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https://issues.apache.org/jira/browse/HADOOP-3941?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12625689#action_12625689
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Doug Cutting commented on HADOOP-3941:
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> Below is a summary of the default getFileChecksum() implementation options [
> ... ]
The default should minimize code duplication, if possible. An abstract method
should only be used for mandatory methods. Since this is an optional method, a
default implementation should be provided.
The choice of an exception or null depends on the expected use. An exception
should be thrown for unusual situations that are best handled non-locally,
somewhere above the call. The absence of a checksum should probably be handled
at the site of the call, so returning null seems a better choice than an
exception here. Another option might be to return a trivial checksum, e.g.,
the file's length.
Perhaps we should include a use of this new feature in the patch, to better
guide its implementation. Should we extend distcp to use this? Or do you have
another canonical application in mind? If we add features without applications
of them, we risk a design that does not meet any needs.
> Extend FileSystem API to return file-checksums/file-digests
> -----------------------------------------------------------
>
> Key: HADOOP-3941
> URL: https://issues.apache.org/jira/browse/HADOOP-3941
> Project: Hadoop Core
> Issue Type: New Feature
> Components: fs
> Reporter: Tsz Wo (Nicholas), SZE
> Attachments: 3941_20080818.patch, 3941_20080819.patch,
> 3941_20080819b.patch, 3941_20080820.patch
>
>
> Suppose we have two files in two locations (may be two clusters) and these
> two files have the same size. How could we tell whether the content of them
> are the same?
> Currently, the only way is to read both files and compare the content of
> them. This is a very expensive operation if the files are huge.
> So, we would like to extend the FileSystem API to support returning
> file-checksums/file-digests.
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