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@@ -314,7 +314,7 @@ document.write("Last Published: " + document.lastModified);
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<a href="#Other+Useful+Features">Other Useful Features</a>
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<ul class="minitoc">
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<li>
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-<a href="#Submitting+Jobs+to+a+Queue">Submitting Jobs to a Queue</a>
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+<a href="#Submitting+Jobs+to+Queues">Submitting Jobs to Queues</a>
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</li>
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<li>
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<a href="#Counters">Counters</a>
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@@ -351,7 +351,7 @@ document.write("Last Published: " + document.lastModified);
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<a href="#Example%3A+WordCount+v2.0">Example: WordCount v2.0</a>
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<ul class="minitoc">
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<li>
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-<a href="#Source+Code-N10F95">Source Code</a>
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+<a href="#Source+Code-N10FA4">Source Code</a>
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</li>
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<li>
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<a href="#Sample+Runs">Sample Runs</a>
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@@ -2283,23 +2283,26 @@ document.write("Last Published: " + document.lastModified);
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<span class="codefrag">FileSystem</span>.</p>
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<a name="N10D0C"></a><a name="Other+Useful+Features"></a>
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<h3 class="h4">Other Useful Features</h3>
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-<a name="N10D12"></a><a name="Submitting+Jobs+to+a+Queue"></a>
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-<h4>Submitting Jobs to a Queue</h4>
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-<p>Some job schedulers supported in Hadoop, like the
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- <a href="capacity_scheduler.html">Capacity
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- Scheduler</a>, support multiple queues. If such a scheduler is
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- being used, users can submit jobs to one of the queues
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- administrators would have defined in the
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- <em>mapred.queue.names</em> property of the Hadoop site
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- configuration. The queue name can be specified through the
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- <em>mapred.job.queue.name</em> property, or through the
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- <a href="api/org/apache/hadoop/mapred/JobConf.html#setQueueName(java.lang.String)">setQueueName(String)</a>
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- API. Note that administrators may choose to define ACLs
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- that control which queues a job can be submitted to by a
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- given user. In that case, if the job is not submitted
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- to one of the queues where the user has access,
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- the job would be rejected.</p>
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-<a name="N10D2A"></a><a name="Counters"></a>
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+<a name="N10D12"></a><a name="Submitting+Jobs+to+Queues"></a>
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+<h4>Submitting Jobs to Queues</h4>
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+<p>Users submit jobs to Queues. Queues, as collection of jobs,
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+ allow the system to provide specific functionality. For example,
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+ queues use ACLs to control which users
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+ who can submit jobs to them. Queues are expected to be primarily
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+ used by Hadoop Schedulers. </p>
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+<p>Hadoop comes configured with a single mandatory queue, called
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+ 'default'. Queue names are defined in the
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+ <span class="codefrag">mapred.queue.names</span> property of the Hadoop site
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+ configuration. Some job schedulers, such as the
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+ <a href="capacity_scheduler.html">Capacity Scheduler</a>,
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+ support multiple queues.</p>
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+<p>A job defines the queue it needs to be submitted to through the
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+ <span class="codefrag">mapred.job.queue.name</span> property, or through the
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+ <a href="api/org/apache/hadoop/mapred/JobConf.html#setQueueName(java.lang.String)">setQueueName(String)</a>
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+ API. Setting the queue name is optional. If a job is submitted
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+ without an associated queue name, it is submitted to the 'default'
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+ queue.</p>
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+<a name="N10D30"></a><a name="Counters"></a>
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<h4>Counters</h4>
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<p>
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<span class="codefrag">Counters</span> represent global counters, defined either by
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@@ -2316,7 +2319,7 @@ document.write("Last Published: " + document.lastModified);
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in the <span class="codefrag">map</span> and/or
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<span class="codefrag">reduce</span> methods. These counters are then globally
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aggregated by the framework.</p>
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-<a name="N10D59"></a><a name="DistributedCache"></a>
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+<a name="N10D5F"></a><a name="DistributedCache"></a>
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<h4>DistributedCache</h4>
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<p>
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<a href="api/org/apache/hadoop/filecache/DistributedCache.html">
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@@ -2387,7 +2390,7 @@ document.write("Last Published: " + document.lastModified);
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<span class="codefrag">mapred.job.classpath.{files|archives}</span>. Similarly the
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cached files that are symlinked into the working directory of the
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task can be used to distribute native libraries and load them.</p>
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-<a name="N10DDC"></a><a name="Tool"></a>
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+<a name="N10DE2"></a><a name="Tool"></a>
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<h4>Tool</h4>
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<p>The <a href="api/org/apache/hadoop/util/Tool.html">Tool</a>
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interface supports the handling of generic Hadoop command-line options.
