[hibernate-dev] 2LC docs

Vlad Mihalcea mihalcea.vlad at gmail.com
Mon Jan 25 07:22:30 EST 2016


Thanks. Well, the User Guide must detail the API and focus on general usage.
We don't want to scare users with internal details that are more useful in
a Developer Guide or Integrator Guide.
So we must find the right balance for what info we supply in the User Guide.


Vlad

On Mon, Jan 25, 2016 at 1:46 PM, Radim Vansa <rvansa at redhat.com> wrote:

> I wish the docs were as descriptive as your blog - thanks Vlad! (so, I
> should make that happen).
>
> So I (hopefully) finally understand what you mean by sync/async:
> * sync implements only certain methods from the *RegionAccessStrategy and
> the two-phase commit requires TM calling 2LC directly (either by
> registering itself as XAResource or Synchronization)
> * async implements all methods and does not need to interact with TM at
> all.
>
> However, this is just an implementation detail which should not relate to
> cache concurrency strategy, should it? CCS should define only the isolation
> level achieved, and that is:
> * nonstrict-read-write: read committed with short window for stale reads
> during commit
> * read-write: read committed
> * transactional: serializable
>
> Radim
>
> On 01/25/2016 11:55 AM, Vlad Mihalcea wrote:
>
>> Hi,
>>
>> I have some sequence diagrams depicting the async/sync behavior if you
>> are interested:
>>
>> For async: NONSTREICT_READ_WRITE and READ_WRITE:
>>
>>
>> http://vladmihalcea.com/2015/05/18/how-does-hibernate-nonstrict_read_write-cacheconcurrencystrategy-work/
>>
>> http://vladmihalcea.com/2015/05/25/how-does-hibernate-read_write-cacheconcurrencystrategy-work/
>>
>> For sync: TRANSACTIONAL
>>
>>
>> http://vladmihalcea.com/2015/06/01/how-does-hibernate-transactional-cacheconcurrencystrategy-work/
>>
>> Only the region strategy differs since it's not Ehcache, but everything
>> else is from Hibernate API.
>>
>> Vlad
>>
>> On Mon, Jan 25, 2016 at 12:48 PM, Radim Vansa <rvansa at redhat.com <mailto:
>> rvansa at redhat.com>> wrote:
>>
>>     On 01/22/2016 05:26 PM, Steve Ebersole wrote:
>>     > On Fri, Jan 22, 2016 at 9:30 AM Radim Vansa <rvansa at redhat.com
>>     <mailto:rvansa at redhat.com>
>>     > <mailto:rvansa at redhat.com <mailto:rvansa at redhat.com>>> wrote:
>>     >
>>     >     On 01/22/2016 03:11 PM, Steve Ebersole wrote:
>>     >     > On Fri, Jan 22, 2016 at 7:21 AM Radim Vansa
>>     <rvansa at redhat.com <mailto:rvansa at redhat.com>
>>     >     <mailto:rvansa at redhat.com <mailto:rvansa at redhat.com>>
>>     >     > <mailto:rvansa at redhat.com <mailto:rvansa at redhat.com>
>>     <mailto:rvansa at redhat.com <mailto:rvansa at redhat.com>>>> wrote:
>>     >     >
>>     >     >     Why should the strategy 'never be used if serializable
>>     >     transaction
>>     >     >     isolation level is required'? What guarantees it
>>     gives, and what
>>     >     >     in ORM
>>     >     >     core depends on this?  When I've asked the last time,
>>     Steve said
>>     >     >     that all modes but the
>>     >     >
>>     >     >     nonstrict one require that the 2LC is absolutely
>>     transparent
>>     >     >     (consistency-wise), so you always get the same answer
>>     as if
>>     >     you were
>>     >     >     directly talking to DB.
>>     >     >
>>     >     >
>>     >     > I would guess this is talking about "serializable
>>     isolation" at the
>>     >     > application layer.  Yes extended across both the
>>     application and
>>     >     > database.  In our original implementations we had no L2 cache
>>     >     > providers that would support serializable isolation. Does
>>     >     > hibernate-infinispan? If we ask for a certain entry from the
>>     >     cache in
>>     >     > T1, T2 adds that entry and commits, and then we ask for it
>>     again
>>     >     in T1
>>     >     > do we still see it as "not existing"?  I'd highly doubt
>>     it, but
>>     >     if it
>>     >     > does then lets make note of that.
>>     >
>>     >     No, without a transactional cache, it does not. Thanks for the
>>     >     example.
>>     >     But will the request get to 2LC, or will it be served
>>     already from
>>     >     Session cache?
>>     >
>>     >
>>     > It won't work even with a transactional cache I believe. It
>>     won't work
>>     > with Infinispan e.g. I do not think. Hibernate does not keep
>>     reference
>>     > to "non-existing" entities.  That's the only way the Session could
>>     > "serve" the fact that the first T1 lookup found nothing.  Again,
>>     this
>>     > gets right back to that idea of consistency. Without L2 caching, in
>>     > this scenario with serializable isolation the database would
>>     return me
>>     > "no row" in both T1 SELECTs.
>>
>>     Infinispan keeps 'transactional context' for the current
>>     transaction and
