[hibernate-dev] 2LC docs

[Radim Vansa]

On 01/25/2016 11:48 AM, Radim Vansa 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>> 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>>> 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.

So it seems that Infinispan handles repeatable reads of null correctly.

>
>>
>>     >  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>
JBoss Performance Team