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@@ -2427,7 +2430,7 @@ document.write("Last Published: " + document.lastModified);
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</span>
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</p>
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-<a name="N10E0E"></a><a name="IsolationRunner"></a>
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+<a name="N10E14"></a><a name="IsolationRunner"></a>
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<h4>IsolationRunner</h4>
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<p>
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<a href="api/org/apache/hadoop/mapred/IsolationRunner.html">
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@@ -2451,7 +2454,7 @@ document.write("Last Published: " + document.lastModified);
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<p>
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<span class="codefrag">IsolationRunner</span> will run the failed task in a single
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jvm, which can be in the debugger, over precisely the same input.</p>
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-<a name="N10E41"></a><a name="Profiling"></a>
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+<a name="N10E47"></a><a name="Profiling"></a>
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<h4>Profiling</h4>
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<p>Profiling is a utility to get a representative (2 or 3) sample
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of built-in java profiler for a sample of maps and reduces. </p>
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@@ -2484,39 +2487,40 @@ document.write("Last Published: " + document.lastModified);
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<span class="codefrag">-agentlib:hprof=cpu=samples,heap=sites,force=n,thread=y,verbose=n,file=%s</span>
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</p>
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-<a name="N10E75"></a><a name="Debugging"></a>
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+<a name="N10E7B"></a><a name="Debugging"></a>
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<h4>Debugging</h4>
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-<p>Map/Reduce framework provides a facility to run user-provided
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- scripts for debugging. When map/reduce task fails, user can run
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- script for doing post-processing on task logs i.e task's stdout,
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- stderr, syslog and jobconf. The stdout and stderr of the
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- user-provided debug script are printed on the diagnostics.
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- These outputs are also displayed on job UI on demand. </p>
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-<p> In the following sections we discuss how to submit debug script
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- along with the job. For submitting debug script, first it has to
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- distributed. Then the script has to supplied in Configuration. </p>
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-<a name="N10E81"></a><a name="How+to+distribute+script+file%3A"></a>
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-<h5> How to distribute script file: </h5>
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+<p>The Map/Reduce framework provides a facility to run user-provided
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+ scripts for debugging. When a map/reduce task fails, a user can run
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+ a debug script, to process task logs for example. The script is
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+ given access to the task's stdout and stderr outputs, syslog and
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+ jobconf. The output from the debug script's stdout and stderr is
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+ displayed on the console diagnostics and also as part of the
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+ job UI. </p>
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+<p> In the following sections we discuss how to submit a debug script
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+ with a job. The script file needs to be distributed and submitted to
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+ the framework.</p>
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+<a name="N10E87"></a><a name="How+to+distribute+the+script+file%3A"></a>
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+<h5> How to distribute the script file: </h5>
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<p>
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- The user has to use
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+ The user needs to use
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<a href="mapred_tutorial.html#DistributedCache">DistributedCache</a>
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- mechanism to <em>distribute</em> and <em>symlink</em> the
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- debug script file.</p>
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-<a name="N10E95"></a><a name="How+to+submit+script%3A"></a>
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-<h5> How to submit script: </h5>
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-<p> A quick way to submit debug script is to set values for the
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- properties "mapred.map.task.debug.script" and
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- "mapred.reduce.task.debug.script" for debugging map task and reduce
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- task respectively. These properties can also be set by using APIs
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+ to <em>distribute</em> and <em>symlink</em> the script file.</p>
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+<a name="N10E9B"></a><a name="How+to+submit+the+script%3A"></a>
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+<h5> How to submit the script: </h5>
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+<p> A quick way to submit the debug script is to set values for the
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+ properties <span class="codefrag">mapred.map.task.debug.script</span> and
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+ <span class="codefrag">mapred.reduce.task.debug.script</span>, for debugging map and
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+ reduce tasks respectively. These properties can also be set by using APIs
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<a href="api/org/apache/hadoop/mapred/JobConf.html#setMapDebugScript(java.lang.String)">
