- From: Edwin Khodabakchian <edwink@collaxa.com>
- Date: Fri, 18 Oct 2002 13:43:53 -0700
- To: "'Burdett, David'" <david.burdett@commerceone.com>
- Cc: <www-ws-arch@w3.org>
- Message-ID: <000701c276e7$14927260$680aa8c0@collaxa.net>
David, I *very* much agree on your point where the coordination of message exchanges is different from the coordination of activities (which is what workflow engine do) but this is not easy to explain. Regarding public versus private, I was *not* talking about company boundaries but component/service boundary. The public face of an ERP system is the information you need to know about that system to integrate it into your application (even if the application that is integrating the functionality lives within your corporation). On the other hand, the private implementation refers to how SAP delivers the service. Separating the 2 faces is important because you do not want to overwhelm the application developer who is doing the integration with the details of how SAP processes order. Also you want to be able to change the internals of SAP without having to update all the applications that integrate to SAP. In the Java world, an Interface is the public face of a component, a class is the private implementation. If you think that within that context, public/private separation is not the right dicothomy, I am open to any other suggestion but I believe that we need to make sure that there is a very clear understanding of the problem we are solving and the value that a choreography spec will deliver: will it increase interoperability between business processes or make the execution of business processes more portable or both. Best, Edwin -----Original Message----- From: Burdett, David [mailto:david.burdett@commerceone.com] Sent: Friday, October 18, 2002 12:32 PM To: 'edwink@collaxa.com'; Burdett, David; 'Mark Baker'; 'Champion, Mike' Cc: www-ws-arch@w3.org Subject: RE: Definition of Choreography Edwin I agree with your descriptions of the two problems that need to be solved. I don't though, think of choreographies as the interface to a complex service. Instead I think of them as "the sequence of messages exchanged between two or more services in order to implement a (business) process". Note that I said "sequence of messages". This means that you could have different types of services with different WSDL definitions sending and receiving those same messages in the same sequence. If you don't do it this way, then you are stopping the re-use of a choreography as the choreography will be tied to particular services. I also think that this type of choreography needs to be defined using a choreography language. This is diffferent from a business process language which defines the sequence in which processes (in this context web services) are excecuted - i.e. not messages. I also don't think that public vs private processes is the best way of thinking of this distinction. For example you could have an ERP system which requires a pre-defined choreography that must be followed and can't be changed if you want to interact with that ERP. This information may not be "public" knowledge. So really, I think of choreographies as constraints on what a business process can do and something which business process languages should recognize. David -----Original Message----- From: Edwin Khodabakchian [mailto:edwink@collaxa.com] Sent: Thursday, October 17, 2002 7:02 PM To: 'Burdett, David'; 'Mark Baker'; 'Champion, Mike' Cc: www-ws-arch@w3.org Subject: RE: Definition of Choreography David, It seems that what you are sharing is the public interface of your service/process. In the example described in [1], you are sharing that the seller exposes a service that understands Order and ChangeOrder messages and also that Change Order only makes sense after Order. What you are not sharing is what happens when you get an Order (our private implementation of this service): that private implementation could be entirely manual or completely automated. It could itself include asynchronous interactions with legacy mainframe application, batch processing etc.. There seems to be 2 problems: [Problem #1] How do you describe the interface of a complex service such as the seller service described in one where a client needs to send and receive multiple message from the service to get its result. [Problem #2] How do you code, execute and run the back end application that implements the public protocol implemented using problem #1. Problem #2 is about control logic and requires a programming language. That programming language can be expressed in XML or a script, that is irrelevant. Standardizing that language will only offer portability between the workflow engines that execute that language, it will *not* increase interoperability. Problem #1 can be solved in various ways: - Extend WSDL so that it not only defines the port types but also the public protocols defined between the port types. - "bake" the public protocol into the interface definition of the service. In the example described in [1] this option would be implemented by having the Order operation return the uri of the ChangeOrder operation forcing the Order to be invoked before the change order. [Paul Prescod posted a couple of email on the merit of that approach] - Coming up with a low tech approach, where the protocol is only available through a documentation: developers have to read the documentation to use the service correctly and have to handle exceptions when they don't (JavaDoc ++ for web services). There are probably many more ways to address this to. By choreography, are we trying to solve problem #1 or problem #2? Edwin -----Original Message----- From: www-ws-arch-request@w3.org [mailto:www-ws-arch-request@w3.org] On Behalf Of Burdett, David Sent: Thursday, October 17, 2002 6:30 PM To: 'Mark Baker'; Champion, Mike Cc: www-ws-arch@w3.org Subject: RE: Definition of Choreography Mark You said ... Why would I ever need to *share* a description with anybody? If you are inside your own business you don't. But choreographies can go between businesses, in which case you definitely do - see [1]. Both sides **need** to know exactly what choreography they are following otherwise you don't get interoperability. For example we have identified 14 different choreographies that can be used to place an order. Without a) a precise definition of the choreography that is actually going to be used, and b) a shared understanding of that choreography by both ends, it just won't work. ... or am I missing something ... Regards David [1] http://lists.w3.org/Archives/Public/www-ws-arch/2002Oct/0217.html -----Original Message----- From: Mark Baker [mailto:distobj@acm.org] Sent: Thursday, October 17, 2002 6:15 PM To: Champion, Mike Cc: www-ws-arch@w3.org Subject: Re: Definition of Choreography On second thought, I'd like to focus on this part of your response, Mike; On Wed, Oct 16, 2002 at 09:50:12PM -0400, Champion, Mike wrote: > reason = prompt("why are you doing this to yourself?") > destination = prompt("where are you going") > departure = prompt("when do you leave") > return = prompt("when do you return") > tripId = TentativelyBookTravel(destination, departure, return) > estimatedCost = getCost(tripId) > if (estimatedCost > managerApprovalLimit) > approved = getVPApproval(reason, estimatedCost) > else > approved = getManagerApproval(reason, estimatedCost) > if (approved) > confirmTrip(tripId) > else > cancelTrip(tripId) This is a good example. And one could certainly specify a language for describing such a flow of operations. But why is a *standardized* language required? Why would I ever need to *share* a description with anybody? As I see it, that flow (minus conditions, which are encapsulated within the service) can be observed at runtime, so doesn't need to be specified earlier, at least for interop reasons. So I invoke "prompt()" on the first service, which returns "why are you doing this to yourself?", which I answer by invoking "answer('because I feel like it')". The response to that invocation is then another question, or perhaps a pointer to the next service which I invoke prompt() on, etc, etc.. Behind the scenes, I could certainly be using some description language to drive this flow. But again, why does it matter if it's standardized or not? The only reason I could think of, is because we're trying to enable somebody to reuse their rules with different tools. But that seems quite different than the motivation I've seen for some of the choreography specs out there. For example, all of them integrate with WSDL, which suggests that choreography is part of the interface, not just the implementation. Can anybody shed some light on this? MB -- Mark Baker, CTO, Idokorro Mobile. Ottawa, Ontario, CANADA. http://www.markbaker.ca http://www.idokorro.com
Received on Friday, 18 October 2002 16:44:05 UTC