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Posted to dev@nutch.apache.org by "Andrzej Bialecki (JIRA)" <ji...@apache.org> on 2006/10/05 02:11:20 UTC
[jira] Updated: (NUTCH-371) DeleteDuplicates should remove
documents with duplicate URLs
[ http://issues.apache.org/jira/browse/NUTCH-371?page=all ]
Andrzej Bialecki updated NUTCH-371:
------------------------------------
Attachment: patch.txt
This patch contains an alternative implementation of DeleteDuplicates, which follows the algorithm described in the previous comment. A new JUnit test was created to test this implementation.
> DeleteDuplicates should remove documents with duplicate URLs
> ------------------------------------------------------------
>
> Key: NUTCH-371
> URL: http://issues.apache.org/jira/browse/NUTCH-371
> Project: Nutch
> Issue Type: Bug
> Components: indexer
> Reporter: Chris Schneider
> Attachments: patch.txt
>
>
> DeleteDuplicates is supposed to delete documents with duplicate URLs (after deleting documents with identical MD5 hashes), but this part is apparently not yet implemented. Here's the comment from DeleteDuplicates.java:
> // 2. map indexes -> <<url, fetchdate>, <index,doc>>
> // partition by url
> // reduce, deleting all but most recent.
> //
> // Part 2 is not yet implemented, but the Indexer currently only indexes one
> // URL per page, so this is not a critical problem.
> It is apparently also known that re-fetching the same URL (e.g., one month later) will result in more than one document with the same URL (this is alluded to in NUTCH-95), but the comment above suggests that the indexer will solve the problem before DeleteDuplicates, because it will only index one document per URL.
> This is not necessarily the case if the segments are to be divided among search servers, as each server will have its own index built from its own portion of the segments. Thus, if the URL in question was fetched in different segments, and these segments end up assigned to different search servers, then the indexer can't be relied on to eliminate the duplicates.
> Thus, it seems like the second part of the DeleteDuplicates algorithm (i.e., deleting documents with duplicate URLs) needs to be implemented. I agree with Byron and Andrzej that the most recently fetched document (rather than the one with the highest score) should be preserved.
> Finally, it's also possible to get duplicate URLs in the segments without re-fetching an expired URL in the crawldb. This can happen if 3 different URLs all redirect to the target URL. This is yet another consequence of handling redirections immediately, rather than adding the target URL to the crawldb for fetching in some subsequent segment (see NUTCH-273).
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