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JobConf.setMapDebugScript(String) </a> and
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<a href="api/org/apache/hadoop/mapred/JobConf.html#setReduceDebugScript(java.lang.String)">
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- JobConf.setReduceDebugScript(String) </a>. For streaming, debug
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- script can be submitted with command-line options -mapdebug,
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- -reducedebug for debugging mapper and reducer respectively.</p>
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-<p>The arguments of the script are task's stdout, stderr,
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+ JobConf.setReduceDebugScript(String) </a>. In streaming mode, a debug
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+ script can be submitted with the command-line options
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+ <span class="codefrag">-mapdebug</span> and <span class="codefrag">-reducedebug</span>, for debugging
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+ map and reduce tasks respectively.</p>
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+<p>The arguments to the script are the task's stdout, stderr,
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syslog and jobconf files. The debug command, run on the node where
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- the map/reduce failed, is: <br>
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+ the map/reduce task failed, is: <br>
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<span class="codefrag"> $script $stdout $stderr $syslog $jobconf </span>
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</p>
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@@ -2526,17 +2530,17 @@ document.write("Last Published: " + document.lastModified);
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<span class="codefrag">$script $stdout $stderr $syslog $jobconf $program </span>
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</p>
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-<a name="N10EB7"></a><a name="Default+Behavior%3A"></a>
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+<a name="N10EC9"></a><a name="Default+Behavior%3A"></a>
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<h5> Default Behavior: </h5>
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<p> For pipes, a default script is run to process core dumps under
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gdb, prints stack trace and gives info about running threads. </p>
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-<a name="N10EC2"></a><a name="JobControl"></a>
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+<a name="N10ED4"></a><a name="JobControl"></a>
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<h4>JobControl</h4>
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<p>
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<a href="api/org/apache/hadoop/mapred/jobcontrol/package-summary.html">
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JobControl</a> is a utility which encapsulates a set of Map/Reduce jobs
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and their dependencies.</p>
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-<a name="N10ECF"></a><a name="Data+Compression"></a>
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+<a name="N10EE1"></a><a name="Data+Compression"></a>
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<h4>Data Compression</h4>
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<p>Hadoop Map/Reduce provides facilities for the application-writer to
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specify compression for both intermediate map-outputs and the
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@@ -2550,7 +2554,7 @@ document.write("Last Published: " + document.lastModified);
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codecs for reasons of both performance (zlib) and non-availability of
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Java libraries (lzo). More details on their usage and availability are
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available <a href="native_libraries.html">here</a>.</p>
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-<a name="N10EEF"></a><a name="Intermediate+Outputs"></a>
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+<a name="N10F01"></a><a name="Intermediate+Outputs"></a>
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<h5>Intermediate Outputs</h5>
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<p>Applications can control compression of intermediate map-outputs
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via the
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@@ -2559,7 +2563,7 @@ document.write("Last Published: " + document.lastModified);
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<span class="codefrag">CompressionCodec</span> to be used via the
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<a href="api/org/apache/hadoop/mapred/JobConf.html#setMapOutputCompressorClass(java.lang.Class)">
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JobConf.setMapOutputCompressorClass(Class)</a> api.</p>
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-<a name="N10F04"></a><a name="Job+Outputs"></a>
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+<a name="N10F16"></a><a name="Job+Outputs"></a>
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<h5>Job Outputs</h5>
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<p>Applications can control compression of job-outputs via the
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<a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#setCompressOutput(org.apache.hadoop.mapred.JobConf,%20boolean)">
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@@ -2576,64 +2580,60 @@ document.write("Last Published: " + document.lastModified);
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<a href="api/org/apache/hadoop/mapred/SequenceFileOutputFormat.html#setOutputCompressionType(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.io.SequenceFile.CompressionType)">
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SequenceFileOutputFormat.setOutputCompressionType(JobConf,
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SequenceFile.CompressionType)</a> api.</p>
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-<a name="N10F31"></a><a name="Skipping+Bad+Records"></a>
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+<a name="N10F43"></a><a name="Skipping+Bad+Records"></a>
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<h4>Skipping Bad Records</h4>
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-<p>Hadoop provides an optional mode of execution in which the bad
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- records are detected and skipped in further attempts.
