[jira] [Commented] (LUCENE-9406) Make it simpler to track IndexWriter's events

["Michael McCandless (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-9406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17278361#comment-17278361 ] 

Michael McCandless commented on LUCENE-9406:
--------------------------------------------

Thanks for taking up the torch here [~zacharymorn]!

This seems like a lot of new API surface area for a somewhat simple start at metrics.  Could we maybe simplify this further?  Why not a single interface like:
{noformat}
interface IndexWriterMetricss {
  beginMergeOnFullFlush(OneMerge merge);
  endMergeOnFullFlush(OneMerge merge);
  ...
} {noformat}
We would mark it experimental so we are free to improve this over time.  We could add additional event driven methods to it as needed ...

> 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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