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https://issues.apache.org/jira/browse/LUCENE-6524?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14580935#comment-14580935
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Michael McCandless commented on LUCENE-6524:
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Thanks Rob, I'll explore that simpler API.
> Create an IndexWriter from an already opened NRT or non-NRT reader
> ------------------------------------------------------------------
>
> Key: LUCENE-6524
> URL: https://issues.apache.org/jira/browse/LUCENE-6524
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Fix For: 5.3, Trunk
>
> Attachments: LUCENE-6524.patch, LUCENE-6524.patch
>
>
> I'd like to add a new ctor to IndexWriter, letting you start from an already
> opened NRT or non-NRT DirectoryReader. I think this is a long missing
> API in Lucene today, and we've talked in the past about different ways
> to fix it e.g. factoring out a shared reader pool between writer and reader.
> One use-case, which I hit in LUCENE-5376: if you have a read-only
> index, so you've opened a non-NRT DirectoryReader to search it, and
> then you want to "upgrade" to a read/write index, we don't handle that
> very gracefully now because you are forced to open 2X the
> SegmentReaders.
> But with this API, IW populates its reader pool with the incoming
> SegmentReaders so they are shared on any subsequent NRT reopens /
> segment merging / deletes applying, etc.
> Another (more expert) use case is allowing rollback to an NRT-point.
> Today, you can only rollback to a commit point (segments_N). But an
> NRT reader also reflects a valid "point in time" view of the index (it
> just doesn't have a segments_N file, and its ref'd files are not
> fsync'd), so with this change you can close your old writer, open a
> new one from this NRT point, and revert all changes that had been done
> after the NRT reader was opened from the old writer.
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