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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-153) skip records that throw exceptions
Date Mon, 10 Mar 2008 22:34:49 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-153?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577231#action_12577231
] 

Doug Cutting commented on HADOOP-153:
-------------------------------------

The tricky bit will be identifying the failed record number, no?  The naive approach would
be to have the child report after each record has been processed, so that the parent can then
know, when it crashes, which record it was on.  But that would probably be too expensive.

So rather we might have the child report in its TaskStatus the range of record numbers it
intends to process before it next reports.  Then, on failure, the parent knows that skipping
that range will skip the offending record.  The child can adaptively choose a range that keeps
the status update time within reason.  It would start by reporting range 0-1, then use the
time required to process that entry to determine how big of a range to send the next time
and still provide timely updates.  If a particular record takes a long time, then it can update
status before the range is exhausted, providing a new range.

Would this be acceptable, to skip a few more than just the offending record?


> skip records that throw exceptions
> ----------------------------------
>
>                 Key: HADOOP-153
>                 URL: https://issues.apache.org/jira/browse/HADOOP-153
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>    Affects Versions: 0.2.0
>            Reporter: Doug Cutting
>            Assignee: Devaraj Das
>             Fix For: 0.17.0
>
>
> MapReduce should skip records that throw exceptions.
> If the exception is thrown under RecordReader.next() then RecordReader implementations
should automatically skip to the start of a subsequent record.
> Exceptions in map and reduce implementations can simply be logged, unless they happen
under RecordWriter.write().  Cancelling partial output could be hard.  So such output errors
will still result in task failure.
> This behaviour should be optional, but enabled by default.  A count of errors per task
and job should be maintained and displayed in the web ui.  Perhaps if some percentage of records
(>50%?) result in exceptions then the task should fail.  This would stop jobs early that
are misconfigured or have buggy code.
> Thoughts?

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