[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-3608) DB sequence numbers are not unique when using the pooled SequenceStyleGenerator in multiple JVMs with the same DB

[Matthias Gommeringer (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-3608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=37057#action_37057 ] 

Matthias Gommeringer commented on HHH-3608:
-------------------------------------------

Hi Steve,
thanks for reading this and for your reply (finally ;-) )

I do not agree (or not understand) with your statement:
>>>>
... The second select bumps the "current value" in the sequence to 11...
<<<<

My understanding is as follows:

DB sequence starts with 1 increment by 10
Obtaining next value for the first time will bump DB sequence to 11 reserving 1-10 for hibernate. With the second select the "current value" will be bumped to 21.

Furthermore my understanding of DB sequences is that they guarantee to generate unique numbers for several different users.
Excerpt from http://download-uk.oracle.com/docs/cd/B13789_01/server.101/b10759/statements_6014.htm:
<<Use the CREATE SEQUENCE statement to create a sequence, which is a database object from which multiple users may generate unique integers.>>

So if sequences are properly used the DB guarantees that there are no duplicates. Sorry, but I'm still convinced that this is a bug. Please correct me if I'm wrong.
But luckily a custom optimizer can be registered with hibernate :-)

> DB sequence numbers are not unique when using the pooled SequenceStyleGenerator in multiple JVMs with the same DB
> -----------------------------------------------------------------------------------------------------------------
>
>                 Key: HHH-3608
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3608
>             Project: Hibernate Core
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.2.6, 3.3.0.GA, 3.3.0.SP1, 3.3.1
>         Environment: Hibernate 3.2.6, Oracle (any version)
>            Reporter: Matthias Gommeringer
>            Priority: Blocker
>         Attachments: CustomPooledOptimizer.java, PooledOptimizerTest.java
>
>
> We have several Application Servers (=JVMs) running each of them using Hibernate-Objects with the SequenceStyleGenerator+pooled configured. In unpredictable time intervals it happens that hibernate assigns the same ID to two completely different objects which results in a UniqueConstraintViolation exception from the database. Here an example with a description where hibernate fails:
> DB-Sequence setup:
> start=0
> increment=2
> PooledOptimizer.generate() with 2 threads (first assignment of hiValue/value):
> JVM-1                                                 JVM-2
> value=0=callback.nextval
>                                                             value=2=callback.nextval
> hiValue=4=callback.nextval
>                                                             hiValue=6=callback.nextval
> The problem's cause is in the PooledOptimizer.generate: when it initializes
> the value+hiValue for the first time it invokes callback.nextValue() twice which 
> may provide values that do not belong to each other. The reason is that 
> between the assignment of "value" and "hiValue" another JVM can retrieve a
> DB sequence value from the callback which leads to an inconsistent "value" and "hiValue"
> relation (see example above).
> A fix that works for multiple JVMs would be to invoke the "callback.getNextValue()" maximum once
> per "optimizer.generate()" call:
>     public synchronized Serializable generate(AccessCallback callback) {
>         if ( hiValue < 0 ) {
>             value = callback.getNextValue();
>             hiValue = value + incrementSize;
>         }
>         else if ( value >= hiValue ) {
>             value = callback.getNextValue();
>             hiValue = value + incrementSize;
>         }
>         return make(value++);
>     }
> I attached a testcase that prooves the described problem (you can see that the IDs "2" and "3" are assigned two times).
> I would be very thankful if this problem could be fixed very soon since it is a showstopper which
> occurs very unpredictably.

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