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https://issues.apache.org/jira/browse/LUCENE-9406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17279439#comment-17279439
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Dawid Weiss commented on LUCENE-9406:
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Looking from the side - wouldn't it be better to separate index writer events
(merge happening, type of merge, segments merged, etc.) from metrics aggregates
(statistics)? I specifically see statistics computation component as an
implementation of index writer events. Then that component can expose
statistical aggregates further down for consumption.
This would make it possible to implement metric aggregators in your own way
(adjusting to one's needs) or use the default implementation for ready-to-use
consumption. It might also simplify event callback interfaces on index writer
side.
> Make it simpler to track IndexWriter's events
> ---------------------------------------------
>
> Key: LUCENE-9406
> URL: https://issues.apache.org/jira/browse/LUCENE-9406
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/index
> Reporter: Michael McCandless
> Priority: Major
>
> This is the second spinoff from a [controversial PR to add a new index-time
> feature to Lucene to merge small segments during
> commit|https://github.com/apache/lucene-solr/pull/1552]. That change can
> substantially reduce the number of small index segments to search.
> In that PR, there was a new proposed interface, {{IndexWriterEvents}}, giving
> the application a chance to track when {{IndexWriter}} kicked off merges
> during commit, how many, how long it waited, how often it gave up waiting,
> etc.
> Such telemetry from production usage is really helpful when tuning settings
> like which merges (e.g. a size threshold) to attempt on commit, and how long
> to wait during commit, etc.
> I am splitting out this issue to explore possible approaches to do this.
> E.g. [~simonw] proposed using a statistics class instead, but if I understood
> that correctly, I think that would put the role of aggregation inside
> {{IndexWriter}}, which is not ideal.
> Many interesting events, e.g. how many merges are being requested, how large
> are they, how long did they take to complete or fail, etc., can be gleaned by
> wrapping expert Lucene classes like {{MergePolicy}} and {{MergeScheduler}}.
> But for those events that cannot (e.g. {{IndexWriter}} stopped waiting for
> merges during commit), it would be very helpful to have some simple way to
> track so applications can better tune.
> It is also possible to subclass {{IndexWriter}} and override key methods, but
> I think that is inherently risky as {{IndexWriter}}'s protected methods are
> not considered to be a stable API, and the synchronization used by
> {{IndexWriter}} is confusing.
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