public synchronized ServerLiveState isServerKnownAndOnline(ServerName serverName) {
    return onlineServers.containsKey(serverName)
      ? ServerLiveState.LIVE
      : (deadservers.isDeadServer(serverName) ? ServerLiveState.DEAD : ServerLiveState.UNKNOWN);
  }
Returns whether the server is online, dead, or unknown.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java