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From "Abdul Qadeer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4182) Streaming Documentation Update
Date Tue, 23 Sep 2008 23:37:47 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12633973#action_12633973

Abdul Qadeer commented on HADOOP-4182:

I agree with you that it is a problem at the application / user level.  I
only wanted to put a simple comment somewhere on the Hadoop Wiki that says
that a line must end with an end of line delimiter.  If not, user might get
different behaviors as I explained earlier.  This simple comment can keep a
user from accidental un-expected results.

> Streaming Documentation Update
> ------------------------------
>                 Key: HADOOP-4182
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4182
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: contrib/streaming
>    Affects Versions: 0.19.0
>            Reporter: Abdul Qadeer
>            Priority: Minor
>             Fix For: 0.19.0
> When Text input data is used with streaming, every line is expected to end with a newline.
 Hadoop results are undefined if input files do not end in a newline.  (The results will depend
on how files are assigned to mappers.)
> Example:
> In streaming if
> mapper = xargs cat
> reducer = cat
> and the input is a two line, where each line is symbolic link in HDFS
> link1\n
> link2\n
> link1 points to a file which contains
> This is line1EOF
> link2 points to a file which  contains
> This is line2EOF
> Now running a streaming job such that, there is only one split, will produce results:
> This is line1This is line2\t\n
> But if there were two splits, the result will be
> This is line1\t\n
> This is line2\t\n
> So in summary, the output depends on the factor that how many mappers were invoked. 
As a caution, it should be recorded in Streaming wiki that users always put a new line at
the end of each line to get away with such problems.

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