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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Fri, 10 Oct 2008 17:15:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12638593#action_12638593
] 

Doug Cutting commented on HADOOP-4044:
--------------------------------------

Owen> I would probably use a thread local variable to measure the depth of the traversal.
It isn't great, but it would be encapsulated in LinkResult.

I see.  Yuck.  It wouldn't in fact be entirely encapsulated, since it would assume that all
the resolution of a path happens in the same thread.  Thus we'd want to mark each of the FileSystem
methods that uses a LinkResult final, add some comments noting that the LinkResult API has
this restriction, etc.  (It'd be odd to ever try to run these in different threads, but folks
have tried odder things!)  In general, thread locals seem like a poor design choice for this.
 They're best for things like caches, to avoid thread contention, rather than for storing
program state, no?

We could instead perhaps add something like a LinkResolution object that keeps a counter internally
that is incremented as the link is resolved.  The SPI would pass this around rather than a
Path?  Is that better than Dhruba's patch?


> Create symbolic links in HDFS
> -----------------------------
>
>                 Key: HADOOP-4044
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4044
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: symLink1.patch, symLink1.patch, symLink4.patch, symLink5.patch,
symLink6.patch, symLink8.patch, symLink9.patch
>
>
> HDFS should support symbolic links. A symbolic link is a special type of file that contains
a reference to another file or directory in the form of an absolute or relative path and that
affects pathname resolution. Programs which read or write to files named by a symbolic link
will behave as if operating directly on the target file. However, archiving utilities can
handle symbolic links specially and manipulate them directly.

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