public int getBackupZooKeeperServerNum() {
    return zooKeeperServers.size() - 1;
  }
Selects a ZK client port.
@param seedPort the seed port to start with; -1 means first time.
@return a valid and unused client port
hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/MiniZooKeeperCluster.java