W3C home > Mailing lists > Public > public-ws-chor@w3.org > April 2003

Re: Progressive Concreteness of Choreography binding

From: Assaf Arkin <arkin@intalio.com>
Date: Wed, 02 Apr 2003 20:27:29 -0800
Message-ID: <3E8BB831.7010506@intalio.com>
To: Ricky Ho <riho@cisco.com>
CC: public-ws-chor@w3.org

Ricky Ho wrote:

> Let me try to rewrite the code using your suggested model and then see 
> if there are values of indirection.  I'm assuming you are questioning 
> the value of separating the implementation binding from abstract 
> process.  I assume you are OK with the "Protocol Binding" and "Address 
> Binding".
>
> PROCESS
> =========
> In this model (which WSCI and BPEL based on), the message exchange is 
> all expressed in terms of WSDL operations.
> So there is no need to define named messages.  The process itself 
> directly point to the WSDL operation being invoked.
> Message properties are defined as an XPATH extract from the message type.
>
> Orchestration POHandlingProcess {
>         Partner declaration {
>                 mySelf --> PortType POHandlingPT  // contains a 
> one-way "submitPO" operation
>                 purchaser --> PortType POCallbackPT // contains 2 
> one-way "confirmPO", "rejectPO" operations
>                 creditCheckProvider --> CreditCheckPT // contains a 
> request/reply "check" operation
>         }
>
>         Message Property declaration {
>                 float PO.dollarAmount = XPATH(//totalAmount/@amount) 
> of request of (PortType = "POHandlingPT", Operation = "submitPO", 
> Part="po");
>                 Customer PO.cust = XPATH(//customer) of request of 
> (PortType = "POHandlingPT", Operation = "submitPO", Part="po");
>                 Customer CreditCheckRequest.cust = XPATH(//customer) 
> of request of (PortType = "CreditCheckPT", Operation = "check", 
> Part="customerInfo");
>                 int CreditCheckResponse.rating = XPATH("//rank") of 
> response of (PortType = "CreditCheckPT", Operation = "check", 
> Part="Return");
>         }
>
>         Process {
>                 receive request of (PortType = "POHandlingPT", 
> Operation = "submitPO", Part="po") from partner "purchaser"
>                 if ("PO.dollarAmount" > 100K) {
>                         assign "PO.cust" to "CreditCheckRequest.cust";
>                         send request of (PortType = "CreditCheckPT", 
> Operation = "check", Part="customerInfo") to partner 
> "creditCheckProvider";
>                         receive response of (PortType = 
> "CreditCheckPT", Operation = "check", Part="customerInfo") from 
> partner "creditCheckProvider";
>                         if ("CreditCheckResponse.rating" > 3) {
>                                 send request of (PortType = 
> "POCallbackPT", Operation = "confirmPO", Part="poConfirmation") to 
> partner "purchaser"
>                         } else {
>                                 send request of (PortType = 
> "POCallbackPT", Operation = "rejectPO", Part="poRejection") to partner 
> "purchaser"
>                         }
>                 }
>                 send request of (PortType = "POCallbackPT", Operation 
> = "confirmPO", Part="poConfirmation") to partner "purchaser"
>         }
> }
>
> Is this a correct representation of your suggestion ?

Perfect

You're expressing the properties in terms of the abstract message, so 
you can talk to the other partners using any protocol you want, and you 
can also support more concrete messages that extend the abstract 
messages defined by the WSDL operations.

