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Posted to dev@nutch.apache.org by "Doug Cutting (JIRA)" <ji...@apache.org> on 2005/09/21 04:12:28 UTC
[jira] Commented: (NUTCH-95) DeleteDuplicates depends on the order of input segments
[ http://issues.apache.org/jira/browse/NUTCH-95?page=comments#action_12330057 ]
Doug Cutting commented on NUTCH-95:
-----------------------------------
A simpler fix would be to simply sort the list of segment names, since segment names are dates. This is imperfect, when, e.g., one merge's segments, but it is very simple!
> DeleteDuplicates depends on the order of input segments
> -------------------------------------------------------
>
> Key: NUTCH-95
> URL: http://issues.apache.org/jira/browse/NUTCH-95
> Project: Nutch
> Type: Bug
> Components: indexer
> Versions: 0.7, 0.8-dev, 0.6
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
>
> DeleteDuplicates depends on what order the input segments are processed, which in turn depends on the order of segment dirs returned from NutchFileSystem.listFiles(File). In most cases this is undesired and may lead to deleting wrong records from indexes. The silent assumption that segments at the end of the listing are more recent is not always true.
> Here's the explanation:
> * Dedup first deletes the URL duplicates by computing MD5 hashes for each URL, and then sorting all records by (hash, segmentIdx, docIdx). SegmentIdx is just an int index to the array of open IndexReaders - and if segment dirs are moved/copied/renamed then entries in that array may change their order. And then for all equal triples Dedup keeps just the first entry. Naturally, if segmentIdx is changed due to dir renaming, a different record will be kept and different ones will be deleted...
> * then Dedup deletes content duplicates, again by computing hashes for each content, and then sorting records by (hash, segmentIdx, docIdx). However, by now we already have a different set of undeleted docs depending on the order of input segments. On top of that, the same factor acts here, i.e. segmentIdx changes when you re-shuffle the input segment dirs - so again, when identical entries are compared the one with the lowest (segmentIdx, docIdx) is picked.
> Solution: use the fetched date from the first record in each segment to determine the order of segments. Alternatively, modify DeleteDuplicates to use the newer algorithm from SegmentMergeTool. This algorithm works by sorting records using tuples of (urlHash, contentHash, fetchDate, score, urlLength). Then:
> 1. If urlHash is the same, keep the doc with the highest fetchDate (the latest version, as recorded by Fetcher).
> 2. If contentHash is the same, keep the doc with the highest score, and then if the scores are the same, keep the doc with the shortest url.
> Initial fix will be prepared for the trunk/ and then backported to the release branch.
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