This is to mtodd:

If you're stuck/forced to use the legacy numbers, Dan's solution below makes sense. That is, attempt to delete the row. If nothing is deleted, presumably another user has that number, so get the next number and retry.

You can do this in a simplified way without transactions or commitment control, by using a delete statement and executeUpdate():

iCount = ps.executeUpdate();

If the count is zero, the operation affected no rows. Notice that you don't get an exception in this case.


Joe Sam

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-----Original Message----- From: Dan Kimmel
Sent: Wednesday, October 10, 2012 11:11 AM
To: Java Programming on and around the IBM i
Subject: RE: Locking of record using SQL in JAVA Websphere application.

What goes on in your deleteMemberNumber() method?

If it was me, I'd just use rs.deleteRow() in its place.

I also think you need a rollback() in your exception handlers, though closing the connection without a commit will force a rollback.

Under commitment control, I don't think "FOR UPDATE" is going to preclude another thread from reading the same record. You have to rely on deleteRow() to throw an exception in the thread that loses the race.

-----Original Message-----
From: java400-l-bounces@xxxxxxxxxxxx [mailto:java400-l-bounces@xxxxxxxxxxxx] On Behalf Of mtodd@xxxxxxxxxxxx
Sent: Tuesday, October 09, 2012 7:44 AM
To: java400-l@xxxxxxxxxxxx
Subject: Locking of record using SQL in JAVA Websphere application.


Have Web app that reads record from iSeries to get a unique number, then deletes that record.

Need to make sure that only one web user gets that unique number.

Application seems to work 99% of the time but sometimes log reports duplicate key error message when using unique number to write to transaction file. This seems to suggest that more than one user retrieved the unique number.

I use SQL to access the iSeries, setting the connection to TRANSACTION_SERIALIZABLE to lock the record.

Appreciate any help on this issue.

Many thanks.

Below is the code :

sqlSelect="SELECT AMMBR# FROM MBPRDDTA.MBAMBRP WHERE AMGRWS= ? FOR UPDATE";
try{
con=AS400ConnectionManager.getInstance().getConnection();
con.setTransactionIsolation
(Connection.TRANSACTION_SERIALIZABLE);
psmt=con.prepareStatement(sqlSelect);
psmt.setString(1,memberData.getTradeIndividual());
ResultSet rs=psmt.executeQuery();
rs=psmt.executeQuery();

if(rs.next()){
response.setMemberNumber(rs.getLong("AMMBR#"));
if (deleteMemberNumber(response.getMemberNumber
(),con)){
response.setStatus
(ApplicationStatus.SUCCESS);
response.setMessage("Successfully retrieved & deleted member number from MBAMBRP");
}
}
con.commit();


}catch (SQLException e){
response.setMessage("SQL exception : Error retrieving & deleting member number from MBAMBRP");
e.printStackTrace();
} catch (PersistenceException e) {
response.setMessage("Persistence exception : Error retrieving & deleting member number from MBAMBRP");
e.printStackTrace();
}
finally{
try {
psmt.close();
con.close();
} catch (SQLException e) {
response.setMessage("SQL final exception : Error retrieving & deleting member number from MBAMBRP");
e.printStackTrace();
}
}


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