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Re: Decoupling of low-level and high-level MEP

From: Assaf Arkin <arkin@intalio.com>
Date: Mon, 21 Jul 2003 12:48:56 -0700
Message-ID: <3F1C43A8.3060809@intalio.com>
To: Ricky Ho <riho@cisco.com>
CC: public-ws-chor@w3.org

This is still not out-of-bands but a transformation of one protocol into 
another.

Out of band would be:

3) The binding layer (this is what you call the "protocol handler", 
consider it to be the client stub of the POHandlerPT) decides to use the 
RM services.  So it invokdes submitPO on PO-HandlerPT (notice: 
business-level port and operation). But it adds all the relevant RM 
headers which are marked as mustUnderstand="true".
3-4) The implementation of submitPO (at received side, part of 
business-level MEP) receives message. Detecting RM headers which it must 
understand, it asks the RM protocol handler to make sense of them 
*before* forwarding the message to the application. (If it has no RM 
support it rejects the message with a standard SOAP fault)
4) The protocol handler, as per the RM protocol specification, reviews 
the RM headers, persists the message, invokes the sendAck operation on 
the sender side, or anything else it needs to do. Then it delivers the 
message to the application.

Note that the business-level MEP is not changed or transformed in any 
way, it is still executed as is. So a monitoring tool would see a 
message exchange as specified in the business-level MEP -- important for 
management purposes or users would have no clue what has just transpired.

The sendAck operation is indeed defined by some RM-ReceiverPT that is 
specified in the RM specification. That same porttype/operation is used 
anywhere RM is used.

arkin

Ricky Ho wrote:

>
> Arkin, your idea here is very interesting to me.  I try to use an 
> example to confirm my understanding ...
>
> Business-level MEP
> ===============
> - Submit PO
> - A one-way message (no response is expected at the business level)
>
> MEP SubmitPO {
>         role: buyer, seller (implements PO-HandlerPT)
>         sequence {
>                 buyer invoke seller (operation = submitPO, portType = 
> PO-HandlerPT)
>         }
> }
>
> PortType PO-HandlerPT {
>         Operation submitPO {
>                 Input: (PO)
>         }
> }
>
> However, since the PO must be send reliably (ie, it has certain QOS 
> requirement).  Therefore, at the implementation level, this 
> "business-layer MEP" is build on the RM MEP
>
> Infrastructure-level MEP
> ==================
>
> MEP RM {
>     role: sender (implements RM-SenderPT), receiver (implements 
> RM-ReceiverPT)
>     sequence {
>         while (no Ack received) {
>             sender invoke receiver (operation = sendMessage, portType 
> = RM-ReceiverPT)
>             switch {
>                 case1: {
>                     receiver invoke sender (operation = sendAck, 
> portType = RM-SenderPT)
>                 }
>                 case2: {
>                     timeout
>                 }
>             }
>         }
>     }
> }
>
> PortType RM-ReceiverPT {
>         Operation sendMessage {
>                 Input: (Message, Ack-CallbackAddress)
>         }
> }
>
> PortType RM-SenderPT {
>         Operation sendAck {
>                 Input: (Ack)
>         }
> }
>
> Here is what happens at runtime ...
>
> 1) The buyer engage in the SubmitPO MEP when it follows the 
> choreography definition and invoke the "submitPO" operation.
> 2) There is separate QOS requirement specified by the buyer (maybe 
> communicate out of band) that the operation must be done reliably.
> 3) The binding layer (this is what you call the "protocol handler", 
> consider it to be the client stub of the POHandlerPT) decides to use 
> the RM services.  So it invoke the "sendMessage" operation of the 
> RM-ReceiverPT instead (the client stub may encapsulate the business 
> SOAP message inside the RM SOAP message).
> 4) The implementation of sendMessage (at receiver side, which you call 
> the "protocol handler") invoke the sendAck operation of the sender 
> side.  At the same time, it also invoke the "submitPO" operation of 
> the receiver side.
>
> The key point here is that the binding between the business-level-MEP 
> and RM-MEP is captured "somewhere else" (what you call "out-of-band"), 
> so that they can essentially decoupled with each other.  This 
> "decoupling" introduce tremendously reusability and flexibility and 
> still retain simplicity.
>
> Do I accurately rephrase what you mean ?
>
> Rgds, Ricky
Received on Monday, 21 July 2003 15:59:11 GMT

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