====== MapReduce Tutorial : If things go wrong ======

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.