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POIn Hadoop where does the framework save the output of the Map task in a normal Map-Reduce Application?
 

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  1. POIn Hadoop where does the framework save the output of the Map task in a normal Map-Reduce Application?
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    <p>I am trying to find out where does the output of a Map task is saved to disk before it can be used by a Reduce task.</p> <p><strong>Note:</strong> - version used is Hadoop 0.20.204 with the new API</p> <p>For example, when overwriting the map method in the Map class:</p> <pre><code>public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String line = value.toString(); StringTokenizer tokenizer = new StringTokenizer(line); while (tokenizer.hasMoreTokens()) { word.set(tokenizer.nextToken()); context.write(word, one); } // code that starts a new Job. } </code></pre> <p>I am interested to find out where does context.write() ends up writing the data. So far i've ran into the:</p> <pre><code>FileOutputFormat.getWorkOutputPath(context); </code></pre> <p>Which gives me the following location on hdfs:</p> <pre><code>hdfs://localhost:9000/tmp/outputs/1/_temporary/_attempt_201112221334_0001_m_000000_0 </code></pre> <p>When i try to use it as input for another job it gives me the following error:</p> <pre><code>org.apache.hadoop.mapreduce.lib.input.InvalidInputException: Input path does not exist: hdfs://localhost:9000/tmp/outputs/1/_temporary/_attempt_201112221334_0001_m_000000_0 </code></pre> <p><strong>Note:</strong> the job is started in the Mapper, so technically, the temporary folder where the Mapper task is writing it's output exists when the new job begins. Then again, it still says that the input path does not exist.</p> <p>Any ideas to where the temporary output is written to? Or maybe what is the location where i can find the output of a Map task during a job that has both a Map and a Reduce stage?</p>
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    1. COBTW, looks like FileOutputhFormat#getWorkOutputPath gives the location of the temporary output folder for the job and not for the map task as per the api documentation. `When i try to use it as input for another job it gives me the following error:` - usually temporary files are deleted once the job is completed or else the node would get full easily.
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      1. USPraveen Sripati
    2. COOK, fair point, but i am starting the job while the other job is executing. I'm doing nested map reduce. So, the outerjob is busy-waiting while the inner job is executing, so there shouldn't be any deletion of temporary files.
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    3. COBut ok, fair enough things get deleted, which means they were somewhere before they were deleted. I need to find out where that is. :D
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