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https://issues.apache.org/jira/browse/LUCENE-743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12533213
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Michael McCandless commented on LUCENE-743:
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> We have to be very careful about cross-referencing multiple readers.
> If for some reason any references between two or more readers are
> not cleared after one was closed, then that reader might not become
> GC'd. I'm not saying it's not possible or even very hard, we just
> have to make sure those things can't ever happen.
One correction: there should be no cross-referencing, right? The only
thing that's happening is incrementing & decrementing the "referrer
count" for a reader? (But, you're right: the "copy on write" approach
to cloning would have cross-referencing).
I think the downside of this proposed "shared readers" (non-cloning)
approach is that you can't delete/setNorm on the clone until you close
the original. But I think that's fairly minor? Also if you really
need a full deep copy of your reader you could just open a new one
(ie, not use "reopen")?
I think the big upside of "shared readers" is reopening becomes quite
a bit more resource efficient: the cost of re-opening a reader would
be in proportion to what has actually changed in the index. EG, if
your index has added a few tiny segments since you last opened then
the reopen would be extremely fas but with cloning you are forced
to make a full deep copy of all other [unchanged] segments.
> IndexReader.reopen()
> --------------------
>
> Key: LUCENE-743
> URL: https://issues.apache.org/jira/browse/LUCENE-743
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Reporter: Otis Gospodnetic
> Assignee: Michael Busch
> Priority: Minor
> Fix For: 2.3
>
> Attachments: IndexReaderUtils.java, lucene-743-take2.patch,
> lucene-743.patch, lucene-743.patch, lucene-743.patch, MyMultiReader.java,
> MySegmentReader.java, varient-no-isCloneSupported.BROKEN.patch
>
>
> This is Robert Engels' implementation of IndexReader.reopen() functionality,
> as a set of 3 new classes (this was easier for him to implement, but should
> probably be folded into the core, if this looks good).
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