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From "Ning Li (JIRA)" <>
Subject [jira] Commented: (LUCENE-847) Factor merge policy out of IndexWriter
Date Wed, 08 Aug 2007 14:24:59 GMT


Ning Li commented on LUCENE-847:

On 8/8/07, Michael McCandless (JIRA) <> wrote:
> Actually I was talking about my idea (to "simplify MergePolicy.merge
> API").  With the simplification (whereby MergePolicy.merge just
> returns the MergeSpecification instead of driving the merge itself) I
> believe it's simple to make a concurrency wrapper around any merge
> policy, and, have all necessary locking for SegmentInfos inside
> IndexWriter.

I agree with Mike. In fact,, which is the counterpart
of MergePolicy.merge in the patch I submitted for concurrent merge,
simply returns a MergeSpecification. It's simple and sufficient to have
all necessary lockings for SegmentInfos in one class, say IndexWriter.
For example, IndexWriter locks SegmentInfos when MergePolicy(MergeSelector)
picks a merge spec. Another example, when a merge is finished, say
IndexWriter.checkin is called which locks SegmentInfos and replaces
the source segment infos with the target segment info.

On 8/7/07, Steven Parkes (JIRA) <> wrote:
> The synchronization is still tricky, since parts of segmentInfos are
> getting changed at various times and there are references and/or
> copies of it other places. And as Ning pointed out to me, we also
> have to deal with buffered delete terms. I'd say I got about 80% of
>the way there on the last go around. I'm hoping to get all the way
> this time.

It just occurred to me that there is a neat way to handle deletes that
are flushed during a concurrent merge. For example, MergePolicy
decides to merge segments B and C, with B's delete file 0001 and
C's 100. When the concurrent merge finishes, B's delete file becomes
0011 and C's 110. We do a simple computation on the delete bit
vectors and check in the merged segment with delete file 00110.

> Factor merge policy out of IndexWriter
> --------------------------------------
>                 Key: LUCENE-847
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Steven Parkes
>            Assignee: Steven Parkes
>         Attachments: concurrentMerge.patch, LUCENE-847.patch.txt, LUCENE-847.txt
> If we factor the merge policy out of IndexWriter, we can make it pluggable, making it
possible for apps to choose a custom merge policy and for easier experimenting with merge
policy variants.

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