[ovirt-devel] hibernate's internal PersistentBag sent to FE

Tomas Jelinek tjelinek at redhat.com
Tue Jun 2 13:08:05 UTC 2015 


----- Original Message -----
> From: "Alexander Wels" <awels at redhat.com>
> To: "Liran Zelkha" <lzelkha at redhat.com>
> Cc: "Tomas Jelinek" <tjelinek at redhat.com>, "Oved Ourfali" <ovedo at redhat.com>, devel at ovirt.org
> Sent: Tuesday, June 2, 2015 2:51:31 PM
> Subject: Re: [ovirt-devel] hibernate's internal PersistentBag sent to FE
> 
> On Tuesday, June 02, 2015 03:05:11 AM Liran Zelkha wrote:
> > ----- Original Message -----
> > 
> > > From: "Tomas Jelinek" <tjelinek at redhat.com>
> > > To: "Oved Ourfali" <ovedo at redhat.com>
> > > Cc: awels at redhat.com, "Liran Zelkha" <lzelkha at redhat.com>,
> > > devel at ovirt.org
> > > Sent: Tuesday, June 2, 2015 9:59:17 AM
> > > Subject: Re: [ovirt-devel] hibernate's internal PersistentBag sent to FE
> > > 
> > > 
> > > 
> > > ----- Original Message -----
> > > 
> > > > From: "Oved Ourfali" <ovedo at redhat.com>
> > > > To: awels at redhat.com
> > > > Cc: "Liran Zelkha" <lzelkha at redhat.com>, devel at ovirt.org
> > > > Sent: Tuesday, June 2, 2015 8:35:56 AM
> > > > Subject: Re: [ovirt-devel] hibernate's internal PersistentBag sent to
> > > > FE
> > > > 
> > > > top-posting:
> > > > Due to the fact that we'll see that all over the place, I really think
> > > > that
> > > > it would be best to support that at the frontend level, and not the
> > > > backend
> > > > level.
> > > 
> > > well, from philosophical perspective I think that sending
> > > org.hibernate.collection.**internal**.PersistentBag from backend to FE
> > > and
> > > than faking it's implementation on FE
> > > is a way to hell. It will bring lots of problems. For example the FE will
> > > be directly dependent on the internal structure of an internal hibernate
> > > class and if we update hibernate, FE can fail miserably.
> > > Also the FE patch (https://gerrit.ovirt.org/#/c/41682/1) seems to be more
> > > tricky than expected - it seems to be working but Omer had issues with it
> > > and in some cases (when the delegating also retainAll()) it caused my JRE
> > > to fail on SIGSEGV.
> > > It can be fixed on FE but it is a hack with all risks this kinds of hacks
> > > brings.
> > 
> > I don't think it really matters where we do it, as long as server code will
> > still use the PersistentBag (otherwise we won't be able to use attached
> > entities, and performance will suffer). I believe that serializing these
> > objects to JSON/XML - will work, we won't need to do 2 level of conversions
> > (PersistentBag-->List-->JSON/XML). So if the issue is only on the GWT side,
> > and we won't to write this code once, and not per entity - where is the
> > best way to put it?
> 
> The original problem comes from the fact that we are serializing an object
> from the backend to the frontend that the frontend cannot handle (The
> persistent bag). This is a backend implementation detail that the frontend
> really doesn't care about.  In essence GWT is Javascript that is made to look
> like Java, but it has a bunch of limitations due to the fact it still ends up
> as javascript in the end.
> 
> > > > Doing it in the backend level will cause a lot of overhead.
> > > 
> > > Is it really lots of overhead? Comparing to all other layers the data has
> > > to pass anyway? When abandoning GWT FE and start to use REST we will
> > > anyway remove the GenericApiGWTServiceImpl which contains this cleaning
> > > (https://gerrit.ovirt.org/#/c/41810/).
> > > 
> 
> The overhead comes from having to scan through every single object that we
> are
> transferring to the front end, even though right now only one or two object
> types have this particular problem. The scanning only happens if we actually
> transfer the object to the frontend. If there was a way to determine if we
> need to scan the objects being returned to the front end that would cut down
> on the overhead. Another question to ask, is the overhead BAD enough to care?
> I honestly don't know without doing some profiling. But my gut is telling me
> no
> it is not.

