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https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12639629#action_12639629
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Doug Cutting commented on HADOOP-4044:
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Sanjay> if someone happens to create a dir called "hdfs:" and valid path
foo/bar under it then the local file system will follow it and not return an
error. Is this a concern?
Folks would have to use a double-slash in the symbolic link (which unix
ignores) for Hadoop to interpret it as an external link. So folks would have
to use single-slash when linking to a directory whose name ends in colon to
keep behavior consistent. That doesn't seem like a big imposition.
Sanjay> Hence, on a "file does not exist" error, the client side lib will have
to check if there are symlinks in the path.
Good point. If we leverage Unix's automatic link following then we'd have to
handle external links by catching FileNotFoundException. We could instead loop
calling lstat(), following links that are internal, and returning when either a
link to a non file:// uri or an actual file is found. This would require
native code, but, this is a unix-only feature and we already provide native
code for linux, so maybe that's acceptable. Or we could call out to a shell,
but that would probably be too slow. Note that Java 7 adds API support for
symbolic links, which will permit such looping logic, so that will provide a
good way to implement this.
FileNotFoundException doesn't actually make this any easier, since it doesn't
contain the path we need, as far as I can tell. We'd still have to loop in
native code or a shell script to find the external link, no? So maybe the
native code route is best? We could implement just this bit of the Java 7 API,
so when that arrives we can use it without changing much?
> 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: HADOOP-4044-strawman.patch, 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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