输出完成后，通过向客户端流写入一个 NULL 对象，而且循环等待下一个客户端，从而刷新流。
When the output is complete, the stream is flushed by writing a NULL object to the client stream and loop awaiting the next client.
流水线复制当某个客户端向HDFS文件写数据的时候，一开始是写入本地临时文件，假设该文件的replication因子设置为3，那么客户端会从Namenode获取一张Datanode列表来存放副本。
When a client is writing data to an HDFS file, its data is first written to a local file as explained in the previous section. Suppose the HDFS file has a replication factor of three.
在设置为after_commit参数，发起事务的客户端仅在服务器向存储引擎写入数据并接受从库得到确认之后才返回状态。
With AFTER_COMMIT, the client issuing the transaction gets a return status only after the server commits to the storage engine and receives slave acknowledgment.
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