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RE: [Requirements] Non-requirement for MEPs

From: Assaf Arkin <arkin@intalio.com>
Date: Thu, 20 Mar 2003 12:11:35 -0800
To: "Patil, Sanjaykumar" <sanjay.patil@iona.com>
Cc: <public-ws-chor@w3.org>
Message-ID: <IGEJLEPAJBPHKACOOKHNOEPHDFAA.arkin@intalio.com>

> I completely agree that it will be useful for the choreography
> layer to handle the business exceptions (product unavailable,
> quote expired, etc).
>
> The issue I was trying to raise is how to deal with the errors
> that originate in the functional layers  underneath the
> choreography layer, and which necessarily need attention of the
> higher layer (choreography in our case) for making progress.
>
> For example, a choreography employing Reliable Messaging layer is
> NOT interested in knowing when the messages are reliable
> delivered by the RM layer, but it IS interested in knowing if a
> message can not be delivered after all attempts in the RM layer.
> Some of its choreography logic subsequent to the activity of
> sending the message would be assuming successful delivery and may
> prefer to take alternative course in the case of failures.

This is where faults come into play. A fault should express not the business
exception but the fact that the operation could not be completed
successfully. Failure to deliver a message (regardless of cause) is a fault.
So the choreography must be able to accept that such a fault may occur and
propose an alternative path.

If you want to know whether the fault occurred because the last delivery
attempt failed, or the message could not be validated, or there is no route
to host, then you need very precise definition of what each fault means. If
you just treat all these faults as "failed to deliver, regardless of cause",
then simply being able to capture a fault may be sufficient.

WSCI has the notion of a fault handler: one of the participants detects a
fault and decides on an alternative path that is formalized in the
choreography definition, so you know what cause leads to what effect. You
can catch very specific faults, but you can also just catch any fault and
switch to some alternative flow. In the later case you lose the fine-grained
distinction between last retry and no route to host, but you can express the
fact that a fault has occurred and some corrective action will be taken.

Is that something that would meet your requirement?

arkin

>
> For example, a real time broker hunting for the best price of an
> item may ignore the vendors who can not be reached by reliable
> messaging and process the responses from the ones who were
> reachable at the time of the query.
>
> I however agree that not each and every error of the low level
> stack would be of any value to the higher layers. However, those
> errors, which if not handled would necessarily violate the
> assumptions (and therefore interfere with the business logic) of
> the higher layers need to be popped up. Perhaps the choreography
> layer needs to be capable of handling such coarse grained/generic
> errors for the functionality down the stack.
>
> Sanjay Patil
> Distinguished Engineer
> sanjay.patil@iona.com
> -------------------------------------------------------
> IONA Technologies
> 2350 Mission College Blvd. Suite 650
> Santa Clara, CA 95054
> Tel: (408) 350 9619
> Fax: (408) 350 9501
> -------------------------------------------------------
> Making Software Work Together TM
>
>
> -----Original Message-----
> From: Assaf Arkin [mailto:arkin@intalio.com]
> Sent: Wednesday, March 19, 2003 11:58 AM
> To: Patil, Sanjaykumar
> Cc: public-ws-chor@w3.org
> Subject: RE: [Requirements] Non-requirement for MEPs
>
>
>
>
> > -----Original Message-----
> > From: public-ws-chor-request@w3.org
> > [mailto:public-ws-chor-request@w3.org]On Behalf Of Patil, Sanjaykumar
> > Sent: Wednesday, March 19, 2003 10:20 AM
> > To: Assaf Arkin
> > Cc: public-ws-chor@w3.org
> > Subject: RE: [Requirements] Non-requirement for MEPs
> >
> >
> >
> >
> > >> <Assaf>
> > >> I'm not sure there is much value in identifying specific
> > faults, I think
> > >> some coarse distinction will suffice.
> >
> > >> For example, the seller may send a message that the buyer
> > cannot validate.
> > >> The buyer can send back some fault. But the seller may
> > validate the message
> > >> on its side before sending it, determine the fault without
> > having to receive
> > >> it from the buyer. The seller may find that it's
> > implementation is wrong and
> > >> it cannot proceed. It's able to fix the implementation but
> > that may take
> > >> longer than the transaction timeout. So suffice that the
> seller can say
> > >> 'oops, something went wrong, let's decide to cancel the
> transaction'. I
> > >> don't think there's need to delve into the what went wrong in
> > much detail,
> > >> coarse grain would work fine for all the scenarios I've seen.
> > >> </Asaaf>
> >
> > Even to say that "oops, something went wrong, let's decide to
> > cancel the transaction", you need to know the exact type of the
> > error. That is, if you wanted to define the behavior of the
> > system to cancel the transaction for certain types of errors, you
> > are already assuming that you can identify the types of errors.
> > Isn't specifying the type of the error already getting into
> > defining details of the error message!
>
> But I can do that in a very generic way.
>
> It's possible that a message has a well formed error, validation error,
> attachment corrupt error, mandatory header not supported error, or
> decryption failure error. At the choreography level do I care
> which of these
> errors prevented the message from being processed? Or can I just forumlate
> the behavior in terms of a very generic error (like error = SOAP server
> fault) ignoring the details?
>
> This is different from business errors, which may have more
> specific effect
> on the flow, errors like:
>
> - The product is not available, please update your catalog
> - The quoted price is no longer accepted, please obtain a new quote
> - Cannot possibly ship by this date, alternative date provided
>
> You would want to clearly define these errors and use suitable messages.
> Usually these are messages, not specifically faults (in the WSDL
> term). Even
> if you decide to use faults, you can still define these faults precisely
> using WSDL and address them specifically using a fault handler.
>
> The issue here - as I understood from reading Edwin's e-mail - is that the
> faults listed above (invalid, unsupported header, etc) are not
> well defined.
> For the implementation this is an issue, but for the choreography
> I have yet
> to see a use case that distinguishes between failure due to invalid vs
> failure due to malformed. I do see a difference between these generic
> "server faults" and other type of errors, but the other type of errors can
> be well defined by creating specific message definitions for them.
>
> arkin
>
>
> > Once we support certain types of errors,  we may also have to
> > augment the error message definitions with the exact information
> > of the error instance. For example, a content-validation-failed
> > error message can include the details of which element in which
> > document was the culprit.
> >
> > Yes, a coarse grained error type is useful in its own merit and
> > we could include such a category to account for unidentified
> > errors. However, for those functionalities that will be directly
> > supported by the choreography specification, we need to clearly
> > support the relevant error messages also.
> >
> >
> > Sanjay Patil
> > Distinguished Engineer
> > sanjay.patil@iona.com
> > -------------------------------------------------------
> > IONA Technologies
> > 2350 Mission College Blvd. Suite 650
> > Santa Clara, CA 95054
> > Tel: (408) 350 9619
> > Fax: (408) 350 9501
> > -------------------------------------------------------
> > Making Software Work Together TM
>
Received on Thursday, 20 March 2003 15:12:27 GMT

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