>>     stores all reads there, even if this is a null read. However, as I've
>>     checked the distribution code, it still does the remote lookup (which
>>     escapes the transaction) and the value could get there even with
>>     so-called repeatable reads. I'll check infinispan-dev why.
>>
>>     >
>>     >     >  Does the ' you should ensure that the transaction is
>>     completed when
>>     >     >     `Session.close()` or `Session.disconnect()` is called'
>>     still
>>     >     hold, or
>>     >     >     does the transactional rework in 5.0 somehow obsolete
>>     this info?
>>     >     >
>>     >     >
>>     >     > I cannot say why this is discussed in a chapter on caching.
>>     >     > Session#disconnect is largely deprecated (its main use case is
>>     >     handled
>>     >     > much more transparently now).  IMO it's always a good idea
>>     to make
>>     >     > sure a transaction against a resource is completed prior
>>     to closing
>>     >     > that transaction. That's no different for a Hibernate Session
>>     >     then it
>>     >     > is for a JDBC Connection, etc.
>>     >
>>     >     Did you meant 'commit the transaction before closing the
>>     session'? If
>>     >     the Session.close() is called with tx open, will the
>>     transaction be
>>     >     committed? But any way, this should be really the same as
>>     without 2LC.
>>     >
>>     >
>>     > I meant to say " make sure a transaction against a resource is
>>     > completed prior to closing that resource". Saying "complete the
>>     > transaction" != "commit the transaction". Completion might be either
>>     > commit or rollback.  But the idea is that it is in a definitive
>>     state.
>>     >
>>     > Historically what a stranded transaction at the time of
>>     Session#close
>>     > meant depended on the JDBC driver.  Most drivers rollback back on a
>>     > stranded transaction; Oracle has always been the notable
>>     exception as
>>     > they would commit a stranded transaction.  But regardless in
>>     terms of
>>     > Session locks etc in the cache that would strand the locks as
>>     well iirc.
>>     >
>>     > In developing 5.0 and the new transaction handling I know we talked
>>     > about making this more deterministic, specifically always handling
>>     > this as if a rollback had been called.  But to be honest, that's not
>>     > what I am seeing in the code. Andrea, do you remember?  If not, we
>>     > should definitely add some tests for this to see what happens
>>     atm and
>>     > make sure its really what we want to have happen moving forward.
>>     >
>>     >
>>     >     > Basically this passage is a poorly worded hint.  What it is
>>     >     trying to
>>     >     > convey is that for "asynchronous" cache access what drives the
>>     >     > interactions with the Cache is the Hibernate transaction,
>>     and in
>>     >     these
>>     >     > case the user should take extra care to make sure that the
>>     >     transaction
>>     >     > is handled properly. That still holds true.
>>     >     >
>>     >     > As a refresher, the idea of "synchronous" versus
>>     "asynchronous" is
>>     >     > simply cache access that is driven by JTA ("synchronous")
>>     versus
>>     >     those
>>     >     > that are driven by local transactions ("asynchronous").
>>     >
>>     >
>>     >     Eh, I probably don't get the exact meaning of 'driving the
>>     access' :-/
>>     >     And I can't find any reference to 'async' in user guide.
>>     >
>>     >
>>     > I keep pointing y'all to
>>     > org.hibernate.cache.spi.access.EntityRegionAccessStrategy,
>>     > org.hibernate.cache.spi.access.CollectionRegionAccessStrategy,
>>     etc as
>>     > the best source for this information.  I spent a lot of time
>>     >  documenting (javadoc) these contracts as I developed them.
>>     >  sync/async is discussed there.  No need for it to be discussed
>>     in the
>>     > user guide IMO, its a concept for developers of cache
>>     implementations
>>     > to understand not users.
>>
>>     Okay, this sync/async. Sure, then it makes sense that it's not in user
>>     guide. But pardon my confusion, that class documents which methods are
>>     used by sync/async strategies, and what's the order of method
>>     invocation, but I never got what is the idea behind the sync/async
>>     strategy differentiation. As I've started messing with ORM only after
>>     the 5.0 tx rework, I always considered the difference between JTA and
>>     local transactions just an implementation detail orthogonal to 2LC.
>>
>>     Radim
>>
>>     --
>>     Radim Vansa <rvansa at redhat.com <mailto:rvansa at redhat.com>>
>>     JBoss Performance Team
>>
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>>
>>
>>
>
> --
> Radim Vansa <rvansa at redhat.com>
> JBoss Performance Team
>
>


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