I agree this is the correct fix on the correct place. On the other hand, it needs some optimizations and tests if it actually works and if it is not a big bottleneck. 
Also we need to consider if we want to have some annotation on classes we want to filter before sending back to frontend or we want this to be done all the time.

Long story short, I think the patch which broke the frontend debugging should be reverted and merged once the https://gerrit.ovirt.org/#/c/41810/ is finished and tested.
@Liran: could you please revert your patch? Because currently the FE debugging is pretty much broken (and I don't trust the web mode either...) and the solution is not completely clear now.


> 
> > > > I'll leave the technical details to the UX experts here to see what's
> > > > the
> > > > right approach to do it in the frontend side.
> > > > 
> > > > Thanks,
> > > > Oved
> > > > 
> > > > ----- Original Message -----
> > > > 
> > > > > From: "Alexander Wels" <awels at redhat.com>
> > > > > To: "Martin Perina" <mperina at redhat.com>
> > > > > Cc: "Liran Zelkha" <lzelkha at redhat.com>, devel at ovirt.org
> > > > > Sent: Tuesday, June 2, 2015 4:16:31 AM
> > > > > Subject: Re: [ovirt-devel] hibernate's internal PersistentBag sent to
> > > > > FE
> > > > > 
> > > > > On Monday, June 01, 2015 10:51:28 AM Martin Perina wrote:
> > > > > > ----- Original Message -----
> > > > > > 
> > > > > > > From: "Alexander Wels" <awels at redhat.com>
> > > > > > > To: "Tomas Jelinek" <tjelinek at redhat.com>
> > > > > > > Cc: "Liran Zelkha" <lzelkha at redhat.com>, devel at ovirt.org
> > > > > > > Sent: Monday, June 1, 2015 4:19:47 PM
> > > > > > > Subject: Re: [ovirt-devel] hibernate's internal PersistentBag
> > > > > > > sent
> > > > > > > to
> > > > > > > FE
> > > > > > > 
> > > > > > > On Monday, June 01, 2015 09:33:07 AM Tomas Jelinek wrote:
> > > > > > > > ----- Original Message -----
> > > > > > > > 
> > > > > > > > > From: "Alexander Wels" <awels at redhat.com>
> > > > > > > > > To: devel at ovirt.org
> > > > > > > > > Cc: "Tomas Jelinek" <tjelinek at redhat.com>, "Liran Zelkha"
> > > > > > > > > <lzelkha at redhat.com> Sent: Monday, June 1, 2015 3:16:34 PM
> > > > > > > > > Subject: Re: [ovirt-devel] hibernate's internal PersistentBag
> > > > > > > > > sent
> > > > > > > > > to
> > > > > > > > > FE
> > > > > > > > > 
> > > > > > > > > On Monday, June 01, 2015 09:08:50 AM Tomas Jelinek wrote:
> > > > > > > > > > Hey all,
> > > > > > > > > > 
> > > > > > > > > > since the org.ovirt.engine.core.common.job.Job/Step... has
> > > > > > > > > > been
> > > > > > > > > > moved
> > > > > > > > > > to
> > > > > > > > > > use
> > > > > > > > > > the JPA we have a problem on frontend. The problem is that
> > > > > > > > > > the
> > > > > > > > > > @OneToMany
> > > > > > > > > > annotations results in a List which is of type
> > > > > > > > > > PersistentBag.
> > > > > > > > > > When
> > > > > > > > > > we
> > > > > > > > > > send
> > > > > > > > > > this to Frontend it fails during deserialization. It
> > > > > > > > > > actually
> > > > > > > > > > fails
> > > > > > > > > > quite
> > > > > > > > > > bad because the FE already has an ui-override of it which
> > > > > > > > > > is
> > > > > > > > > > not
> > > > > > > > > > correct
> > > > > > > > > > resulting in a ton of NPEs in development mode.
> > > > > > > > > > 
> > > > > > > > > > So, there are 2 nasty fixes I have made where none of them
> > > > > > > > > > should
> > > > > > > > > > be
> > > > > > > > > > merged
> > > > > > > > > > but demonstrate the possibilities: 1: extend the FE to be
> > > > > > > > > > able
> > > > > > > > > > to
> > > > > > > > > > work
> > > > > > > > > > with
> > > > > > > > > > the PersistentBag (https://gerrit.ovirt.org/#/c/41682/) not
> > > > > > > > > > really
> > > > > > > > > > good
> > > > > > > > > > solution since the PersistenBag is an internal Hibernate
> > > > > > > > > > class
> > > > > > > > > > which
