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Subject: Re: [wsbpel] Issue - 118 - When are Correlation Sets Mandatory?


See below

Maciej Szefler wrote:

> 
> 
> Yaron,
> 
> On Thu, 2004-04-15 at 12:58, ws-bpel issues list editor wrote:
> 
>  > Normative Change - The schemas for pick and receive make correlation
>  > sets optional. That would appear to be wrong.
>  >
> This change would preclude start activities without a correlation set;
> for example if every time I get a message on some port I want to start a
> new process but there is nothing unique in the received message (the
> operation may have an input message with no unique parts). To make this
> more concrete, if I have a process for making pizza's, I might want the
> makePizza(toppings) operation start the process. In this case there is
> nothing in the makePizza input message to uniquely identify the new
> pizza process (topping not being unique), so there is nothing to
> initiate a correlation set with.  This is explictly allowed in the spec
> in sec 6.5:
> 
>          "If exactly one start activity is expected to instantiate the 
> process,
> the use of correlation sets is unconstrained. This includes a pick with
> multiple onMessage branches; each such branch can use different
> correlation sets or no correlation sets."
> 
> Are you of the opinion that such usage should not be permitted?
> 

There had to be some sort of correlation or the message would never have 
reached the BPEL instance in the first place. For the scenario you 
describe I would recommend Issue 96, engine managed correlation. In this 
case the 'correlation' is just the URI assigned to the process instance. 
This would fully support the scenario you describe but be consistent 
with our correlation model.

Another possibility is to specify the absence of a correlation set as 
meaning that correlation is being handled by the engine but that leads 
to ambiguities (which seems to be my theme for this week). For example, 
if I specify a single correlation set then did I meant to do correlation 
exclusively on that correlation set or on a combination of the 
correlation set and some unspecified machine specific correlation mechanism?

Still, as ambiguities go I think I might be able to live with this one. 
I really need to noodle on our correlation set model some more. It just 
seems a big, clunky. The whole start activities mess is another symptom 
of the clunkiness.

> 
>  > Also note, that the WSDL 1.1 spec quite clearly states that
>  > request/responses do not have to be sent over synchronous transports
>  > so there may be values we could use for correlation sets. In other
>  > words, the situation is inconsistent. In some cases a request/response
>  > uses a synchronous transport and in other cases it could be using an
>  > asynchronous transport with some message based correlation. Do we want
>  > to distinguish these cases or do we want to just say that we presume
>  > that any time a request/response pattern is used there is some
>  > correlation mechanism implicitly known to the engine and therefore
>  > correlation sets are always optional on the incoming message? Reply
>  > the same issue as responses on invokes.
>  > Changes: 15 Apr 2004 - new issue
> The transport being asynchronous is an irrelevant implementation detail
> (at least as far as the BPEL language is concerned): the fact that the
> operation is declared synchronous means that the transport (not the
> engine) has some (transport-specicific) means of matching up the
> response to the request. For the simple HTTP case this is simple: the
> response is received on the same socket. For an asynchronous transport
> like JMS, something like the correlationId property of the JMS message
> would need to be used match up the response to the request; the setting
> and interperetation of such a property would need to be a feature of the
> JMS protocol binding. This applies to both in the invoke case and the
> receive/reply case. 
> 
> -Maciej
> 

I happen to agree with you but this is not clearly spelled out one way 
or another in the spec.

> 
> 
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> 


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