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Question ID 12469

Which describes how a client reads a file from HDFS?

Option A

The client queries the NameNode for the block location(s). The NameNode returns the block location(s) to the client. The client reads the data directory off the DataNode(s).

Option B

The client queries all DataNodes in parallel. The DataNode that contains the requested data responds directly to the client. The client reads the data directly off the DataNode.

Option C

The client contacts the NameNode for the block location(s). The NameNode then queries the DataNodes for block locations. The DataNodes respond to the NameNode, and the NameNode redirects the client to the DataNode that holds the requested data block(s). The client then reads the data directly off the DataNode.

Option D

The client contacts the NameNode for the block location(s). The NameNode contacts the DataNode that holds the requested data block. Data is transferred from the DataNode to the NameNode, and then from the NameNode to the client.

Correct Answer A
Explanation Reference: 24 Interview Questions & Answers for Hadoop MapReduce developers, How the Client communicates with HDFS?


Question ID 12470

You are developing a MapReduce job for sales reporting. The mapper will process input
keys representing the year (IntWritable) and input values representing product indentifies
(Text).
Indentify what determines the data types used by the Mapper for a given job.

Option A

The key and value types specified in the JobConf.setMapInputKeyClass and JobConf.setMapInputValuesClass methods

Option B

The data types specified in HADOOP_MAP_DATATYPES environment variable

Option C

The mapper-specification.xml file submitted with the job determine the mappers input key and value types.

Option D

The InputFormat used by the job determines the mapper’s input key and value types.

 

 

Correct Answer D
Explanation Explanation: The input types fed to the mapper are controlled by the InputFormat used. The default input format, "TextInputFormat," will load data in as (LongWritable, Text) pairs. The long value is the byte offset of the line in the file. The Text object holds the string contents of the line of the file. Note: The data types emitted by the reducer are identified by setOutputKeyClass() andsetOutputValueClass(). The data types emitted by the reducer are identified by setOutputKeyClass() and setOutputValueClass(). By default, it is assumed that these are the output types of the mapper as well. If this is not the case, the methods setMapOutputKeyClass() and setMapOutputValueClass() methods of the JobConf class will override these. Reference: Yahoo! Hadoop Tutorial, THE DRIVER METHOD

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