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 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.