How data or file is written into HDFS?
When a client wants to write a file to HDFS, it communicates to namenode for metadata. The Namenode responds with details of a number of blocks, replication factor. Then, on basis of information from NameNode, client split files into multiple blocks. After that client starts sending them to first DataNode. The client sends block A to Datanode 1 with other two Datanodes details.
When Datanode 1 receives block A sent from the client, Datanode 1 copy same block to Datanode 2 of the same rack. As both the Datanodes are in the same rack so block transfer via rack switch. Now Datanode 2 copies the same block to Datanode 3. As both the Datanodes are in different racks so block transfer via an out-of-rack switch.
After the Datanode receives the blocks from the client. Then Datanode sends write confirmation to Namenode. Now Datanode sends write confirmation to the client. The Same process will repeat for each block of the file. Data transfer happen in parallel for faster write of blocks.
Read HDFS file write operation workflow in detail