> > > > > > > > > > is
> > > > > > > > > > really not meant to be passed around
> > > > > > > > > > 
> > > > > > > > > > 2: fix on the backend to not send the PersistentBag but an
> > > > > > > > > > ArrayList.
> > > > > > > > > > This
> > > > > > > > > > is only a PoC fixed on a command we face the problem
> > > > > > > > > > (https://gerrit.ovirt.org/#/c/41797/) Obviously this is not
> > > > > > > > > > going
> > > > > > > > > > to
> > > > > > > > > > work
> > > > > > > > > > for other commands accessing the same Job nor for other
> > > > > > > > > > entities.
> > > > > > > > > > 
> > > > > > > > > > So, the first option is generic but very very bad. The
> > > > > > > > > > second
> > > > > > > > > > option
> > > > > > > > > > should
> > > > > > > > > > be used but not sure how to do this in a cheep way (e.g.
> > > > > > > > > > without
> > > > > > > > > > using
> > > > > > > > > > reflection to deep traverse everything sent back to
> > > > > > > > > > frontend
> > > > > > > > > > checking
> > > > > > > > > > if
> > > > > > > > > > it
> > > > > > > > > > does not have a PersistentBag in it.
> > > > > > > > > 
> > > > > > > > > Tomas,
> > > > > > > > > 
> > > > > > > > > Thanks, I was investigating the same issue, I noticed it last
> > > > > > > > > Friday
> > > > > > > > > just
> > > > > > > > > before leaving, so I was investigating the problem to see
> > > > > > > > > what
> > > > > > > > > was
> > > > > > > > > going
> > > > > > > > > on. You are right we should not be sending PersistentBag to
> > > > > > > > > the
> > > > > > > > > frontend
> > > > > > > > > at all. So how about we do a combination of [1] and [2], but
> > > > > > > > > instead
> > > > > > > > > of
> > > > > > > > > delegating in [1] we actually simple throw an exception
> > > > > > > > > stating
> > > > > > > > > don't
> > > > > > > > > sent PersistentBag to the front end. that way anyone
> > > > > > > > > inadvertently
> > > > > > > > > using
> > > > > > > > > it will be notified immediately (since their code won't
> > > > > > > > > work).
> > > > > > > > 
> > > > > > > > Throwing the exception would help us in debugging but the main
> > > > > > > > question
> > > > > > > > is
> > > > > > > > how will we make it work? Since we are planning to move more
> > > > > > > > and
> > > > > > > > more
> > > > > > > > to
> > > > > > > > JPA so we will face this issue more and more often. Solving it
> > > > > > > > one
> > > > > > > > by
> > > > > > > > one
> > > > > > > > on backend in each command is not going to work.
> > > > > > > 
> > > > > > > How about something like this [1]? It appears to use some aspects
> > > > > > > to
> > > > > > > translate
> > > > > > > the hibernate internal classes into normal java util classes.
> > > > > > > 
> > > > > > > [1] https://code.google.com/p/dehibernator/
> > > > > > 
> > > > > > Well, not sure about that from performance point of view.
> > > > > > 
> > > > > > If we really move away from GWT completely in 4.0, then
> > > > > > implementing
> > > > > > all
> > > > > > hibernate inner collections in "uioverride" in a similar way as
> > > > > > Tomas
> > > > > > did
> > > > > > seems to me as the best solution from performance and "easiest to
> > > > > > remove
> > > > > > when not needed" point of view.
> > > > > 
> > > > > I took some inspiration from that code and wrote my own fairly simple
> > > > > hibernate persistent collection replacer, and I put the patch up here
> > > > > [3].
> > > > > It
> > > > > still uses reflection so it is probably not the fastest thing ever
> > > > > written.
> > > > > 
> > > > > [3] https://gerrit.ovirt.org/#/c/41810/
> > > > > 
> > > > > > > > > Alexander
> > > > > > > > > 
> > > > > > > > > > Any better ideas?
> > > > > > > > > > Thanx,
> > > > > > > > > > Tomas
> > > > > > > > > > _______________________________________________
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> > > > > > > 
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