public void shutdownCluster() {
    String statusStr = "Cluster shutdown requested of master=" + this.master.getServerName();
    LOG.info(statusStr);
    this.clusterShutdown.set(true);
    if (onlineServers.isEmpty()) {
      // we do not synchronize here so this may cause a double stop, but not a big deal
      master.stop("OnlineServer=0 right after cluster shutdown set");
    }
  }
Check if a server is unknown. A server can be online, or known to be dead, or unknown to this
manager (i.e, not online, not known to be dead either; it is simply not tracked by the master
any more, for example, a very old previous instance).
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java