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From Apache Wiki <wikidi...@apache.org>
Subject [Lucene-hadoop Wiki] Update of "HowToDebugMapReducePrograms" by OwenOMalley
Date Mon, 18 Jun 2007 05:48:40 GMT
Dear Wiki user,

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The following page has been changed by OwenOMalley:

  This can be extremely useful to display debug information about the current record being
handled, or setting certain debug flags about the status of the mapper. While running locally
on a small data set can find many bugs, large data sets may contain pathological cases that
are otherwise unexepcted. This method of debugging can help catch those cases.
+ == How to debug Hadoop Pipes programs
+ In order to debug Pipes programs you need to keep the downloaded commands. 
+ First, to keep the !TaskTracker from deleting the files when the task is finished, you need
to set either keep.failed.task.files (set to true if the task you want to debug fails) or
keep.task.files.pattern (set to a regex of the task name you want to debug).
+ Second, your job should set hadoop.pipes.command-file.keep to true in the JobConf. This
will cause all of the tasks in the job to write their command stream to a file in the working
directory named downlink.data. This file will contain the JobConf, the task information, and
the task input, so it may be large. But it provides enough information that your executable
will run without any interaction with the framework. 
+ Third, go to the host where the problem task ran, go into the work directory and
+ {{{
+ setenv hadoop.pipes.command.file downlink.data
+ }}}
+ and run your executable under the debugger or valgrind. It will run as if the framework
was feeding it commands and data and produce a output file downlink.data.out with the binary
commands that it would have sent up to the framework. I guess eventually, I should have the
output file be written in text rather than binary...

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