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What Steps Occur When a Client Reads a File in Hadoop Distributed File System?
Master the HDFS file read operation for your Hadoop certification. Discover how clients interact with NameNodes to retrieve metadata and directly stream block data from DataNodes to ensure high-speed parallel processing.
Question
How does HDFS handle reading a file?
A. The client retrieves metadata from the NameNode and then accesses DataNodes for blocks
B. Files are downloaded fully before processing starts
C. The client reads all blocks directly from the NameNode
D. Data is always read sequentially from one DataNode
Answer
A. The client retrieves metadata from the NameNode and then accesses DataNodes for blocks
Explanation
When reading a file in HDFS, the process begins with the client contacting the NameNode to retrieve the file’s metadata. This metadata contains critical information, specifically the locations of the data blocks and the specific DataNodes where those blocks are stored. Once the client has these addresses, it interacts directly with the closest DataNodes to read the actual data blocks, bypassing the NameNode for the data transfer itself. This architecture prevents the NameNode from becoming a bottleneck and allows for efficient, parallel processing rather than downloading entire files sequentially or pulling data directly through the master node.