgroup/RSGroupInfoManagerImpl.java