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https://issues.apache.org/jira/browse/MAPREDUCE-5184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13675905#comment-13675905
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Jason Lowe commented on MAPREDUCE-5184:
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bq. I'm a little confused by this. What does it mean that "hadoop -jar
hadoop-examples-1.x.x.jar" is still using the 2.x.x.jar? Where is the 2.x.x jar
coming from?
It's coming from the classpath. If someone tries to run the 1.x examples jar,
the classes from the 2.x jar will end up being picked up *before* the 1.x jar
because the Hadoop framework jars appear before the user's stuff in the
classpath by default. Therefore even though it looks like you're running the
1.x version of wordcount, you're really running the 2.x version because of the
default classpath setup.
That's why HADOOP_USER_CLASSPATH_FIRST=true *and*
-Dmapreduce.job.user.classpath.first=true are necessary to make sure you're
really running the 1.x code. The environment variable is to make sure the job
client picks up the code from the 1.x jar, and the property is to make sure the
tasks pick up the 1.x classes when they run on the nodes.
> Document MR Binary Compatibility vis-a-vis hadoop-1 and hadoop-2
> ----------------------------------------------------------------
>
> Key: MAPREDUCE-5184
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5184
> Project: Hadoop Map/Reduce
> Issue Type: Sub-task
> Components: documentation
> Reporter: Arun C Murthy
> Assignee: Zhijie Shen
> Fix For: 2.1.0-beta
>
> Attachments: MAPREDUCE-5184.1.patch
>
>
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