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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Wed, 08 Oct 2008 21:51:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12638108#action_12638108
] 

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

> why don't we accept the solution that does not require any fundamental changes to the
FileSystem API

But it *does* change the API fundamentally.  All of these methods are now conditional.  They
now either return the declared value or a boolean indicating that the declared value is to
be ignored and that one should follow up with a request for the link value.  That's a substantial
change, that shouldn't be cloaked in exception handling.

We might instead consider changing the contract of these methods to permit the return of a
null value, and interpret a null value as the need to request the link value.  Unfortunately
that won't work with the boolean methods, and it's a fragile approach, since folks might forget
to check for null.  It's better to make it explicit with a datastructure.

> My proposal does "not double the number of RPCs per file operation".

I did not mean to imply that it did.  I'm sorry that you thought I did.  When I referred to
doubling the number of RPCs I was contrasting with the naive approach of calling resolveLink()
or somesuch explicitly before open(), so that open() is never passed a path that contains
a link, as this is implemented in Unix and other VFSs.

Your approach would add an RPC whenever a symbolic link is encountered over the other approaches,
since the link value would not be returned directly with the call to open() but would require
a second RPC, right?

> I keep wondering what is your selective criteria for ignoring some arguments and answering
other.

What arguments am I ignoring?  I am sorry if you feel I've ignored some of your arguments.
 Please highlight them, if you like.

> 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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