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https://issues.apache.org/jira/browse/HADOOP-7183?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13007180#comment-13007180
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Tom White commented on HADOOP-7183:
-----------------------------------
Before HADOOP-6881 the only way to add a WritableComparator to the cache was by
explicitly calling WritableComparator.define(). HADOOP-6881 changed
WritableComparator.get() to add a generic WritableComparator to the comparators
cache when there was none registered for the class. This patch simply restores
the previous behaviour.
Making WritableComparator thread-safe by using a thread-local buffer would be a
good enhancement but isn't necessary for this fix.
> WritableComparator.get should not cache comparator objects
> ----------------------------------------------------------
>
> Key: HADOOP-7183
> URL: https://issues.apache.org/jira/browse/HADOOP-7183
> Project: Hadoop Common
> Issue Type: Bug
> Affects Versions: 0.22.0
> Reporter: Todd Lipcon
> Assignee: Tom White
> Priority: Blocker
> Fix For: 0.20.3, 0.21.1, 0.22.0
>
> Attachments: HADOOP-7183.patch
>
>
> HADOOP-6881 modified WritableComparator.get such that the constructed
> WritableComparator gets saved back into the static map. This is fine for
> stateless comparators, but some comparators have per-instance state, and thus
> this becomes thread-unsafe and causes errors in the shuffle where multiple
> threads are doing comparisons. An example of a Comparator with per-instance
> state is WritableComparator itself.
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