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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Thu, 09 Oct 2008 00:31:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12638152#action_12638152

Konstantin Shvachko commented on HADOOP-4044:

> All of these methods are now conditional. They now either return the declared value or
a boolean

To make sure we are on the same page - open() does not literally return a boolean, you mean
that throwing an exception should be treated as an (boolean) indicator to whether the link
should be retrieved, right?
But the methods have always been conditional in *that sense*, because they are throwing AccessControlException,
QuotaExceededException, and FileNotFoundException. And XMountPointException is just another
exception of the same nature.
I do not see it as a fundamental change to the FileSystem API.

> When I referred to doubling the number of RPCs I was contrasting with the naive approach

I think everybody agrees that the naive approach is unacceptable.

> the link value would not be returned directly with the call to open() but would require
a second RPC, right?

Correct. The point is that there is no degradation of performance in regular case, which is
when there are no links or the link is local, but there is an extra rpc when the link points
to another cluster. 
And I do not think we should optimize it exactly because this leads to the fundamental change
of the api, just as you describe.
And because an application can avoid this inefficiency by directly resolving the link before
calling open() if it knows about the mount point.

> 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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