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From "Eli Collins (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-245) Create symbolic links in HDFS
Date Wed, 06 Jan 2010 21:05:54 GMT

    [ https://issues.apache.org/jira/browse/HDFS-245?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12797311#action_12797311

Eli Collins commented on HDFS-245:

Here's the final link resolution behavior based on discussion in HADOOP-6427. Basically we
dropped (for now) links that resolve relative to the default file system of the file context
accessing the link. Updated description follows:

Given a symbolic link of form:


X is the scheme and authority that identify the file system, and Y is the
path leading up to the symlink "link". If Y is a symlink itself then
let Y' be the target of Y and X' be the scheme and authority of Y'.

Then link targets of the following form resolve as follows:

1. Fully qualified URI

 fs://hostX/A/B/file   Resolved according to the target file system.

2. Partially qualified (scheme but no host) URI

 fs:///A/B/file        Resolved according to the target file sytem. Eg
                       resolving a link to hdfs:///A will result in an 
                       exception because HDFS URIs must be fully qualified, 
                       while a link to file:///A will not since Hadoop's 
                       local file systems require partially qualified URIs.

3. Relative path

 path                  Resolves to <Y'><path>. Eg if Y resolves to 
                       hdfs://host/A and path is "../B/file" then
                       <Y'><path> is hdfs://host/B/file

4. Absolute path

 path                  Resolves to <X'><path>. Eg if Y resolves
                       hdfs://host/A/B and path is "/file" then
                       <X'><path> is hdfs://host/file

> Create symbolic links in HDFS
> -----------------------------
>                 Key: HDFS-245
>                 URL: https://issues.apache.org/jira/browse/HDFS-245
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: dhruba borthakur
>            Assignee: Eli Collins
>         Attachments: 4044_20081030spi.java, designdocv1.txt, designdocv2.txt, designdocv3.txt,
HADOOP-4044-strawman.patch, symlink-0.20.0.patch, symlink-25-hdfs.patch, symlink-26-hdfs.patch,
symlink-26-hdfs.patch, symLink1.patch, symLink1.patch, symLink11.patch, symLink12.patch, symLink13.patch,
symLink14.patch, symLink15.txt, symLink15.txt, symlink16-common.patch, symlink16-hdfs.patch,
symlink16-mr.patch, symlink17-common.txt, symlink17-hdfs.txt, symlink18-common.txt, symlink19-common-delta.patch,
symlink19-common.txt, symlink19-common.txt, symlink19-hdfs-delta.patch, symlink19-hdfs.txt,
symlink20-common.patch, symlink20-hdfs.patch, symlink21-common.patch, symlink21-hdfs.patch,
symlink22-common.patch, symlink22-hdfs.patch, symlink23-common.patch, symlink23-hdfs.patch,
symlink24-hdfs.patch, symlink27-hdfs.patch, symlink28-hdfs.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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