[hibernate-dev] ReadWrite 2LC "read uncommitted" data anomaly

[Vlad Mihalcea]

Hi,

I'd definitely fix it for the refresh operation, which does an implicit
cache eviction too.
In this case, the proposed PR solution is fine since it simply locks the
entry right after it is evicted from the cache and releases the lock after
the transaction is ended.
This way, we won't push an uncommitted entity into 2PL during the two-phase
loading phase that is triggered by the refresh operation.

For sessionFactory.getCache.evictEntity/evictCollection, if there is a
current Session going on, we could propagate a
CacheEvictEvent/CollectionCacheEvictEvent which can apply the lock on that
particular entity/collection, and we release it right after the current
transaction is ended, similar to what refresh should do as well.

For every other use case, like evictAll/evictRegion, we should just
document the behavior.

I saw that Radim has added such a warning for Infinispan in the new User
Guide:

read-write mode is supported on non-transactional distributed/replicated
> caches, however, eviction should not be used in this configuration. Use of
> eviction can lead to consistency issues.


Unfortunately, the EhCache documentation
<http://www.ehcache.org/documentation/2.8/integrations/hibernate.html#read-write>
 makes a wrong assumption:

read-write - Caches data that is sometimes updated while maintaining the
> semantics of “read committed” isolation level.


Vlad

On Tue, Apr 5, 2016 at 4:30 PM, Sanne Grinovero <sanne at hibernate.org> wrote:

> On 5 April 2016 at 14:11, Vlad Mihalcea <mihalcea.vlad at gmail.com> wrote:
> > Hi,
> >
> > While reviewing the PR for this issue:
> >
> > https://hibernate.atlassian.net/browse/HHH-10649
> >
> > I realized that the ReadWrite cache concurrency strategy has a flaw that
> > permits "read uncommitted" anomalies.
> > The RW cache concurrency strategy guards any modifications with Lock
> > entries, as explained in this post that I wrote some time ago:
> >
> >
> http://vladmihalcea.com/2015/05/25/how-does-hibernate-read_write-cacheconcurrencystrategy-work/
> >
> > Every time we update/delete an entry, a Lock is put in the cache under
> the
> > entity key, and, this way, "read uncommitted" anomalies should be
> prevented.
> >
> > The problem comes when entries are evicted either explicitly:
> >
> > session.getSessionFactory().getCache().evictEntity( CacheableItem.class,
> > item1.getId() );
> >
> > or implicitly:
> >
> > session.refresh( item1 );
>
> Good catch!
>
> I think this is caused as we generally don't expect the evict
> operation to be controlled explicitly.
> In my personal experience, I would use the evictAll method to nuke the
> cache state after some significant operation, like restoring a
> backup.. and no other Session would have been opened in the meantime.
> I never used an explicit single-shot evict so I can't say what the use
> case would be.
>
> But of course you're right that it might be used differently, or at
> least such a limitation should be clear.
>
> >
> > During eviction, the 2PL will remove the Lock entry, and if the user
> > attempts to load the entity anew (in the same transaction that has
> modified
> > the entity but which is not committed yet), an uncommitted change could
> be
> > propagated to the 2PL.
> >
> > This issue is replicated by the PR associated to this Jira issue, and I
> > also replicated it with manual eviction and entity loading.
> >
> > To fix it, the RW cache concurrency strategy should not delete entries
> from
> > 2PL upon eviction, but instead it should turn them in Lock entries.
>
> I'm not sure I understood this part. Shouldn't it rather be allowed to
> delete everything, except any existing locks?
> Then rather than turn the remaining locks into locks, it would be
> enough to leave them.
>
> > For the evict method, this is not really a problem, but evictAll would
> > imply taking all entries and replacing them with Locks, and that might
> not
> > perform very well in a distributed-cache scenario.
> >
> > Ideally, lock entries would be stored separately than actual cached value
> > entries, and this problem would be fixed in a much cleaner fashion.
>
> I'd leave this as a detail to the Cache implementation, some might be
> able to perform some operation more efficiently.
> Probably a good idea to clarify this expectation on the javadocs of
> the SPI methods.
>
> Thanks,
> Sanne
>
>
> >
> > Let me know what you think about this.
> >
> > Vlad
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>