public boolean isServerUnknown(ServerName serverName) {
    return serverName == null
      || (!onlineServers.containsKey(serverName) && !deadservers.isDeadServer(serverName));
  }
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