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courses:mapreduce-tutorial:if-things-go-wrong [2012/02/05 21:45]
straka 		courses:mapreduce-tutorial:if-things-go-wrong [2012/02/06 13:55] (current)
straka
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 ====== MapReduce Tutorial : If things go wrong ====== ====== MapReduce Tutorial : If things go wrong ======
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 +A lot can go wrong in the process of creating cluster and submitting the Hadoop job:
 +  * ''Hadoop::Runner.pm'' module not found: The Perl Hadoop package is not configured, see [[.:step-1|Setting the environment]].
 +  * ''ipc.Client: Retrying connect to server: IP_ADDRESS:PORT. Already tried ? time(s)'': The jobtracker cannot be contacted. If using ''-jt jobtracker:port'' flag, check, that the jobtracker address is correct.
 +  * ''/net/projects/hadoop/bin/hadoop-cluster'' fails to start a cluster: Look where the jobtracker was scheduled by SGE using ''qstat''. Login to that machine and investigate logs in ''/var/log/hadoop/$USER/$SGE_JOBID/''.
 +
 +If the cluster works, but your job crashes, you can:
 +  * run the computation locally in single threaded mode (i.e., without specifying ''-c'' and ''-jt'' flag). This is more useful for Hadoop jobs written in Java, because then you can use a debugger. When using Perl API, new subprocess are created for Perl tasks anyway.
 +  * use the web interface to see Java exceptions / Perl return codes for each failed task attempt.
 +  * use standard error output for log messages. You can access the stderr logs of all Hadoop tasks using the web interface.

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