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Re: Correlation and the need for causality

From: Assaf Arkin <arkin@intalio.com>
Date: Mon, 15 Sep 2003 19:18:48 -0700
Message-ID: <3F667308.7070206@intalio.com>
To: Andrew Berry <andyb@whyanbeel.net>
Cc: public-ws-chor@w3.org

I disagree on the point that the correlation must be done in the 
choreography engine distinct from the application. I don't see how this 
would stick as a general rule.

If the choreography says that customer sends PO to supplier, does that 
mean that the choreography engine is responsible to construct a PO 
message and send it? Without involvement of the application?

In our case there would be an application that handles resources and 
makes the decision to construct and send a message. The choreography 
engine plays a role, but it is still the application that sends the 
message, and does so in compliance with the choreography definition.

In fact, I believe that in the context of choreography we are talking 
about multiple applications interacting with each other, not 
choreography engines. All the interactions I can imagine are based on 
application needs, all the complexity comes from the complexity of the 
application.

arkin

Andrew Berry wrote:

>
> I wanted to raise an issue related to the content of correlation 
> elements in a
> message and thought it probably belonged in a separate thread from the
> discussions on the header/body and CDL/technology-binding issues 
> associated with
> correlation.
>
> One of the key characteristics of a choreography (for me) is that its 
> behaviour
> is intrinsically distributed.  In any distributed application, 
> causality or a
> good approximation of causality must be the basis of any correlation 
> decisions
> otherwise you have the potential for anomalous and often incorrect 
> behaviour if
> causality is not respected.  This is because messages can arrive at a
> destination out of causal order, even if point-to-point ordering is 
> guaranteed.
> For example, if a purchase order (PO) from a new customer must be 
> accompanied by
> a guarantee of credit-worthiness from a trusted third party, you could 
> have
> something like:
>
> 1. Customer sends PO to supplier
> 2. Customer asks third party to validate credit-worthiness to supplier
> 3. Third party sends credit validation directly to supplier
> 4. Supplier receives credit validation from third party
> 5. Supplier receives PO from customer
>
> This is a simple case, but a "dumb" supplier would probably discard the
> credit-worthiness message from the third party because it doesn't know 
> anything
> about the customer, yet the causality is quite clear and implicit in the
> interactions.  It gets more complex when you add the possibilities 
> associated
> with multiple distinct POs.  Here are two ways of solving this problem:
>
> 1. Modify the supplier to hold credit validations until a PO arrives.
>    Problem is, how do you match them when a PO does arrive?  You could 
> use
>    a PO identifier that is unique to the customer.  But how do we specify
>    correlation with application-specific information?  Does it belong 
> in the
>    choreography?  If so, then the CDL needs semantics to specify these
>    correlation rules and the choreography "engine" must be able to 
> interpret
>    them.  If not, the correlation must done by the application so why 
> bother
>    putting it in the CDL at all?
>
>    So, now that we've solved this particular problem, how do we solve 
> the next,
>    different correlation problem?  We need another specific solution 
> with the
>    same problem of representing the correlation logic in the CDL.
>
> 2. Have a CDL semantics with an explicit notion of causality.  Specify 
> that the
>    receipt of an order must be accompanied by a causally-succeeding 
> credit
>    validation in the CDL.  Have an implementation engine or 
> infrastructure that
>    can interpret and/or enforce such constraints.
>
> There may be other ways, but they all eventually fall back to the 
> fundamental
> issue of identifying the actual and required causal relationships between
> messages/events.  The first solution above is doing that in an
> application-specific manner and is not particularly friendly or 
> portable.  I
> would argue, therefore, that the CDL must have a notion of causal 
> relationships
> and the ability to specify required causal relationships.  In an 
> implementation,
> a representation of causality will be the content of any correlation 
> header or
> body element.  There is a considerable body of research in this area 
> but at the
> end of the day you require vector clocks or a reasonable approximation 
> to get
> usable semantics.
>
> Other thoughts?
>
> AndyB
Received on Monday, 15 September 2003 22:34:17 GMT

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