Re: [hibernate-dev] ReadWrite 2LC "read uncommitted" data anomaly
On 5 April 2016 at 14:11, Vlad Mihalcea <mihalcea.vlad(a)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-cachecon...
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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