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https://issues.apache.org/jira/browse/LUCENE-847?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12518165
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Steven Parkes commented on LUCENE-847:
--------------------------------------
I
think we ideally would like concurrency to be fully independent of the
merge policy.
I thought of that, too, while taking a fresh look at things again. It's my
current approach, though I'm not yet sure there won't be stumbling blocks. More
soon, hopefully.
I think with one change to your MergePolicy API & control flow, we
could make this work very well: instead of requiring the MergePolicy
to call IndexWriter.merge, and do the cascading, it should just return
the one MergeSpecification that should be done right now.
Hmm ... interesting idea. I thought about it briefly, though I didn't pursue it
(see below). It would end up changing the possible space of merge policies
subtly. You wouldn't be able to have any state in the algorithm. Arguably this
is a good thing. There is also a bit more overhead, since starting the
computation of potential merges from scratch each time could imply a little
more computation, but I suspect this is not significant.
When the inner MergePolicy wants
to do a merge, the ConcurrentMergePolicy would in turn kick off that
merge in the BG but then return null to the IndexWriter allowing
IndexWriter to return to its caller, etc.
I'm a little unsure here. Are you saying the ConcurrentMergePolicy does the
merges itself, rather than using the writer? That's going to mean a
synchronization dance between the CMP and the writer. There's no question but
that there has to be some synch dance, but my current thinking was to try to
keep as cleanly within one class, IW, as I could.
> Factor merge policy out of IndexWriter
> --------------------------------------
>
> Key: LUCENE-847
> URL: https://issues.apache.org/jira/browse/LUCENE-847
> 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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