public synchronized boolean isServerDead(ServerName serverName) {
    return serverName == null || deadservers.isDeadServer(serverName);
  }
Check if a server is known to be dead. 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