[jira] Resolved: (HADOOP-4182) Streaming Documentation Update

["Owen O'Malley (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HADOOP-4182?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Owen O'Malley resolved HADOOP-4182.
-----------------------------------

    Resolution: Won't Fix

There isn't much that streaming can do. In the first case, your application gives the streaming framework:

line1line2EOF

In the second case you give it:

line1EOF in one map

and 

line2EOF in the second map

streaming needs line based data, so the entire input is treated as a line. Leading to the differences that you observed.

> 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
> EOF
> 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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