- From: Tony Fletcher <tony.fletcher@choreology.com>
- Date: Tue, 13 Jul 2004 11:55:50 +0100
- To: <public-ws-chor@w3.org>
- Cc: "Robin Milner" <Robin.Milner@cl.cam.ac.uk>, "'Kohei Honda'" <kohei@dcs.qmul.ac.uk>, "Nobuko Yoshida" <yoshida@doc.ic.ac.uk>
- Message-ID: <001001c468c7$f62c8570$6501a8c0@corp.choreology.com>
Dear Colleagues, I hope this is a nightmare and someone can wake me up and re-assure me that everything is all right really! I know that Nick (who could answer this concern as one of our main language designers) is on holiday for awhile now, so I have copied this message to your resident experts as well, though Steve, Gary or somebody else may well be able to answer simply. I started writing this email a few days ago before Gary Brown's message on 'Perform' came out. Having now had a look at that I wonder if my concern is a more extreme form of his. Following discussions with Peter last week I have a major concern about how we are designing the choreography description language. I am no mathematician, but my understanding is that pi-calculus describes each 'node' (a partner according to our current definitions) as a process and the allowed message sequences (I think mathematicians call these 'traces') are generated by mathematically 'composing' each node process with the others and letting the maths tell you what the various allowed message exchanges are (etc.) Now I am more familiar with state machines, and I will therefore continue the discussion based on these, but I think the same principles apply to process algebras. Consider a relationship between two nodes that we wish to describe. The usual way to describe this fully suing state machines is to develop a state machine for each end and see how they interact. Suppose one state machine has N states and the other M (N and M will be close to the same integer value but not necessarily equal). So we have had to use N + M (approx 2*N) states to describe this relationship precisely. However, there are altogether N*M (approx N squared) states that the system can be in. Now depending on the whether each of our state machines are relatively simple and only have entries on the diagonal cells (or nearly so) or a very complicated with entries in most cells the number of valid, reachable states may be near to 2N or to N squared). The answer is usually a lot more than 2N even if less than the worst case. Now it is possible, in principle (I think) to have a single state machine that describes the permitted message sequences for the relationship but this is usually only attempted for very simple cases due to the state 'explosion' described above. This single state machine has to represent all the valid states which is between N+M and N*M and usually well above N+M for any 'interesting' protocol. In the current version of CDL we do not seem to be describing the process at each node then letting these interact with the others, but describing the interactions directly. My concern is that this approach will suffer from state space explosion when one tries to include every conceivable message sequence that could happen, and that actually the current CDL may be fine for describing message sequence charts in XML (which is useful but not I thought what we were attempting to achieve) which are fine for illustrating message flows, but do not, in general, cover every possible case the protocol designer needs to allow for, but will become unwieldy / impossible to use for a complete description. I hope the point is clear and that someone can answer it. Best Regards Tony A M Fletcher Cohesions (TM) Business transaction management software for application coordination www.choreology.com <http://www.choreology.com/> Choreology Ltd., 68 Lombard Street, London EC3V 9LJ UK Tel: +44 (0) 1473 729537 Fax: +44 (0) 870 7390077 Mobile: +44 (0) 7801 948219 tony.fletcher@choreology.com (Home: amfletcher@iee.org)
Received on Tuesday, 13 July 2004 08:46:29 UTC