Grokking the Advanced System Design Interview
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Anatomy of a Read Operation

HDFS read process

HDFS read process can be outlined as follows:

  1. When a file is opened for reading, HDFS client initiates a read request, by calling the open() method of the Distributed FileSystem object. The client specifies the file name, start offset, and the read range length.
  2. The Distributed FileSystem object calculates what blocks need to be read based on the given offset and range length, and requests the locations of the blocks from the NameNode.
  3. NameNode has metadata for all blocks' locations




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