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https://issues.apache.org/jira/browse/MAPREDUCE-1288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12789153#action_12789153
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Devaraj Das commented on MAPREDUCE-1288:
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All I am saying is that irrespective of the file being public or not, in the 
current codebase, we localize the file exactly once per TaskTracker. On a given 
tasktracker, users cannot share the same hdfs file as a distributed cache 
file.. 
What I thought earlier was that the same file would be localized twice in such 
a case (in their respective private directories).

> DistributedCache localizes only once per cache URI
> --------------------------------------------------
>
>                 Key: MAPREDUCE-1288
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1288
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: security, tasktracker
>    Affects Versions: 0.21.0
>            Reporter: Devaraj Das
>            Priority: Blocker
>             Fix For: 0.21.0
>
>
> As part of the file localization the distributed cache localizer creates a 
> copy of the file in the corresponding user's private directory. The 
> localization in DistributedCache assumes the key as the URI of the cachefile 
> and if it already exists in the map, the localization is not done again. This 
> means that another user cannot access the same distributed cache file. We 
> should change the key to include the username so that localization is done 
> for every user.

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