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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4044) Create symbolic links in HDFS
Date Tue, 07 Oct 2008 19:40:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12637620#action_12637620
] 

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

> Another trueism is that returning unrelated values is API pollution.

Sure, but the values here are not unrelated.  They're core.

> I guess one end of the opinion is that retuning link is normal and other end thinks it
isn't.

Okay, so, if that's the case, how is it abnormal?  The traditional way to model this would
be something like:
{code}
public FileHandle open(Path path) {
  for (element in path) {
    FileStatus stat = fsImpl.getStatus(element);
    if (stat.isLink()) {
      return open(stat.getLinkTarget());
    } else if stat.isFile() {
      return fsImpl.open(element);
    }
  }
{code}
I don't think anyone would argue that the values of isLink() and getLinkTarget() are not normal
data.  But we don't want to make this many calls to our FS implementations, since they're
RPCs.  So, for performance, we want to package that same data in a different way, with something
like:
{code}
public FileHandle open(Path path) {
    LinkOrFileHandle linkOrFileHandle = getNextLinkOrOpen(path);
    while (linkOrFileHandle.isLink()) {
       linkOrFileHandle = getNextLinkOrOpen(linkOrFileHandle.getLink());
    }
    return linkOrFileHandle.getFileHandle();
}
{code}
Why does this suddenly make the that same data non-normal?


> 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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