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https://issues.apache.org/jira/browse/LUCENE-7410?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15414942#comment-15414942
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Robert Muir commented on LUCENE-7410:
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{{getCombinedCoreAndDeletesKey()}}: what uses this one? Can we remove it?
> Make cache keys and closed listeners less trappy
> ------------------------------------------------
>
> Key: LUCENE-7410
> URL: https://issues.apache.org/jira/browse/LUCENE-7410
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Adrien Grand
>
> IndexReader currently exposes getCoreCacheKey(),
> getCombinedCoreAndDeletesKey(), addCoreClosedListener() and
> addReaderClosedListener(). They are typically used to manage resources whose
> lifetime needs to mimic the lifetime of segments/indexes, typically caches.
> I think this is trappy for various reasons:
> h3. Memory leaks
> When maintaining a cache, entries are added to the cache based on the cache
> key and then evicted using the cache key that is given back by the close
> listener, so it is very important that both keys are the same.
> But if a filter reader happens to delegate get*Key() and not
> add*ClosedListener() or vice-versa then there is potential for a memory leak
> since the closed listener will be called on a different key and entries will
> never be evicted from the cache.
> h3. Lifetime expectations
> The expectation of using the core cache key is that it will not change in
> case of deletions, but this is only true on SegmentReader and LeafReader
> impls that delegate to it. Other implementations such as composite readers or
> parallel leaf readers use the same key for "core" and "combined core and
> deletes".
> h3. Throw-away wrappers cause cache trashing
> An application might want to either expose more (with a ParrallelReader or
> MultiReader) or less information (by filtering fields/docs that can be seen)
> depending on the user who is logged in. In that case the application would
> typically maintain a DirectoryReader and then wrap it per request depending
> on the logged user and throw away the wrapper once the request is completed.
> The problem is that these wrappers have their own cache keys and the
> application may build something costly and put it in a cache to throw it away
> a couple milliseconds later. I would rather like for such readers to have a
> way to opt out from caching on order to avoid this performance trap.
> h3. Type safety
> The keys that are exposed are plain java.lang.Object instances, which
> requires caches to look like a {{Map<Object, ?>}} which makes it very easy to
> either try to get, put or remove on the wrong object since any object would
> be accepted.
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