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https://issues.apache.org/jira/browse/LUCENE-3569?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13150233#comment-13150233
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Shai Erera commented on LUCENE-3569:
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bq. "Maintenance" of what?
Segment management is all that IW does (in the context of MergePolicy) - it
merges them from time to time, and deletes unused segments. I don't mind if we
call it SegmentMaintenacePolicy to make it more clear. In general I find terms
like 'management' not always clarifying what the object is about, but in this
case I think it's not so bad. So if others like the name, I'm ok with it as
well.
bq. how exactly would the MP specified on the IWC interact with the MP passed
explicitly to the merge(MP) method?
Why would they need to interact? I think that the MP passed to IWC should be
used for ongoing merges that IW initiates due to flushing new segments, and the
one that you pass to merge() should do explicit actions, that are usually not
done as part of the ongoing merges (like optimize, expunging deletes etc.).
Mike suggested that we keep MP passed to IWC, and introduce this new class for
the explicit merge() call. I like that idea, even though they will both have
similar API signatures: findSegmentsForMerge, findSegmentsForMaintenance. But,
it will also solve Robert's concern -- if we develop an
OptimizingMaintenancePolicy, you cannot pass it to IWC.
Yet, if someone wants to pass such a policy to IWC, he can create a MergePolicy
wrapper to it -- but that is an explicit action someone does and not a naive
mistake.
> 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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