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https://issues.apache.org/jira/browse/HADOOP-3063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12581128#action_12581128
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Doug Cutting commented on HADOOP-3063:
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> I think it would be best to import these classes into
> org.apache.hadoop.util.bloom package [ ...]
+1
The license should also be added to our top-level LICENSE.txt, in the style of
http://svn.apache.org/repos/asf/httpd/httpd/trunk/LICENSE
> BloomMapFile - fail-fast version of MapFile for sparsely populated key space
> ----------------------------------------------------------------------------
>
> Key: HADOOP-3063
> URL: https://issues.apache.org/jira/browse/HADOOP-3063
> Project: Hadoop Core
> Issue Type: Improvement
> Components: io
> Affects Versions: 0.17.0
> Reporter: Andrzej Bialecki
> Fix For: 0.17.0
>
> Attachments: bloommap.patch
>
>
> The need for this improvement arose when working with large ancillary
> MapFile-s (essentially used as external dictionaries). For each invokation of
> map() / reduce() it was necessary to perform several look-ups in these
> MapFile-s, and in case of sparsely populated key-space the cost of finding
> that a key is absent was too high.
> This patch implements a subclass of MapFile that creates a Bloom filter from
> all keys, so that accurate tests for absence of keys can be performed quickly
> and with 100% accuracy.
> Writer.append() operations update a DynamicBloomFilter, which is then
> serialized when the Writer is closed. This filter is loaded in memory when a
> Reader is created. Reader.get() operation first checks the filter for the key
> membership, and if the key is absent it immediately returns null without
> doing any further IO.
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