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https://issues.apache.org/jira/browse/HADOOP-4044?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12630123#action_12630123
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dhruba borthakur commented on HADOOP-4044:
------------------------------------------
@Raghu: The FSDataInputStream will implement another interface StreamType that
will store the information on whether this is a symbolc link. No extra RPC is
needed.
@Doug: By definition, the filesystem has no control about the target of a
symbolic link. The user could make it point to a non-existent path too. There
will be no enforcement of whether the target is a directory or not.
@Allen: This proposal should not introduce additional grief for namenode. If
you replace a file with a symbolic link, it still occupies one inode. No
additional inodes are necessary. I agree that admins might like symbolic links
to directories, but maybe we can do that at a later date when the user-case
becomes more concrete. Does it sound acceptable?
The simplicity for the filesystem is that no "kickbacks" while traversing paths
are needed.
{noformat}
/**
* Opens an FSDataInputStream at the indicated Path.
* This does not follow symbolic links.
* @param f the file name to open
* @param bufferSize the size of the buffer to be used.
*/
public abstract FSDataInputStream openfs(Path f, int bufferSize)
throws IOException;
/**
* Opens an FSDataInputStream at the indicated Path.
* If the specified path is a symbolic link, then open the
* target of the symbolic link.
* @param f the file to open
*/
public FSDataInputStream open(Path f) throws IOException {
return open(f, getConf().getInt("io.file.buffer.size", 4096));
}
/**
* Opens an FSDataInputStream at the indicated Path.
* @param f the file to open
* @param bufferSize the size of the buffer to be used.
*/
public FSDataInputStream open(Path f, int bufferSize)
throws IOException {
FileSystem fs = this;
FSDataInputStream in = fs.openfs(f, bufferSize);
while (in.isSymlink()) {
// construct new path pointed to by symlink
Path newpath = new Path(in.getSymlink());
if (!newpath.isAbsolute()) {
newpath = new Path(f.getParent(), newpath);
}
in.close();
f = newpath;
fs = f.getFileSystem(getConf());
LOG.warn("XXX Opening symlink at " + f);
in = fs.openfs(f, bufferSize);
}
return in;
}
public class FSDataInputStream extends DataInputStream
implements Seekable, PositionedReadable, StreamType {
....
}
/** Types of streams */
interface StreamType {
/**
* Is this stream a symbolic link?
*/
public boolean isSymlink() throws IOException;
/**
* Return the contents of the symlink
*/
public String getSymlink() throws IOException;
}
{noformat}
> 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
>
>
> 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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