[hibernate-dev] Bytecode enhanced, Reference Cached immutable Entities

Steve Ebersole steve at hibernate.org
Thu Mar 26 12:16:36 EDT 2015

Apparently I hit some key combo that means send :)  To finish up...

On Thu, Mar 26, 2015 at 10:58 AM, Steve Ebersole <steve at hibernate.org>

> On Thu, Mar 26, 2015 at 10:20 AM, Sanne Grinovero <sanne at hibernate.org>
> wrote:
>> Concurrency
>>    Since the goal is to share the ImmutableEntityEntry instance among
>> multiple threads reading it, I'd rather make sure that it is really
>> immutable. For example it now holds onto potentially lazy initialized
>> fields, such as getEntityKey().
>> If it's not possible to make it really immutable (all final fields),
>> we'll need to make it threadsafe and question the name I'm proposing.
> The specific use-case John is interested in does indeed need to be
> completely thread-safe and fully concurrent.
>> LockMode
>>   From a logical perspective of users, one might think that an entity
>> being "immutable" doesn't necessarily imply I can't lock on it..
>> right? I'm not sure how far it could be a valid use case, but these
>> patches are implying that one can't lock an immutable entity anymore,
>> as the lock state would be as immutable as anything else on the
>> EntityEntry.
>> Are we good with that? Alternatively one might need to think to
>> separate the lock state handling from the EntityEntry.
> Conceptually there is nothing wrong with requesting a READ lock on an
> immutable entity.  But like you said, what is the logical expectation
> there?  IMO there should be none.  But if we decide that it is OK to req

But if we decide that it is OK to request a lock on an immutable entity,
that is problematic for the idea of a "SharedEntityEntry" or an
"ImmutableEntityEntry" because a lock is associated with a Session which is
what the EntityEntry is meant to model... an entity's information in
relation to a Session.  Aka, it now needs to hold state.

> Notice I said immutable here and not READ_ONLY which is a specific
> distinction which is important to other parts of your email that I will
> address in a second email.
>> On smaller details:
>> # org.hibernate.engine.internal.SharedEntityEntry is hosted in an
>> .internal package, I don't think it's right to refactor all the public
>> API javadoc which was referring to EntityEntry to now refer to the
>> internal implementation.
>> # things like EntityEntryExtraState should probably get moved to
>> .internal packages as well now - we couldn't do that before without
>> breaking either encapsulation or APIs.

>> In terms of git patches, the complexity of the changeset risks to get
>> a bit our of hand. What about we focus on creating a clean pull
>> request which focuses exclusively on making EntityEntry an interface,
>> and move things to the right packages and javadoc?
Agree 100%

> You'd have a trivial EntitEntryFactory, and we can then build the
>> evolution on top of that, not least maybe helping out by challenging
>> some points in parallel work.
>> These are the things I'd leave for a second iteration:
>>  - add various implementations of EntityEntry iteratively, as needed
>>  - the strategy such a Factory would use the pick an implementation
>>  - ultimately, make it possible for an integrator to override such a
>> Factory
 This all seems reasonable.  For 5.0 I think the most important thing is to
nail down the idea of EntityEntry as an interface, introduce a Factory for
building them and agree on the signature for building them.  Granted we may
need iteration to finalize the actual Factory signature, especially as OGM
finally starts to use it, but I think that in general we can get it pretty
close.  Worst case we just pass high-level constructs like the
EntityPersister (which OGM supplies custom impls) and the Session and all
the "EntityEntry state".  For the purpose of starting a discussion:

public interface EntityEntryFactory {
    public EntityEntry createEntityEntry(
            PersistenceContext persistenceContext,
            EntityPersister persister,
            Status status,
            Serializable id,
            Object version,
            Object[] loadedState,
            Object rowId,
            LockMode lockMode,
            boolean existsInDatabase,
            boolean disableVersionIncrement,
            boolean lazyPropertiesAreUnfetched);

I would suggest a "creation context" method param object to pass in here,
but seeing as how we are trying to stop instantiations...

I would prefer the definition of the EntityEntryFactory to use be defined
via SessionFactoryBuilder interface.  I already have plans to have a
auto-loaded hook for integrators to be able to adjust the
SessionFactoryBuilder.  But as a Service is fine too.

>> For example with Hibernate OGM we might want to override / re
>> configure the factories to use custom EntityEntry implementations -
>> requirements are not fully clear at this point but it seems likely.
>> The priority being to define the API as that would be a blocker for
>> 5.0, we have then better choices to leave more smarter and advanced
>> EntityEntry implementations for the future; we'd still need to
>> implement at least the essential ones to make sure the API of the
>> EntityEntryFactory has all the context it needs.
>> Thanks,
>> Sanne
>> On 24 March 2015 at 09:27, John O'Hara <johara at redhat.com> wrote:
>> > Steve,
>> >
>> > Have you had chance to look at this? Do you have any
>> comments/observations?
>> >
>> > Thanks
>> >
>> > John
>> >
>> >
>> > On 17/03/15 09:24, John O'Hara wrote:
>> >
>> > Steve,
>> >
>> > I have been having a think about the EntityEntry interface, and have
>> forked
>> > a branch here:
>> >
>> > https://github.com/johnaoahra80/hibernate-orm/tree/EntityEntryInterface
>> >
>> > I know it is nowhere near complete, but was this the sort of idea you
>> had in
>> > mind?
>> >
>> > Thanks
>> >
>> > John
>> >
>> >
>> > On 13/03/15 09:44, John O'Hara wrote:
>> >
>> > EntityEntry retains a reference to a persistenceContext internally that
>> > org.hibernate.engine.spi.EntityEntry#setReadOnly makes calls to, is this
>> > where the session reference is kept?  As
>> > org.hibernate.engine.spi.PersistenceConext is an interface could we
>> have a
>> > different implementation for this use case? e.g. an
>> > ImmutablePersistenceContext that could be shared across sessions?
>> >
>> > For the bytecode enhancement, could we change the enhancer so that it
>> adds
>> > an EntityEntry interface with javassist.
>> > ClassPool.javassist.ClassPool.makeInterface()() as opposed to adding a
>> class
>> > javassist.ClassPool.makeClass()? I need to have a look at javassit to
>> > confirm what javassist.ClassPool.makeInterface() does.
>> >
>> > Thanks
>> >
>> > John
>> >
>> > On 12/03/15 18:52, Steve Ebersole wrote:
>> >
>> > It is possible.  Although some of the changes are particularly painful.
>> > Most of EntityEntry, if it is an interface, can be made to work with
>> your
>> > use case.  org.hibernate.engine.spi.EntityEntry#setReadOnly I think is
>> the
>> > one exception, because:
>> > 1) your use case needs it
>> > 2) it expects the Session to be available internally (its not passed)
>> >
>> > The bigger thing I am worried about for you is the bytecode stuff, as
>> that
>> > ties very tightly with EntityEntry.
>> >
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