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- Applications can control various settings via
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+<p>Hadoop provides an option where a certain set of bad input
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+ records can be skipped when processing map inputs. Applications
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+ can control this feature through the
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<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html">
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- SkipBadRecords</a>.</p>
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-<p>This feature can be used when map/reduce tasks crashes
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- deterministically on certain input. This happens due to bugs in the
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- map/reduce function. The usual course would be to fix these bugs.
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- But sometimes this is not possible; perhaps the bug is in third party
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- libraries for which the source code is not available. Due to this,
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- the task never reaches to completion even with multiple attempts and
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- complete data for that task is lost.</p>
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-<p>With this feature, only a small portion of data is lost surrounding
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- the bad record. This may be acceptable for some user applications;
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- for example applications which are doing statistical analysis on
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- very large data. By default this feature is disabled. For turning it
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- on refer <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setMapperMaxSkipRecords(org.apache.hadoop.conf.Configuration, long)">
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+ SkipBadRecords</a> class.</p>
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+<p>This feature can be used when map tasks crash deterministically
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+ on certain input. This usually happens due to bugs in the
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+ map function. Usually, the user would have to fix these bugs.
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+ This is, however, not possible sometimes. The bug may be in third
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+ party libraries, for example, for which the source code is not
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+ available. In such cases, the task never completes successfully even
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+ after multiple attempts, and the job fails. With this feature, only
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+ a small portion of data surrounding the
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+ bad records is lost, which may be acceptable for some applications
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+ (those performing statistical analysis on very large data, for
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+ example). </p>
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+<p>By default this feature is disabled. For enabling it,
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+ refer to <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setMapperMaxSkipRecords(org.apache.hadoop.conf.Configuration, long)">
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SkipBadRecords.setMapperMaxSkipRecords(Configuration, long)</a> and
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<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setReducerMaxSkipGroups(org.apache.hadoop.conf.Configuration, long)">
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SkipBadRecords.setReducerMaxSkipGroups(Configuration, long)</a>.
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</p>
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-<p>The skipping mode gets kicked off after certain no of failures
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+<p>With this feature enabled, the framework gets into 'skipping
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+ mode' after a certain number of map failures. For more details,
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see <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setAttemptsToStartSkipping(org.apache.hadoop.conf.Configuration, int)">
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- SkipBadRecords.setAttemptsToStartSkipping(Configuration, int)</a>.
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- </p>
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-<p>In the skipping mode, the map/reduce task maintains the record
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- range which is getting processed at all times. For maintaining this
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- range, the framework relies on the processed record
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- counter. see <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#COUNTER_MAP_PROCESSED_RECORDS">
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+ SkipBadRecords.setAttemptsToStartSkipping(Configuration, int)</a>.
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+ In 'skipping mode', map tasks maintain the range of records being
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+ processed. To do this, the framework relies on the processed record
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+ counter. See <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#COUNTER_MAP_PROCESSED_RECORDS">
|
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SkipBadRecords.COUNTER_MAP_PROCESSED_RECORDS</a> and
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<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#COUNTER_REDUCE_PROCESSED_GROUPS">
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SkipBadRecords.COUNTER_REDUCE_PROCESSED_GROUPS</a>.
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- Based on this counter, the framework knows that how
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- many records have been processed successfully by mapper/reducer.
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- Before giving the
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- input to the map/reduce function, it sends this record range to the
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- Task tracker. If task crashes, the Task tracker knows which one was
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- the last reported range. On further attempts that range get skipped.
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- </p>
|
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-<p>The number of records skipped for a single bad record depends on
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- how frequent, the processed counters are incremented by the application.
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- It is recommended to increment the counter after processing every
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- single record. However in some applications this might be difficult as
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- they may be batching up their processing. In that case, the framework
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- might skip more records surrounding the bad record. If users want to
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- reduce the number of records skipped, then they can specify the
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- acceptable value using
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+ This counter enables the framework to know how many records have
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+ been processed successfully, and hence, what record range caused
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+ a task to crash. On further attempts, this range of records is
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+ skipped.</p>
|
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+<p>The number of records skipped depends on how frequently the
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+ processed record counter is incremented by the application.
