On 11/14/2011 06:06 PM, Mapred Learn wrote:
Hi,
I have a use case where I want to pass a threshold value to a map-reduce
job. For eg: error records=10.
I want map-reduce job to fail if total count of error_records in the job
i.e. all mappers, is reached.
How can I implement this considering that each mapper would be processing
some part of the input data ?
Thanks,
-JJ
1) Pass in the threshold value as configuration value of the M/R job.
(i.e., job.getConfiguration().setInt("error_threshold", 10) )
2) Make your mappers implement the Configurable interface. This will
ensure that every mapper gets passed a copy of the config object.
3) When you implement the setConf() method in your mapper (which
Configurable will force you to do), retrieve the threshold value from
the config and save it in an instance variable in the mapper. (i.e.,
int errorThreshold = conf.getInt("error_threshold") )
4) In the mapper, when an error record occurs, increment a counter and
then check if the counter value exceeds the threshold. If so, throw an
exception. (e.g., if (++numErrors >= errorThreshold) throw new
RuntimeException("Too many errors") )
The exception will kill the mapper. Hadoop will attempt to re-run it,
but subsequent attempts will also fail for the same reason, and
eventually the entire job will fail.
HTH,
DR
