Parallel flow using same Partnerlink

Question

Hi all,
This is what i'm trying to do: i have a process that splits up in two parallel processes which are EXACTLY the same (only input differs very slightly). Later on these processes converge again to one.
So, I decided to build a process A that calls upon process B twice, using flow to have it in parallel. I realized I had to do something about getting the receive right. The thing to use looks to be the Correlation Set. However, Correlation set documentation is very meagre. I can't seem to understand the working of the 109.CorrelationSets example, as it looks to use a double correlation (?). It just confuses me.
I really need some help here, and would appreciate a very simple working example.
What I really need to know is:
- what steps to take (BPELDEV guide isn't enough help)
- where to place correlation (on invoke or receive in process A ?, on receive or reply in process B?)
- how to set the correlation value, or is it always derived on the basis of the "query" in the bpws:PropertyAlias?
Thanks in advance!

Answer 1

Hi,
This is probably related to the fact that you cannot have two concurrently enabled activities with the same partner link, operation, and correlation set (this constraint can be found in the BPEL spec.) The correlation sets won't help you here because correlation sets are there to do instance routing (i.e. route an inbound message to the right process instance) but the problem here is not to route to the right process instance, but to route to the appropriate "receive" action... If there are two competing receive actions, the engine just go nuts, and this is why the above constraint was introduced in the BPEL spec.
One way to go around (assuming that the interaction between A and B is based on asynchronous send/receive), is to first send the two messages in a flow, and then wait for the response with a single receive (rather than two separate receive actions). Something like this:
<sequence>
<flow>
<invoke (send asynchronous message to B), no output variable here!>
<invoke (send asychrnous message to B), again, no output variable here!>
</flow>
set variable counter to 1
<while counter <= 2>
<receive one message from B>
increment variable counter by 1
</while>
</sequence>
With a colleague, we have written a more complicated version of this scenario, where we effectively can receive messages through the same partner link not from 2 process instances, but from any arbitrary number of process instances. See http://www.serviceinteraction.com and download the code sample for "one-to-many send/receive with dynamically determined partners".
I can send you the full Oracle BPEL project for this example if you think it would help. But I think that with the above information you should be able to proceed.
Regards
Marlon Dumas (m . dumas @ qut . edu . au)