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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Tue, 07 Oct 2008 19:00:48 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12637610#action_12637610

Konstantin Shvachko commented on HADOOP-4044:

Dhruba> In current Hadoop RPC, the only way to serialize/deserialize an exception object
is to serialize its message string.

I am not sure my proposal was understood completely.
- I have an exception XMountPointException, which does not encode path (or any special fields)
in its message.
- If I call {{getFileStatus(src)}} and src crosses a mount point the XMountPointException
is thrown by the name-node, which is passed through RPC to the application level.
- The application catches XMountPointException and calls {{linkSrc = getSymLink(src)}}, which
returns the first link on the path.
- The application then calls {{getFileStatus(linkSrc)}}. 
- Encoding linkSrc inside XMountPointException would be an optimization: unnecessary here
because crossing mount point
is not common.
- If an application knows and expects to cross a mount point it cam call getSymLink() avoiding
catching XMountPointException.

Is this the right usage of exceptions?

Doug> we combined the value of many methods into the value of a single method with a very
generic name.

The point of RPC is to make transparent the types passed and returned by a method.
As said {{Object execute(Object request)}} replaces all PRCs by one generic call.
It is like sending and receiving blob buffers and casting them into appropriate structures.
Bad practice in C, unavoidable in Cobol, cannot qualify for a remote procedure call.

> 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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