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Hadoop, mail # user - Managing stdout in streaming


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Managing stdout in streaming
Keith Wiley 2011-02-01, 20:55
So streaming uses stdout to organize the mapper/reducer output, one record per line with each key/val split at the first TAB.

(Presumably multiple TABS are permitted and become embedded in the value string, I haven't experimented with this yet).

Obviously, one must be very careful not to write any debugging or logging output to stdout.  It seems fairly straight-forward to simply use stderr instead, such that all associated output appears in the job tracker logs.

Buuuuut, what if I'm using a third-party library and I can't tell it to send output elsewhere?  I know that it is possible to redirect stdout using tricks like freopen(), but I believe it can be quite tricky to redirect stdout back to its original stream.  So if I directed stdout away from the original stream for processing, I'm not sure how I would latch it back onto the stream for the purpose of generating my mapper/reducer output data (in the Hadoop streaming TAB-delimited line-per-record format).

Any thoughts on this?  The cluster is running Linux incidentally.  I realize details like that become important when one starts fiddling with redirecting streams and such.

Thank you.

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Keith Wiley               [EMAIL PROTECTED]               www.keithwiley.com

"What I primarily learned in grad school is how much I *don't* know.
Consequently, I left grad school with a higher ignorance to knowledge ratio than
when I entered."
  -- Keith Wiley
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