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+ It is recommended that this counter be incremented after every
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+ record is processed. This may not be possible in some applications
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+ that typically batch their processing. In such cases, the framework
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+ may skip additional records surrounding the bad record. Users can
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+ control the number of skipped records through
|
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|
<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setMapperMaxSkipRecords(org.apache.hadoop.conf.Configuration, long)">
|
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|
SkipBadRecords.setMapperMaxSkipRecords(Configuration, long)</a> and
|
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|
<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setReducerMaxSkipGroups(org.apache.hadoop.conf.Configuration, long)">
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|
SkipBadRecords.setReducerMaxSkipGroups(Configuration, long)</a>.
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- The framework tries to narrow down the skipped range by employing the
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- binary search kind of algorithm during task re-executions. The skipped
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- range is divided into two halves and only one half get executed.
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- Based on the subsequent failure, it figures out which half contains
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- the bad record. This task re-execution will keep happening till
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+ The framework tries to narrow the range of skipped records using a
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+ binary search-like approach. The skipped range is divided into two
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+ halves and only one half gets executed. On subsequent
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+ failures, the framework figures out which half contains
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+ bad records. A task will be re-executed till the
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acceptable skipped value is met or all task attempts are exhausted.
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To increase the number of task attempts, use
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<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxMapAttempts(int)">
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@@ -2641,16 +2641,15 @@ document.write("Last Published: " + document.lastModified);
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<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxReduceAttempts(int)">
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JobConf.setMaxReduceAttempts(int)</a>.
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</p>
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-<p>The skipped records are written to the hdfs in the sequence file
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- format, which could be used for later analysis. The location of
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- skipped records output path can be changed by
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+<p>Skipped records are written to HDFS in the sequence file
|
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|
+ format, for later analysis. The location can be changed through
|
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|
<a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setSkipOutputPath(org.apache.hadoop.mapred.JobConf, org.apache.hadoop.fs.Path)">
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SkipBadRecords.setSkipOutputPath(JobConf, Path)</a>.
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</p>
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</div>
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-<a name="N10F7B"></a><a name="Example%3A+WordCount+v2.0"></a>
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+<a name="N10F8A"></a><a name="Example%3A+WordCount+v2.0"></a>
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|
<h2 class="h3">Example: WordCount v2.0</h2>
|
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<div class="section">
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|
<p>Here is a more complete <span class="codefrag">WordCount</span> which uses many of the
|
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@@ -2660,7 +2659,7 @@ document.write("Last Published: " + document.lastModified);
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<a href="quickstart.html#SingleNodeSetup">pseudo-distributed</a> or
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<a href="quickstart.html#Fully-Distributed+Operation">fully-distributed</a>
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Hadoop installation.</p>
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-<a name="N10F95"></a><a name="Source+Code-N10F95"></a>
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+<a name="N10FA4"></a><a name="Source+Code-N10FA4"></a>
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<h3 class="h4">Source Code</h3>
|
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|
<table class="ForrestTable" cellspacing="1" cellpadding="4">
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@@ -3870,7 +3869,7 @@ document.write("Last Published: " + document.lastModified);
|
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</tr>
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</table>
|
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-<a name="N116F7"></a><a name="Sample+Runs"></a>
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|
+<a name="N11706"></a><a name="Sample+Runs"></a>
|
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|
<h3 class="h4">Sample Runs</h3>
|
|
|
<p>Sample text-files as input:</p>
|
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|
<p>
|
|
|
@@ -4038,7 +4037,7 @@ document.write("Last Published: " + document.lastModified);
|
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|
<br>
|
|
|
|
|
|
</p>
|
|
|
-<a name="N117CB"></a><a name="Highlights"></a>
|
|
|
+<a name="N117DA"></a><a name="Highlights"></a>
|
|
|
<h3 class="h4">Highlights</h3>
|
|
|
<p>The second version of <span class="codefrag">WordCount</span> improves upon the
|
|
|
previous one by using some features offered by the Map/Reduce framework:
|
Christopher Douglas