>
> My other comments are embedded ...
>
> At 02:21 PM 4/2/2003 -0800, Assaf Arkin wrote:
>
>> I am in support of the use of properties, but I don't understand the 
>> level of indirection or what it brings to the table.
>>
>> Some properties are interesting to express in the context of a 
>> choreography. Rules that affect the flow (as given in this example), 
>> partners that are communicated, time-outs that are communicated (as 
>> opposed to pre-decided), etc.
>>
>> Let's say that service seller X decides to extract the property in a 
>> very specific way that is different from how service seller Y 
>> extracts it. Any buyer would have to understand the different ways in 
>> which it is extracted in order to put that value in the message to 
>> begin with. So there's another level of complexity here that affects 
>> all the buyers.
>
>
> For a particular buyer talk to a particular seller, he needs to know
> 1) The abstract process A1
> 2) The implementation binding B1
> 3) The protocol binding C1
> 4) The address binding D1

Why do I need to know about the implementation binding?

If I have the protocol binding and address binding I can talk to it. And 
I understand what it does from the abstract definition. Why do I care 
about specific implementation details that do not reflect on the manner 
in which I communicate with that service?

> But I don't see how eliminating the layer 2 will help.  I think it is 
> even worse.  Lets say the seller want to talk to another buyer who use 
> EDI rather than web services, then exchanging ...
> 1) The abstract process A1
> 2) The implementation binding B2
> 3) The protocol binding C2
> 4) The address binding D2
>
> is better than exchanging
> 1) The process A2
> 2) The protocol binding C2
> 3) The address binding D2

Why?

EDI is a protocol. So if the other service talks EDI I need to know the 
EDI protocol binding and the EDI address binding to send an EDI message 
rather than a SOAP message. But if it participates in the choreography, 
it does so regardless of which protocol I use, and regardless of how 
someone chose to implement it.

I don't see where EDI is different from any other protocol that you need 
B2. Can you point to that difference?

>> It's much easier if you write the choreography such that all seller 
>> services (X, Y, future ones) would extract that information in much 
>> the same way, e.g. by referencing the abstract (not concrete) 
>> message. That allows all buyers to build their side of the 
>> choreography in a generic way because they understand the abstract 
>> requirements regardless of which specific service they end up talking 
>> to.
>>
>> In practice, that's how I would write choreographies. Write only the 
>> abstract parts that do not depend on the specific service you end up 
>> using, so you can have one buyer that talks to any potential seller 
>> and vice versa. If I follow that best practice, I don't see any value 
>> for this level of indirection.
>
>
> I completely agree with you that the extra indirection is unnecessary 
> if XML Schema and WSDL are *ABSTRACT ENOUGH*. I post some questions 
> about that in the following but haven't got any feedback yet.  
> http://lists.w3.org/Archives/Public/www-ws-desc/2003Apr/0007.html
>
> Since I'm not sure, so I propose this extra indirection layer that I 
> know for sure will work.

Conceptually I don't see why this problem would exist. You can define 
abstract messages with WSDL, and you can define abstract types with XSDL 
and then extend them in various ways (derivation, any content, 
substitution, etc).  So my gut instict is that it's going to be a wild 
goose chase to work out a solution to a problem that does not exist. I 
would be more interested to see a specific use case which cannot be 
solved using these technologies and then try to tackle, rather then 
spend much time trying to work out a problem that can't be identified.

I'm not saying WSDL and XSDL are perfect. We know there are a lot of 
issues with WSDL 1.1 particularly in relation to abstraction (e.g. 
inheritence is something that needs to be solved). The question is: are 
these issues that cannot possibly solved in the framework of WSDL and 
XSDL? Or are these things that can be solved. In the later case, should 
every specification that depends on WSDL/XSDL work around these 
limitations at the risk of further complication, or can we ask these 
working group to tackle specific problems we have identified.

My personal opinion is that there is no conceptual issue that prevents 
WSDL from solving these problems, and further, that these problems are 
not specific to choreography. Abstraction is not something you care 
about only because you are doing choreography. Abstraction is important 
in a variety of other cases. So that's something the WSD working group 
need to address, and if we can identify a real use case we can give tham 
that information as an input.

Otherwise, we're just spinning a lot of wheels and not getting any work 
done.

arkin

>
> Rgds, Ricky
>
Received on Wednesday, 2 April 2003 23:28:49 UTC

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