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https://issues.apache.org/jira/browse/LUCENE-8963?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16923284#comment-16923284
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Michael McCandless commented on LUCENE-8963:
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Do we have examples of collectors in Lucene today that are single-threaded?
The core collectors, at least {{TopFieldCollector}} and {{TopDocsCollector}}
seem to be OK since {{IndexSearcher}} makes a {{CollectorManager}} that uses
{{TopDocs.merge}} in the end.
So maybe as long as a {{CollectorManager}} is available that implies it is
thread safe?
> Allow Collectors To "Publish" If They Can Be Used In Concurrent Search
> ----------------------------------------------------------------------
>
> Key: LUCENE-8963
> URL: https://issues.apache.org/jira/browse/LUCENE-8963
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Atri Sharma
> Priority: Major
>
> There is an implied assumption today that all we need to run a query
> concurrently is a CollectorManager implementation. While that is true, there
> might be some corner cases where a Collector's semantics do not allow it to
> be concurrently executed (think of ES's aggregates). If a user manages to
> write a CollectorManager with a Collector that is not really concurrent
> friendly, we could end up in an undefined state.
>
> This Jira is more of a rhetorical discussion, and to explore if we should
> allow Collectors to implement an API which simply returns a boolean
> signifying if a Collector is parallel ready or not. The default would be
> true, until a Collector explicitly overrides it?
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