[hibernate-dev] Cache key containing composite ID with a many-to-one

[Sanne Grinovero]

Hi Gail,

going for a disassembled ID would seem logical, but we'll need some
special care to deal with custom implementations of equals/hashcode.

Clearly a composite ID object would require the users to implement a
custom equals(); going for a solution based on a disassembled ID we
would need to either:

A# trust the equals implementation is "the obvious one"

B# hydrate both sides of the equals check for each time there's need
to invoke an equals (and same for hashcode)

I'm afraid only B would be backwards compatible and bullet-proof, and
yet its performance overhead would make it a terrible choice.

Perhaps the best solution is to constrain this, and warn that such a
model is not a good fit for caching?

Unless someone has a better idea; thinking out of the box: could be
interesting to explore not allowing users to implement custom equality
contracts as that's the root of many problems, but that would require
much more careful thought, and for sure a significant breaking change.

Or allow people to implement any custom equals, but ignore them and
apply "the obvious one" consistently.

Thanks,
Sanne



On Wed, 4 Dec 2019 at 19:40, Gail Badner <gbadner at redhat.com> wrote:
>
> When an entity is cached with a composite ID containing a many-to-one
> association, the cache key will contain the many-to-one associated entity.
> If the associated entity is not enhanced, then it could be an uninitialized
> proxy.
>
> I've created a test case [1] that illustrates this using Infinispan. The
> test case is for 5.1 branch, since hibernate-infinispan is still included
> in that branch. The same would happen for master as well.
>
> Aside from the obvious issue with increased memory requirements to store an
> entity, there are other problems as well.
>
> What I've found so far is that caching a key with an uninitialized entity
> proxy can cause some big problems:
>
> 1) A lookup will never find this key unless the lookup is done with a cache
> key containing the same entity proxy instance.
>
> 2) Calling EntityManager#find with a composite ID containing a detached
> uninitialized entity proxy will result in a LazyInitializationException.
> This does not happen with second-level cache disabled.
>
> 3) Calling EntityManager#find with a composite ID containing a managed
> uninitialized entity proxy will result in the proxy being initialized. This
> does not happen with second-level cache disabled.
>
> I have not looked into what happens when the associated entity is enhanced
> and uninitialized (i.e., enhancement-as-proxy).
>
> IIUC, disassembling the ID that gets stored in the cache key would be far
> more efficient, and would avoid these issues. We would only want to do this
> when a composite ID contains an association. This would require changes in
> some SPIs though, to account for the disassembled ID type.
>
> I've been discussing necessary changes with Scott to cache a
> disassembled ID. Before we get too far down this road, I'd like to get some
> feedback.
>
> In the first place, should an entity instance be stored in a cache key?
>
> Is disassembling primary keys that contain an association the appropriate
> way to go? If so, I'll continue with a POC for doing this.
>
> Thanks,
> Gail
>
> [1]
> https://github.com/gbadner/hibernate-core/blob/753e36edf5137296d28b2a07cee3daffc16c6d1e/hibernate-infinispan/src/test/java/org/hibernate/test/cache/infinispan/CacheKeysWithEagerEntityFactoryTest.java
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