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Subject: Re: T2 - Assertions and Questions
Date: Tue, 14 Aug 2001 13:19:31 -0400 From: Martin W Sachs <mwsachs@us.ibm.com> If X intends the message to go to Y but is not concerned with what happens between C and Y, it either isn't reliable messaging. Well, I think I disagree (that it isn't reliable messaging), in the context of the "chasm model". In David S.'s "chasm model", from the point of view of the Message Service protocol, the "To Party" is implemented by two computers, C and Y. The boundaries between C and Y, and the way they divide their responsibilities, are modularly hidden from the Message Service level of abstraction. As far as the Message Service protocol is concerned, it doesn't matter whether C and Y are two CPU's of an SMP multiprocessor, or two nodes of a shared-disk cluster, or two workstations on an Ethernet. From the point of view of MS, there is just one To Party. The CPP for the To Party contains a PartyInfo/Transport/Endpoint element, in which the To Party asserts "this is the URI that I want you to use to send stuff to me". The To Party takes the responsibility of selecting the URI, and it asserts that if the Message Service, using its reliable messaging protocol, delivers the message to that URI, then the To Party promises that it will process the message. Here's an analogy. Suppose I want to cancel my American Express card, and so I send a letter through the U.S. Postal Service, using their "delivery confirmation" feature (http://www.usps.com/shipping/deliver.htm). I address it to whatever address American Express tells me that I should send such things to; I just look at my latest bill and there's this address on it. Now, for all I know, the address is some generic company in the midwest that receives mail for lots of different companies and forwards it. Or maybe that address is Amex's central North American corporate mailroom, which will receive my mail and dynamically figure out which Amex employee (George Foobar) ought to handle my request, and forward it to him, perhaps indirectly through some other route. So there are all kinds of "intermediaries" happening here, entirely invisible to me. Months later, it turns out Amex hasn't cancelled my card, which has caused me to lose money for some reason, so I sue them, and I want to present evidence in court that I really did ask them to cancel my card and they really did receive my mail. I present my "delivery confirmation". Does the judge say "Oh, that delivery confirmation doesn't mean anything at all, because it's not an end-to-end confirmation from George Foobar himself"? I don't think so. From a business point of view, my letter was reliably delivered to the To Party, namely American Express Corp. What happens to it from that point on is not part of the protocol. The main drawback (in a sense) to the "chasm model" is that it takes the position that the protocol doesn't deal with a concept of "intermediaries" at all. Consider Chris F.'s use case, there's an intermittently connected SME (what does SME stand for?) that "uses an intermediary as a way station". In the "chasm model", the SME and the way station would be considered part of a single From Party, and the communication between the two of them would *not be specified* by the Message Service protocol. In the "chasm model", the Message Service protocol relinquishes responsibility for that kind of communication: it's the job of some lower-level protocol that is not specified. So the "chasm model" implies either that the ebXML standard just doesn't standardize or specify how that kind of communication works, or else that ebXML needs to specify a new protocol, at a lower level than Message Services, that would govern this kind of communication. (David S., if I'm not representing your ideas accurately, I apologize and please correct me.) Is this making any sense? -- Dan
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