Dan,
I think the bottom line is that the EJB server has to provide transaction
isolation to the simultaneous operations on our beans. In GemStone/J we do
this by having separate instances of the bean per transaction, and the
datasource handles the concurrency issues. This works as long as we have
over lapping transactions.
If you pull data out of the beans, copy it to the client, update the data,
and write it back to the bean in a separate transaction than the original
read, then you must use optimistic concurrency control (i.e. "dirty
detection"...).
Other servers will handle this by synchronizing access to a single bean
instance at the Java level. This will perform very poorly for update intense
applications.
Regards,
-Chris.
> -----Original Message-----
> From: dan benanav [SMTP:[EMAIL PROTECTED]]
> Sent: Wednesday, February 09, 2000 7:50 AM
> To: [EMAIL PROTECTED]
> Subject: This should be easy an obvious, but it's not.
>
> I have a question about how to implement something using EJB. I am sure
> that this problem is very common and that it has come up in discussions
> in various forms on this list, but I don't think the right approach has
> been clarified. This is surprising since the spec should be able to
> handle such a situation rather easily. It appears to me that many
> people have a misunderstanding about EJB that leads to an incorrect way
> to solve this problem. Including in that misunderstanding is the Sun
> BluePrints guide. In the following I will assume we are talking EJB 1.1
> spec and I am interested in a solution that conforms to that spec.
>
> Problem: Suppose you have an Account class that is the remote interface
> to an Account bean. AccountBean is the entity bean implementation
> class. You want to write a method increment(int x) that adds x to the
> balance on the account represented by the Account instance. There are 2
> approaches that come to mind about how to do this. The first is the
> most obvious but I believe there are problems with it.
>
> 1) public void increment(int x) {balance += x;};
>
> 2) public void increment(int x) { //use jdbc to increment the balance in
> the db using sql like "updateaccount_table set balance = balance + ?
> where account_key = ?". The first question mark is set to x and the
> second question mark is set to the primary key of the bean.}
>
> Why isn't 1) good? Containers are free to use multiple bean instances
> to handle concurrent calls to an entity object as long as those calls
> are occurring under a different transaction. This means that you can
> have bean instance A and bean instance B both handling calls to
> increment x, where both A and B represent the same entity object (same
> primary key). Some containers my choose not to do this, however there
> is no guarantee that the container provider won't change their approach
> in the future. Furthermore if you rely on the container not doing that
> your code will not work on any EJB server. This would make it difficult
> for EJB bean providers (like the Theory Center) for providing server or
> container independent beans. So if you agree now that your code should
> work if the Container provider uses multiple bean instances then I think
> there is a problem. Suppose that in two separate threads calls are
> made to an entity object. One thread calls increment(1) and the other
> increment(2). These calls occur concurrently. Assume also that the
> initial balance before the calls is 3. After the call the new balance
> should be 6. Here is what can occur:
>
> T1:Container calls ejbLoad on Bean A: Balance gets set to 3.
> T2:Container calls ejbLoad on Bean B: Balance still is 3. (This
> happens in a separate thread and separate transaction context).
> T3:Container calls increment(1) on Bean A: Balance gets set to 4 in Bean
> A.
> T4:Container calls increment(2) on Bean B: Balance gets set to 5 in Bean
> B.
> T5:Container calls ejbStore on Bean A: balance in database is updates to
> 4.
> T6:Container calls ejbStore on Bean B: what happens??
>
> What happens may depend on what the Transaction isolation level is and
> perhaps on the underlying database. If you use Oracle and the default
> transaction isolation level (read committed) then the balance gets set
> to 5 in the database. (The wrong answer!). If you use serializable
> transaction isolation level then an exception is thrown and the client
> will have to react appropriately to the exception. So it would appear
> that the only way 1) could possibly work (in some sense) is to use
> transaction level serializable. So 1) with transaction serializable is
> an approach but it is incompletely specified since we haven't said how
> the client should handle this. I imagine there are ways to handle it.
> (Catch the exception and try again for example). Furthermore I have
> been told by WLS not to use transaction serializable due to a bug in
> Oracle so 1) does not currently work for Oracle. Even if this would
> work it seems like it would be better to just force the container or to
> specify in the deployment descriptor that calls to increment should be
> serialized. Currently there is no way to do that in the spec.
>
> The second solution would work however there are also problems with it.
> You need to be careful to make sure that when ejbStore is called the
> correct value is stored in the database. For example, you cannot just
> increment the value in the db. You would also need to set balance in the
> bean appropriately.
>
> So the question is, how should one implement this? Surely we should be
> able to reach a consensus on this rather simple question?
>
> dan
>
> ==========================================================================
> =
> To unsubscribe, send email to [EMAIL PROTECTED] and include in the
> body
> of the message "signoff EJB-INTEREST". For general help, send email to
> [EMAIL PROTECTED] and include in the body of the message "help".
===========================================================================
To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
of the message "signoff EJB-INTEREST". For general help, send email to
[EMAIL PROTECTED] and include in the body of the message "help".
