Re: Concurrency question

[Michael A. Hovan III]

You may want to use optimistic locking.  You will have to write code to 
handle the lock failure situations when storing and it requires an 
additional column on your tables.  But, it does not hold locks in the 
tables and is a more general mechanism for handling concurrency.  We 
are using it to good effect in a moderately loaded distributed 
simulation application to moderate our database concurrency.

Make sure that you are using OJB 1.0 or later if you decide to use 
Optimistic Locking with a Timestamp column an Oracle Database and Java 
1.4.x.  There is an ugly interaction between the new Java Timestamp and 
the Oracle Driver which I believe was worked around just before the OJB 
1.0 release.

Mike
On Jul 6, 2004, at 12:00, Fiona Magner wrote:
Hi there:
I am just beginning to use OJB in a Web application. The particular 
web application (when it goes live with OJB) will be required for high 
availability. I have a question about a concurrency issue that I have 
come across. Consider the following example. Two users, a and b are 
accessing the customers table and updating the customers balance.

// 1.User A gets customer record 123 (bean) from database.
Customer has balance of 0
        // 2. User B gets customer record 123 (accountbalance = 0)
// 3.User A make changes to bean object using retrieved values
    eg. c.setAccountbalance(c.getAccountbalance() + 20.00)
    (customer will now have balance of 20)
        // 4. User B makes changes to account balance (balance now 20)
// 5. User A sends updates to database (stored balance is now 20)
        // 6. User B sends updates to database (stored balance is now 
20 *** should be 40)

Note that steps 1, 3 and 5 actually all happen within the one script 
(as do 2, 4 and 6) but I have just outlined them as separate steps 
above for clarity. As you can see it is possible for two concurrent 
users to cause inconsistency in the data. Is it necessary to have a 
markDirty() step in the script before applying the changes? Also can 
you tell me if this markDirty() will put a lock on the table until the 
changes are committed? Like I say this will be a high throughput site 
and therefore I would like to avoid any lengthy locks on the tables 
while updates are being carried out. I hope my email is clear, if you 
require any more info please let me know.

Thanks!
Fiona

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