I'm updating two different instances of Oracle while running under Tomcat. The increment generator is using the highest value from the first instance when it generates a new key for the same table in the second instance. This results in a unique key constraint violation.

Running a simplified test program in batch works, and Hibernate debug shows 2 calls that fetch the first free id (one for each db):
id.IncrementGenerator::getNext(76): first free id: 123461

But the real app, running under Tomcat, only shows one such trace. In this case, the first update, to db#1, runs under an http-processor thread; that generates the "first free id" trace. Later, a worker thread running a Quartz-scheduled task opens both db#1 and db#2, and adds a new "Offer" to db#2. No "first free id" trace appears. I've included a condensed version of the failing task's code below.

Hibernate version: 2.1.8
database: Oracle 9i

Mapping documents:
<hibernate-mapping package="com.xyz.admin.data">
<class name="Offer" table="OFFER">
<id name="id" type="java.lang.Long">
<column name="OFFER_ID" sql-type="number" not-null="true"/>
<generator class="increment"/>
</id>

Code from openSession() to failure:
localSessionFactory = _hibernateEnvironments.getLocalSessionFactory();
localSession = localSessionFactory.openSession();
localTransaction = localSession.beginTransaction();
localSession.update(_offerJob); // re-associate this with our session
_offerJob.setMessage( "initializing" );
targetSessionFactory = _hibernateEnvironments.getSessionFactory( _offerJob.getTargetEnvironmentName() );
targetSession = targetSessionFactory.openSession();
offers = _offerJob.getDeployableOffers();
for (Iterator iter = offers.iterator(); iter.hasNext(); ) {
DeployableOffer deployOffer = (DeployableOffer)iter.next();
Offer sourceOffer = deployOffer.getOffer();
Long sourceId = sourceOffer.getId();
// it seemed necessary to do a 'get' here to avoid LazyInitialization errors later.
sourceOffer = (Offer)localSession.get(Offer.class, sourceId);
<calls method:>
_targetSessionFactory = targetSession.getSessionFactory();
Offer targetOffer = null;
sourceOffer.setSynchronizationKey(syncKey);
ClassMetadata offerMetadata = _targetSessionFactory.getClassMetadata( Offer.class );
Transaction targetTransaction = targetSession.beginTransaction();
targetOffer = (Offer)offerMetadata.instantiate(null);
targetOffer.setSynchronizationKey(syncKey);
/* ... update targetOffer fields... */
targetOffer.setWebStatus(...);
targetSession.saveOrUpdate( targetOffer );
targetTransaction.commit();
/* update of target offer fails here due to unique key constraint violation on Offer_id */

stack trace:
* ERROR 10:23:49.671 [Promotion Admin_Worker-2] deploy.OfferSynchronizer::synchronizeOffer(225): error synchronizing;
net.sf.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
at net.sf.hibernate.exception.ErrorCodeConverter.convert(ErrorCodeConverter.java:73)
at net.sf.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:29)
at net.sf.hibernate.impl.BatcherImpl.convert(BatcherImpl.java:328)
at net.sf.hibernate.impl.BatcherImpl.executeBatch(BatcherImpl.java:135)
at net.sf.hibernate.impl.BatcherImpl.prepareStatement(BatcherImpl.java:61)
at net.sf.hibernate.impl.BatcherImpl.prepareStatement(BatcherImpl.java:58)
at net.sf.hibernate.impl.BatcherImpl.prepareBatchStatement(BatcherImpl.java:111)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:454)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:436)
at net.sf.hibernate.impl.ScheduledInsertion.execute(ScheduledInsertion.java:37)
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2449)
at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2435)
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2392)
at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2261)
at net.sf.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:61)
at com.xyz.promotion.admin.deploy.OfferSynchronizer.synchronizeOffer(OfferSynchronizer.java:214)
at com.xyz.promotion.admin.deploy.OfferDeployer.execute(OfferDeployer.java:156)
at com.xyz.promotion.admin.deploy.DeployQuartzJob.execute(DeployQuartzJob.java:72)
at org.quartz.core.JobRunShell.run(JobRunShell.java:191)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:516)
Caused by: java.sql.BatchUpdateException: ORA-00001: unique constraint (TSI.SYS_C002728) violated

at oracle.jdbc.dbaccess.DBError.throwBatchUpdateException(DBError.java:459)
at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:3907)
at com.mchange.v2.sql.filter.FilterPreparedStatement.executeBatch(FilterPreparedStatement.java:260)
at net.sf.hibernate.impl.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:54)
at net.sf.hibernate.impl.BatcherImpl.executeBatch(BatcherImpl.java:128)
... 16 more

Debug level Hibernate log excerpt:
* DEBUG 12:32:34.968 [http-9980-Processor24] id.IncrementGenerator::getNext(62): fetching initial value: select max(OFFER_ID) from OFFER
* DEBUG 12:32:34.984 [http-9980-Processor24] id.IncrementGenerator::getNext(76): first free id: 123462
[the above occurs during creation of new Offer on db#1]
* DEBUG 12:33:01.437 [http-9980-Processor24] id.SequenceGenerator::generate(76): Sequence identifier generated: 124
* INFO 12:34:14.109 [Promotion Admin_Worker-1] deploy.DeployQuartzJob::execute(60): Deploy Job executing
* INFO 12:34:14.234 [Promotion Admin_Worker-1] deploy.OfferDeployer::execute(151): processing offerid=123462
* WARN 12:34:15.093 [Promotion Admin_Worker-1] util.JDBCExceptionReporter::logExceptions(57): SQL Error: 1, SQLState: 23000
* ERROR 12:34:15.109 [Promotion Admin_Worker-1] util.JDBCExceptionReporter::logExceptions(58): ORA-00001: unique constraint (TSI.SYS_C002728) violated