HDFS - Read Anatomy

Following is the read anatomy of HDFS file:

                    

1. Client request the document

2. NN, checks the permissions and sends back the list of blocks and datanodes list (including port number to talk) for each block. 

3-6. "DFSClient" class on client-side picks up first block and requests the block from first datanode on the list. It tries two times and if no response then it adds the datanode to "deadnodes" list. And requests block from next datanode on the list.

7-8. After usccessful read of all the blocks, "DFSClient" send the deadnodes list back to NN for it to take action. 

I will talk about Write-anatomy in next post. Please keep reading...