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https://issues.apache.org/jira/browse/LUCENE-3569?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13150255#comment-13150255
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Shai Erera commented on LUCENE-3569:
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I get your point. Today, it is also possible for IW to initiate a merge
(ongoing) and you call optimize(). Both calls generate MergeSpec which tell IW
what to do. They 'interact' through IW's running and pending merges lists.
Whatever is currently running is managed by a runningMerges<OneMerge> list and
there is another list for pendingMerges<OneMerge). Whatever MP returns is added
to the pendingMerges list, when MergeScheduler takes one merge from the list it
is added to the runningMerges list, and that's more or less it.
I think that if we make both MPs (Merge + Maintenance) return a MergeSpec, then
we will be able to reuse to same interaction that exists today. Though I'm not
sure how MergeSpec + MergeScheduler + IndexWriter could understand a OneMerge
that denotes "just expunge deletes, don't merge", but that's either a bridge
we'll need to cross when we make some progress here, or a requirement that
we'll let go (i.e., someone could argue "if you're already rewriting segments
X, Y and Z cuz they have deletes, why not merge them?").
> Consolidate IndexWriter's optimize, maybeMerge and expungeDeletes under one
> merge(MP) method
> --------------------------------------------------------------------------------------------
>
> Key: LUCENE-3569
> URL: https://issues.apache.org/jira/browse/LUCENE-3569
> Project: Lucene - Java
> Issue Type: Improvement
> Components: core/index
> Reporter: Shai Erera
>
> Today, IndexWriter exposes 3 methods for 'cleaning up' / 'compacting' /
> 'optimizing' your index:
> * optimize() -- merges as much segments as possible (down to 1 segment), and
> is discouraged in many cases because of its performance implications.
> * maybeMerge() -- runs 'subtle' merges. Attempts to balance the index by not
> leaving too many segments, yet not merging large segments if unneeded.
> * expungeDeletes() -- cleans up deleted documents from segments and on the go
> merges them.
> * a default MP that can be set on IndexWriterConfig, for ongoing merges IW
> performs (i.e. as a result of flushing a new segment).
> These methods are confusing in several levels:
> * Their names are misleading, see LUCENE-3454.
> * Why does expungeDeletes need to merge segments?
> * Eventually, they really do what the MergePolicy decides that should be
> done. I.e., one could write an MP that always merges all segments, and
> therefore calling maybeMerge would not be so subtle anymore. On the other
> hand, one could write an MP that never merges large segments (we in fact have
> several of those), and therefore calling optimize(1) would not end up with
> one segment.
> So the proposal is to replace all these methods with a single one
> merge(MergePolicy) (more on the names later). MergePolicy will have only one
> method findSegmentsForMerge and the caller will be responsible to configure
> it in order to perform the needed merges. We will provide ready-to-use MPs:
> * LightMergePolicy -- for setting on IWC and doing the ongoing merges IW
> executes. This one will pick segments respecting various parameters such as
> mergeFactor, segmentSizes etc.
> * HeavyMergePolicy -- for doing the optimize()-style merges.
> * ExpungeDeletesMergePolicy -- for expunging deletes (my proposal is to drop
> segment merging from it, by default).
> Now about the names:
> * I think that it will be good, API-backcompat wise and in general, if we
> name that method doMaintenance (as expungeDeletes does not have to merge
> anything).
> * Instead of MergePolicy we call it MaintenancePolicy and similarly its
> single method findSegmentsForMaintenance, or getMaintenanceSpecification.
> * I called the MPs Light and Heavy just for the text, I think a better name
> should be found, but nothing comes up to mind now.
> It will allow us to use this on 3.x, by deprecating MP and all related
> methods.
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