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

[Matthias Gommeringer (JIRA)]

DB sequence numbers are not unique when using the pooled SequenceStyleGenerator in multiple JVMs with the same DB
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                 Key: HHH-3608
                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3608
             Project: Hibernate Core
          Issue Type: Bug
          Components: core
    Affects Versions: 3.3.1, 3.3.0.SP1, 3.3.0.GA, 3.2.6
         Environment: Hibernate 3.2.6, Oracle (any version)
            Reporter: Matthias Gommeringer
            Priority: Blocker
         Attachments: 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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