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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Thu, 09 Oct 2008 21:56:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12638405#action_12638405
] 

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

Konstantin> this argument is about the programming style rather than performance or anything
else

It's clearly about both of those.  We wish to make a change motivated by performance, and
the question is how best to implement that change.  We have two proposals on the table, one
that is less invasive but uses a discouraged style, and one that involves substantial changes
but avoids that style problem.  I tried earlier to remove the discussion from style guidelines
to the underlying reason for those guidelines --that an API's purpose is to model expected
control and data flow--but that argument seems to have caused more confusion than it has helped,
so I've mostly reverted to citing style guidelines.

Konstantin> FileSystem is the first layer for which link values make sense.

The fact that that layer is fully-resolved and known indicates that links should be a first-class
value in the API.  All of the other layers you present are part of a single module (HDFS)
whose job is to present itself as a FileSystem.  If HDFS is implementing the FileSystem API,
then FileSystem's datatypes are not alien to HDFS.  This is the same twist that Sanjay pursued.
 Yes, the link URI is to a file outside of that particular HDFS filesystem, but link URIs
are normal data that HDFS accepts as parameters (when adding a symbolic link), stores, and
returns (when dereferencing a link).  So this is not an alien object or an unexpected situation.
 A link to another filesystem is a sensible, first class value within HDFS.

> 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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