DeleteDuplicates should remove documents with duplicate URLs
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Key: NUTCH-371
URL: http://issues.apache.org/jira/browse/NUTCH-371
Project: Nutch
Issue Type: Bug
Components: indexer
Reporter: Chris Schneider
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 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 some subsequent segment (see NUTCH-273).
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