Databases
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    <p>We have found that the Eclipselink L2 entity cache doesn't seem to do what it's supposed to do at all. We have relatively few objects to keep in the L2 cache from the database, yet our memory usage spirals out of control over the course of 24 hours until we run out of heap (4GB worth). I have profiled the application and confirmed that the memory usage is coming from Eclipselink's L2 Cache. We are using SOFT-WEAK, and it doesn't ever evacuate old objects as far as we can tell (something the docs suggest it's supposed to do), and creates multiple instance of objects whose properties are the same (which the docs suggest it's NOT supposed to do), this is a read-only application for goodness sake. We have also been utterly unsuccessful getting any manner of cache-coordination in a clustered environment to work. We've had three senior Java devs with over a decade of experience each, one of which actually worked at IBM on their Java implementation remain unsuccessful in getting any cache-coordination to function, spending time periodically on the problem over the course of the last six months even tracing through the Eclipselink source to figure out what was going wrong. It's getting pretty tiresome rebooting our application every 24 hours, and I'm just about done with L2 caching at this point. I only hope we can get acceptable performance without it, at least Eclipselink JPA 1 implementations could manage to do parallel reads which seem to have evaporated in JPA 2. Database implementors solved this stuff years ago with MVCC, why is it that a set of smart Java devs can't manage the same feat given that it's already been done? All we're doing is moving it upstream essentially and expressing the data as objects instead of tuples and pages.</p>
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    1. POJPA/Eclipselink cache life span
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    1. COIf you objects are not garbage collecting from the cache then you most likely still have references to them somewhere in your application. You can use a memory profiler to find to root reference holding onto the objects.
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    2. COFor an example on how to configure cache coordination see, http://wiki.eclipse.org/EclipseLink/Examples/JPA/CacheCoordination
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    3. COYeah - it turned out that the custom entity manager container we wrote wasn't closing entity managers properly, which was leaving references to objects all over the place. It also meant that the objects never got sent through cache coordination.
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