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https://issues.apache.org/jira/browse/LUCENE-3736?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13202459#comment-13202459
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Uwe Schindler commented on LUCENE-3736:
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bq. Another question: do we even need a ParallelCompositeReader? Can't we
have only the Builder, and that returns a MultiReader?
The problem why we need ParallelCompositeReader is that the refCounting and
close logic is different. MultiReader will close/refcount its childs, but the
ParallelCompositeReader must close/refcount the parallel sub readers. If you
return a simple MultiReader, all tests fail, because something is closing the
subreaders of the original parallel Composite/DirectoryReaders (as they are
closed by the wrappers), but the original CompositeReaders stay open.
If the parallel readers are DirectoryReaders it goes very bad, as the wrapped
SegmentReaders are closed...
The change does the following: It returns a BaseMultiReader of
ParallelAtomicReaders that wrap e.g. the SegmentReaders of a DirectoryReader.
On close it decRefs or closes the original wrapped DirectoryReader, but does
not touch the subreaders. In contrast, a normal MultiReader would close/decRef
the inner ParallelAtomicReaders, which itsself cose the SegmentReaders they
wrap. The DirectoryReaders would still be open.
> ParallelReader is now atomic, rename to ParallelAtomicReader and also add a
> ParallelCompositeReader (that requires LogDocMergePolicy to have identical
> subreader structure)
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-3736
> URL: https://issues.apache.org/jira/browse/LUCENE-3736
> Project: Lucene - Java
> Issue Type: Sub-task
> Components: core/index
> Reporter: Uwe Schindler
> Assignee: Michael McCandless
> Fix For: 4.0
>
> Attachments: LUCENE-3736.patch, LUCENE-3736.patch, LUCENE-3736.patch
>
>
> The plan is:
> - Move all subreaders to ctor (builder-like API. First build reader-set, then
> call build)
> - Rename ParallelReader to ParallelAtomicReader
> - Add a ParallelCompositeReader with same builder API, but taking any
> CompositeReader-set and checks them that they are aligned (docStarts
> identical). The subreaders are ParallelAtomicReaders.
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