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Anatomy of a Read Operation
HDFS read process
HDFS read process can be outlined as follows:
- When a file is opened for reading, HDFS client initiates a read request, by calling the
open()
method of theDistributed FileSystem
object. The client specifies the file name, start offset, and the read range length. - 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. - NameNode has metadata for all blocks' locations
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