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Posted to common-dev@hadoop.apache.org by "Abdul Qadeer (JIRA)" <ji...@apache.org> on 2008/09/24 01:37:47 UTC
[jira] Commented: (HADOOP-4182) Streaming Documentation Update
[ 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
> 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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