Incorrect update

Question

Hello
Oracle 11.1.0.6.0
We have a financial app developed in house using java and tomcat. Under very high load we have noticed some anomalies.
There are multiple customers and only one vendor. We see something like this
Vendor balance = 0
Customer 1 balance = 5
Customer 2 balance = 5
1) Customer 1 initiates transaction - opens cursor and inserts into customer table
2) Customer 2 initiates transaction. - opens cursor and inserts into customer table
3) Customer 1 updates customer balance (-1) and vendor balance (+1)
4) Customer 2 updates customer balance (-1) and vendor balance (+1)
5) Customer 1 commit
6) Customer 2 commit
Vendor Balance = 1
Customer 1 balance = 4
Customer 2 balance = 4
So Vendor Balance should be 2. Looks like when the update takes place the update statements must both be reading a value of 0. That is the updates are taking place at the SAME time?
The application developer is unhappy about this ;-)
I have suggested use of select for update to serialise the transactions.
This is felt to cause too much of an overhead as they are no longer concurrent. Which is the point....
Any thoughts or ideas ??
Regards
Ali
Code used
create or replace procedure doPaymentTransaction
( paymentResponseId IN VARCHAR2
) AS
cursor getPaymentDetails is
SELECT
req.customer_play_ref customerPlayRef
,req.order_description orderDescription
,req.vendor_id vendorId
,req.vendor_transaction_id vendorTransactionId
,req.price amount
,c.id customerId
,c.balance customerBalance
,v.balance vendorBalance
FROM
payment_response resp
,payment_request req
,customer c
,vendor v
where
resp.id = paymentResponseId
and
req.id = resp.payment_request_id
and
c.play_ref = req.customer_play_ref
and
v.id = req.vendor_id;
rec getPaymentDetails%ROWTYPE;
ctxId VARCHAR2(255);
vtxId VARCHAR2(255);
begin
OPEN getPaymentDetails;
FETCH getPaymentDetails INTO rec;
    select hibernate_sequence.nextval INTO ctxId FROM sys.dual;
    insert into customer_transaction (
        id
        , version
        , amount
        , customer_play_ref
        , date_created
        , transaction_description
        , description
        , new_balance
        , old_balance
        , payment_response_id
        , vendor_id
        , class
         values (
        ctxId
        , 0
        , rec.amount
        , rec.customerPlayRef
        , sysdate
        , rec.orderDescription
        , 'debit'
        , rec.customerBalance - rec.amount
        , rec.customerBalance
        , paymentResponseId
        , rec.vendorId
        ,'CustomerDebit');
    update customer
        set balance= balance - rec.amount
            ,version = version + 1
        where id=rec.customerId;
    insert into customer_customer_transaction
         customertx_idx
        ,customer_customertx_id
        , customer_transaction_id
    values
        hibernate_sequence.nextval
        , rec.customerId
        , ctxId
    select hibernate_sequence.nextval INTO vtxId FROM sys.dual;
    insert into vendor_transaction
    id
    , version
    , customer_play_ref
    , date_created
    , description
    , new_balance
    , old_balance
    , payment_response_id
    , vendor_id
    values (
    vtxId
    , 0
    , rec.customerPlayRef
    , sysdate
    , 'credit'
    , rec.vendorBalance + rec.amount
    , rec.vendorBalance
    , paymentResponseId
    , rec.vendorId
    insert into vendor_vendor_transaction
        vendortx_idx
        ,vendor_vendortx_id
        , vendor_transaction_id
    values
       hibernate_sequence.nextval
        ,rec.vendorId
        ,vtxId
    update vendor set
     balance= balance + rec.amount
    , version = version + 1
    where id=rec.vendorId;

Answer 1

Thank you - that is a very useful article. As a DBA i was aware of pessimistic v optimistic locking but was unaware of how to programmatically ensure concurrency with an optimisitic model.
Regards
Ali