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Can we have Hadoop job output in multiple directories?

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Can we have Hadoop job output in multiple directories?

Yes, it is possible to have the output of Hadoop MapReduce Job written to multiple directories. In Hadoop MapReduce, the output of Reducer is the final output of a Job, and thus its written in to the Hadoop Local File System(HDFS).

When HDFS is being used what happens when a file is deleted from the command line?

Q 20 – When using HDFS, what occurs when a file is deleted from the command line? A – It is permanently deleted if trash is enabled.

How can I run Mapper and Reducer in Hadoop?

Your answer

  1. Now for exporting the jar part, you should do this:
  2. Now, browse to where you want to save the jar file. Step 2: Copy the dataset to the hdfs using the below command: hadoop fs -put wordcountproblem​
  3. Step 4: Execute the MapReduce code:
  4. Step 8: Check the output directory for your output.
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How does MapReduce Work?

A MapReduce job usually splits the input datasets and then process each of them independently by the Map tasks in a completely parallel manner. The output is then sorted and input to reduce tasks. Both job input and output are stored in file systems. Tasks are scheduled and monitored by the framework.

Why does Hadoop create multiple output files?

MultipleOutputs class provide facility to write Hadoop map/reducer output to more than one folders. Basically, we can use MultipleOutputs when we want to write outputs other than map reduce job default output and write map reduce job output to different files provided by a user.

In which file of output directory output is getting written in Hadoop?

The way these key-value pairs are written in Output files by RecordWriter is determined by the OutputFormat. OutputFormat instances provided by the Hadoop are used to write to files on the local disk or in HDFS. FileOutputFormat.

What happens when write attempt to HDFS fails?

If block write fails in the first datanodes, it’ll abandon the block write and ask namenode a new set of datanodes where it can attempt to write again.

On which machine does combiner run?

The Combiner class is used in between the Map class and the Reduce class to reduce the volume of data transfer between Map and Reduce. Usually, the output of the map task is large and the data transferred to the reduce task is high. The following MapReduce task diagram shows the COMBINER PHASE.

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How does reducer work in Hadoop?

Reducer in Hadoop MapReduce reduces a set of intermediate values which share a key to a smaller set of values. In MapReduce job execution flow, Reducer takes a set of an intermediate key-value pair produced by the mapper as the input. The user decides the number of reducers in MapReduce.

What is the output of the reducer?

In Hadoop, Reducer takes the output of the Mapper (intermediate key-value pair) process each of them to generate the output. The output of the reducer is the final output, which is stored in HDFS. Usually, in the Hadoop Reducer, we do aggregation or summation sort of computation.

What is MapReduce job in Hadoop?

During a MapReduce job, Hadoop sends the Map and Reduce tasks to the appropriate servers in the cluster. The framework manages all the details of data-passing such as issuing tasks, verifying task completion, and copying data around the cluster between the nodes.

What outcomes can you achieve by running MapReduce jobs in Hadoop?

Benefits of Hadoop MapReduce

  • Speed: MapReduce can process huge unstructured data in a short time.
  • Fault-tolerance: The MapReduce framework can handle failures.
  • Cost-effective: Hadoop has a scale-out feature that enables users to process or store data in a cost-effective manner.
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How to create the output directory in Hadoop?

Solution:-Always specify the output directory name at run time(i.e Hadoop will create the directory automatically for you. You need not to worry about the output directory creation). As mentioned in the above example the same command can be run in following manner –

How many JobTracker processes are there in Hadoop MapReduce?

There is only One Job Tracker process run on any hadoop cluster. Job Tracker runs on its own JVM process. In a typical production cluster its run on a separate machine. Each slave node is configured with job tracker node location. The JobTracker is single point of failure for the Hadoop MapReduce service.

Why is my output directory not available in HDFS?

-1 You are getting above exception because your output directory (/Users/msadri/Documents/files/linkage_output)is already created/existing in the HDFS file system Just remember while running map reduce job do mention the output directory which is already their in HDFS.

What is the Hadoop distributed file system?

The Hadoop Distributed File System (HDFS) is a distributed file system designed to run on commodity hardware. It has many similarities with existing distributed file systems. However, the differences from other distributed file systems are significant. Following are differences between HDFS and NAS