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RE: New Paper available for PDF download: Workflow is just a Pi process (or WFM is not BPM)

From: Jean-Jacques Dubray <jeanjadu@Attachmate.com>
Date: Mon, 17 Nov 2003 08:17:58 -0800
Message-ID: <D15269CBED76D51185280008C73323FA023E9484@exch-bel6.attachmate.com>
To: 'Andrew Berry' <andyb@whyanbeel.net>
Cc: public-ws-chor@w3.org

I like your point and I am not trying to disagree with it. John Yunker made
a similar comment in ebBP, as a possible evolution to ebXML BPSS which does
not yet have this notion of partial state or "distributed shared states".
(it can be emulated, but the idea is to make them first class citizens of
the spec).

I think your point also raises a fundamental question, which I thought I
knew the answer, but I now have some doubts.

I thought up until now that a choreography (or even an orchestration) of
messages could be expressed, in abstraction of all other concepts like
partial state. In particular, I am convinced that as soon as you bring
partial state in, you also have to bring something like an "object" or a
business object into the mix (PO.paid), as most of the states are often
related to the state of such object (e.g. purchase order paid). Then where
does it ends, is the "B" layer the lowest common denominator to SOA?

So I believed that there is this pi-underground world (that has nothing to
do with the "B" or "W" words). At this level, we deal only with message
exchange (the "state" of the choreography can be derived, not only from the
messages that have been exchanged, but also from their content, described in
a very abstract and atomic way). One can formally demonstrate that a
choreography is a state machine in the mathematical sense. 

Unfortunately, if other concepts are needed at the lowest level, then pi
might not work. I can see the wonders it does to model a variable or a const
as a process, but unfortunately this is of limited value to solve the "C",
"B" and "W" problems. 

What is your opinion on this? Reading your comment, I would conclude that a
pi-layer is not possible or are you saying that a message-layer is possible
but pi is not the right formalism for that. 


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-----Original Message-----
From: Andrew Berry [mailto:andyb@whyanbeel.net] 
Sent: Monday, November 17, 2003 2:59 AM
To: Howard N Smith
Cc: public-ws-chor@w3.org; W.M.P.v.d.Aalst@tm.tue.nl
Subject: Re: New Paper available for PDF download: Workflow is just a Pi
process (or WFM is not BPM)


You have a fundamental problem with the choice of Pi Calculus: there is no
concept of locality or partial state. In choreography and web services in
general, you can guarantee that participants (processes) are physically
distributed and need to make choices based on a partial view of state.  To
successfully model, program and reason about these processes, you need to be
able to identify and reason about partial states.

Consider your deferred choice semantics.  If the processes identified as
choices are physically distributed, you *cannot* make a choice without
synchronisation of processes because distinct choices can be made in a truly
concurrent fashion.  Pi Calculus has no way of identifying this issue, let
alone reasoning about it.  Explicit synchronisation processes, while solving
the problem for a given process, require that the programmer reason about
distribution and locality outside the bounds of the Pi Calculus semantics.
I would therefore argue that a worflow and in particular a choreography is
not a Pi Process.



On Wednesday, November 12, 2003, at 03:00  AM, Howard N Smith wrote:

> Choreography pioneers,
> Following a short conversation with Steve R-T, he agreed for me to 
> send you this paper.
> It is intended as a draft for discussion.
> The paper is new information. It shows how, based on BPML, it is 
> possible to model all of the advanced workflow patterns identified by 
> workflow theorists, whereas most workflow engines only support approx 
> 50% of patterns directly and very few of the advanced patterns.
> In addition, it gives insights into the BPML implementation inherent 
> to a BPMS, and how a BPMS is able to support many process models not 
> supported by workflow technology.
> Screenshots from Intalio|n3 BPMS are given as examples. Further, the 
> workflow engine itself can be modelled in BPML, as reusable processes 
> for use in end-to-end processes. The paper was written to more fully 
> explain the work of BPMI.org and its direction in creating BPMS 
> foundation technologies.
> Peter Fingar and I have taken great care with this paper, and do hope 
> it adds to the understanding of BPML/BPMI/BPMS direction. While the 
> paper cannot present proof of these claims, you can consider it a 
> report on the work so far.
> The paper can be downloaded from:
> http://www.bpm3.com/picalculus/workflow-is-just-a-pi-process.pdf
> Regards,
> Howard
> ---
> New Book - Business Process Management: The Third Wave www.bpm3.com
> Howard Smith/CSC/BPMI.org
> cell             +44 7711 594 494 (worldwide)
> home office +44 20 8660 1963
Received on Monday, 17 November 2003 11:23:35 UTC

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