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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Watch out for memory leaks with EhCache
Date Sun, 22 Dec 2019 11:47:07 GMT


> On Dec 11, 2019, at 11:18 PM, John Huss <johnthuss@gmail.com> wrote:
> 
> My use case is very limited in scope. I want to have fresh data basically
> all the time, but not fetch the same data twice in the same request. Once
> the request is over and the request's context is out of scope, then the
> local cache for that context can be cleared. So this cached data is
> extremely short lived, and as a consequence the size of it doesn't really
> matter (though I'm only caching small query results anyway).

Thanks for clarifying. I've dealt a lot with "request-scope cache" scenarios. Here is my approach:

1. Set the lowest reasonable upper limit on the number of entries in the main cache (parent
of local caches). E.g. 3x of your average requests / min or something.
2. Set fairly short expiration times. E.g. 10x of your average response time (say 10-30 sec).

(1) ensures quick turn and prevents leaks
(2) ensures your request doesn't fetch the same data twice, and will not result in any stale
data, as every new context will start its own "region" for the local cache

So you may end up using a bit more memory than absolutely needed, but you avoid the leaks,
and get the desired freshness/caching balance. 

> I've narrowed it down - the "extra" memory being retained by the
> ObjectContext is: entries in ObjectStore.objectMap - but not DataObjects
> themselves (those get cleared), it's just the references to those objects:
> the mapping from ObjectId -> WeakReference. These entries stay present
> after the WeakReference is cleared. All those ObjectIds (though small) add
> up to a significant amount of memory over time. 

Good catch. DataContext getting stuck in the local cache does seem to leave a lot of unintended
garbage floating around. While it doesn't change the equation above, it is still a waste that
we may need to address.

So to me the approach above is "good enough". Though we may still come up with a design that
allows for more control (and hence the lowest possible memory footprint). I suspect any such
solution would require ObjectContext.close() method to allow the user to delineate context
scope. 

Andrus



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