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https://issues.apache.org/jira/browse/LUCENE-2474?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12982604#action_12982604
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Michael McCandless commented on LUCENE-2474:
--------------------------------------------

Ahh, I get it -- invoking the listeners (on cache evict) is dangerous to do 
under a global lock since they could conceivably be costly.

I had switched to Set to try to prevent silliness in the event that an app adds 
same listener over & over (w/o removing it), and also to not have O(N^2) cost 
when removing listeners.  I mean, it is an expert API, but I still think we 
should attempt to be defensive against silliness?

How about CHM?  (There is not builtin CHS, right?  And HS just wraps an HM 
anyway....).

> Allow to plug in a Cache Eviction Listener to IndexReader to eagerly clean 
> custom caches that use the IndexReader (getFieldCacheKey)
> ------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-2474
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2474
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Shay Banon
>         Attachments: LUCENE-2474.patch, LUCENE-2474.patch
>
>
> Allow to plug in a Cache Eviction Listener to IndexReader to eagerly clean 
> custom caches that use the IndexReader (getFieldCacheKey).
> A spin of: https://issues.apache.org/jira/browse/LUCENE-2468. Basically, its 
> make a lot of sense to cache things based on IndexReader#getFieldCacheKey, 
> even Lucene itself uses it, for example, with the CachingWrapperFilter. 
> FieldCache enjoys being called explicitly to purge its cache when possible 
> (which is tricky to know from the "outside", especially when using NRT - 
> reader attack of the clones).
> The provided patch allows to plug a CacheEvictionListener which will be 
> called when the cache should be purged for an IndexReader.

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