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From "Ivan Mitic (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-6357) MultipleOutputs.write() API should document that output committing is not utilized when input path is absolute
Date Wed, 29 Jul 2015 18:36:05 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-6357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14646575#comment-14646575
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Ivan Mitic commented on MAPREDUCE-6357:
---------------------------------------

Thanks [~cotedm], please feel free to take it up.

> MultipleOutputs.write() API should document that output committing is not utilized when
input path is absolute
> --------------------------------------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-6357
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6357
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: documentation
>    Affects Versions: 2.6.0
>            Reporter: Ivan Mitic
>            Assignee: Ivan Mitic
>
> After spending the afternoon debugging a user job where reduce tasks were failing on
retry with the below exception, I think it would be worthwhile to add a note in the MultipleOutputs.write()
documentation, saying that absolute paths may cause improper execution of tasks on retry or
when MR speculative execution is enabled. 
> {code}
> 2015-04-28 23:13:10,452 WARN [main] org.apache.hadoop.mapred.YarnChild: Exception running
child : java.io.IOException: File already exists:wasb://full20150320@bgtstoragefull.blob.core.windows.net/user/hadoop/some/path/block-r-00299.bz2
>        at org.apache.hadoop.fs.azure.NativeAzureFileSystem.create(NativeAzureFileSystem.java:1354)
>        at org.apache.hadoop.fs.azure.NativeAzureFileSystem.create(NativeAzureFileSystem.java:1195)
>        at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:908)
>        at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:889)
>        at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:786)
>        at org.apache.hadoop.mapreduce.lib.output.TextOutputFormat.getRecordWriter(TextOutputFormat.java:135)
>        at org.apache.hadoop.mapreduce.lib.output.MultipleOutputs.getRecordWriter(MultipleOutputs.java:475)
>        at org.apache.hadoop.mapreduce.lib.output.MultipleOutputs.write(MultipleOutputs.java:433)
>        at com.ancestry.bigtree.hadoop.LevelReducer.processValue(LevelReducer.java:91)
>        at com.ancestry.bigtree.hadoop.LevelReducer.reduce(LevelReducer.java:69)
>        at com.ancestry.bigtree.hadoop.LevelReducer.reduce(LevelReducer.java:14)
>        at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:171)
>        at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:627)
>        at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389)
>        at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:163)
>        at java.security.AccessController.doPrivileged(Native Method)
>        at javax.security.auth.Subject.doAs(Subject.java:415)
>        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
>        at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)
> {code}
> As discussed in MAPREDUCE-3772, when the baseOutputPath passed to MultipleOutputs.write()
is an absolute path (or more precisely a path that resolves outside of the job output-dir),
the concept of output committing is not utilized. 
> In this case, the user read thru the MultipleOutputs docs and was assuming that everything
will be working fine, as there are blog posts saying that MultipleOutputs does handle output
commit. 



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