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


Singletons are not a concept supported by the current specification.
The current assumption is that a process always operates through
instances and therefore message-instance correlation must be specified
for all inbound messages, (optionally) except for the one that
instantiates the process.

I have not seen any compelling argument for supporting singletons.  Can
somebody post a potential canonical example? 

-----Original Message-----
From: Ugo Corda [mailto:UCorda@SeeBeyond.com] 
Sent: Thursday, April 22, 2004 9:00 AM
To: Satish Thatte; ygoland@bea.com; Maciej Szefler
Cc: wsbpel@lists.oasis-open.org
Subject: RE: [wsbpel] Issue - 118 - When are Correlation Sets Mandatory?

Please see my answers below.

Ugo

> -----Original Message-----
> From: Satish Thatte [mailto:satisht@microsoft.com] 
> Sent: Thursday, April 22, 2004 7:33 AM
> To: Ugo Corda; ygoland@bea.com; Maciej Szefler
> Cc: wsbpel@lists.oasis-open.org
> Subject: RE: [wsbpel] Issue - 118 - When are Correlation Sets 
> Mandatory?
> 
> 
> OK, by now I am lost so I will respond here, just to clarify 
> what the current spec was meant to say.
> 
> 1.  As Yaron said in the issue def, the current assumption is 
> that all correlation must be explicit.  However it is not 
> true that all correlation must be specified by correlation 
> sets.  The exception is responses in synchronous operations.  
> The assumption for synchronous ops is that the infrastructure 
> will correlate request and response in both inbound and 
> outbound cases.  

I now see where the term "synchronous" came for in the BPEL spec. As I
already mentioned, the use of the term with that meaning is quite
unusual, and prone to generate confusion. A different term should be
adopted (and I think you already agree with that modification).

> Thus the response is always properly 
> correlated to the request by definition, but a request to an 
> existing instance must be correlated by a cset.  

Not if you are dealing with a singleton. In that case, any new request
has only one place to go.

> All this 
> regardless of the nature of the transport binding.  Of course 
> a response may carry a cset, usually to initiate it (see 
> below for the pizza example).
> 
> 2.  For a start activity, there is no requirement to have an 
> inbound correlation set specified.  There are two cases of 
> correlation establishment for start activities.  Case 1: The 
> cset is specified by the message that starts a new instance.  
> Case 2: The cset is created by the instance.  In Case 2, the 
> typical pattern is that the start activity will be 
> synchronous and the response will initiate the cset -- in 
> Maciej's pizza case, the instance would create the "order 
> number" that is later used to let the customer know that the 
> order is ready.  The alternative is Case 1 -- the customer 
> gives his/her (hopefully unique) name in the inbound start 
> message and then that is used to inform the customer about 
> the order completion.  In my experience these are exactly the 
> two alternatives used by real Pizza parlors.  Note that Case 
> 2 takes advantage of the implicit correlation of responses 
> discussed in Point 1 above.

You seem to imply here that a cset is always there (even though it might
not be initialized at start). But, according to your point 1, I should
be able to have cases that are neither case 1 nor case 2 (i.e. no cset
is even defined throughout the process). Could you please clarify? 

> 
> Does that sound more consistent?
> 
> Satish
> 
>  
> -----Original Message-----
> From: Ugo Corda [mailto:UCorda@SeeBeyond.com] 
> Sent: Friday, April 16, 2004 6:09 PM
> To: ygoland@bea.com; Maciej Szefler
> Cc: wsbpel@lists.oasis-open.org
> Subject: RE: [wsbpel] Issue - 118 - When are Correlation Sets 
> Mandatory?
> 
> > this is not clearly spelled out one way or another in the spec.
> 
> I agree. I think the problem with synchronous vs. 
> asynchronous in the BPEL spec is that the terms are used 
> somewhat differently than their usual meaning in the context 
> of request-response MEPs. 
> 
> Usually people think of a synchronous request-response as one 
> where the client has to wait for the response, and an 
> asynchronous request-response as one where the client can do 
> other things between the request and the response. (I am 
> aware that some people have other understanding of the terms, 
> as I learned after countless discussions on this subject in 
> the W3C Web Services Architecture WG, but I think this is the 
> most common understanding).
> 
> In the case of BPEL, the term synchronous makes sense in the 
> case of a request-response <invoke>. The BPEL process 
> instance (the client) actually waits until the response comes 
> back. That is true regardless of whether the transport 
> binding is synchronous or not.
> 
> But in the case of a BPEL <receive>/<reply> the term 
> synchronous seems to make much less sense. It is true that, 
> given a synchronous transport binding, the client (i.e. the 
> Web service interacting with the process) would have to wait 
> until it gets the response from the process. But as soon as 
> the transport binding is asynchronous, this assumption is not 
> valid any more. For instance, a JAX-RPC 2.0 client can issue 
> a request against a BPEL/WSDL request-response port, then 
> happily start some other activities, and finally receive the 
> BPEL response on a different thread.
> 
> So my understanding of the use of the terms synchronous and 
> asynchronous in the BPEL spec is that it made sense at the 
> time a WSDL request-response port implied a synchronous 
> transport. (Even though WSDL does not require that, it is 
> what implementations have been supporting so far). As 
> implementations evolve to support asynchronous transport 
> bindings, I think the current BPEL terminology makes much less sense.
> 
> Ugo
> 
> > -----Original Message-----
> > From: Yaron Y. Goland [mailto:ygoland@bea.com]
> > Sent: Friday, April 16, 2004 2:27 PM
> > To: Maciej Szefler
> > Cc: wsbpel@lists.oasis-open.org
> > 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.
> > 
> > > 
> > > 
> > > To unsubscribe from this mailing list (and be removed from
> > the roster
> > > of
> > > the OASIS TC), go to
> > > 
> > http://www.oasis-open.org/apps/org/workgroup/wsbpel/members/le
> ave_workgroup.php. 
> > 
> > 
> 
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