- From: Burdett, David <david.burdett@commerceone.com>
- Date: Tue, 25 Nov 2003 16:57:49 -0800
- To: 'Steve Ross-Talbot' <steve@enigmatec.net>, public-ws-chor@w3.org
- Message-ID: <99F57F955F3EEF4DABA7C88CFA7EB45A0C0C8AAE@c1plenaexm04-b.commerceone.com>
Steve Although I can see the benefit of your ideas on semantics, I also think there is a much more prosaic requriement. Where: 1. Businesses want to integrate with their partners by building links so that they can exchange business documents 2. The links will only be set up if there is some pre-existing business arrangement between them that decides, in principle that they will "do business" 3. The analysts at each business who are going to do the integration need to understand exactly how they should respond to each message building a "private" business process to support it, or alternatively, identifying a business process that already exists that works. 4. They want a common or global definition (i.e. the choreography) of how they will interact with each other so that they can independently build their own business processes with the confidence that they will then be able to interoperate. 5. The Choreography definition they use, must have sufficient explanation in a form (e.g. english) that the analysts involved can understand so that they can do the work successfully. Thoughts? Also see comment in line about Ps and Qs and Rs ... David -----Original Message----- From: Steve Ross-Talbot [mailto:steve@enigmatec.net] Sent: Tuesday, November 25, 2003 6:06 AM To: public-ws-chor@w3.org Subject: Semantics and choreographies What can semantics be used for in a choreography description language? To explore this question we need to define a few basic concepts and terms. I make the distinction between locating a service and a choreography. A service is a Web Service that exhibits a WSDL interface. A choreography is not a Web Service but a contract between two or more services. In order for a service to be part of a choreography it must agree to abide by the contract and so exhibit the required external observable behaviour. Choreographies are non-executable descriptions of observable behaviour. They are loosely bound to the services that they describe which allows services to join and leave a choreography dynamically. A description of a choreography fully elaborates the externally observable message exchanges between a collection of services. Consider a service P that sends a message to service Q and waits for a response while Q sends a message to service R and then continues doing something else but is now able to receive a response from R asynchronously, which is required for Q to respond to P. This is exactly the observable behaviour that a CDL should describe. It includesl all of the dependencies that are externally visible. In this case the response from R back to Q is a dependency for P. So a CDL will describe the visible "interactions" that occur and their visible dependencies. <DB> I'm just wondering. Does this *need* to be one choreography. In a diagram, what you are saying is the following (I think) ... P Q R msg1 ------> (waits) msg 2 ------> (does something) ... <------ msg 3 <------ msg 4 (continues) My question is why does P need to know anything that goes on between Q and R? More specifically, would P's behavior or processing change in any way if the messages between Q and R were done differently, e.g. with Q waiting for R to respond. Although these can be defined in one Choreography, would it be a "better design" if they were defined as two as then if Q and R wanted to change their choreography, then the choreography between P and Q would not need to change. Thoughts? On the other hand, the following would have to be defined in one choreography ... P Q R msg1 ------> (waits) msg 2 ------> (does something) ... <------ msg 3 <-------------------- msg 4 (continues) ... as P needs to be aware of R, unless, of course R was pretending to be Q ;) </DB> A choreography may include certain system level interaction that are also externally observable. To a large extent the external observable behaviour as at the level of a business model as opposed to a system model [aka MDA]. System model behaviour that is observable might be said to be transaction boundaries in either a conventional ACID based approaches or long running transactions of some flavour. System model behaviour might include any observable exceptions that are propagated by the participant services. Service location/selection Semantics in a CDL are most likely to be used in support of a query that may be used to dynamically locate a choreography by a Web Service. This is akin to a conversation (that may be in play already) in which someone enters a room listens for a while and decides they wish to participate or not - in which case they might move to another room and listen in on another conversation and so on. The sort of thing we do at parties. This sort of conversation surfing that we do at parties is a good analogy for the way in which web services might come together to accomplish some user-driven goal. They also surf for conversations that we call choreography descriptions. They evaluate the conversation, not by listening, but by engaging in a short chat to identify if the conversation makes sense for them (i.e. do they understand it at all), that they are not going to get into trouble by entering (i.e. It all sounds too political for me to enter into this conversation) or that they do understand, feel comfortable with but decide that one of the individuals they don't like and so move on. For a Web Service it might look more like the following: I am a Web Service called P. I would like to participate in a choreography to sell paint? (do I understand what they are talking about) I would like to participate in a choreography to sell paint such that the choreography is lockfree? (am I comfortable) I would like to participate in a choreography to sell paint such that the choreography supports a role called "paint mixer"? (do I have a way into this conversation) I would like to participate in a choreography to sell paint such that the choreography supports the an exchange between a paint broker role and a paint shipper role where the broker and shipper never directly communicate to me as a supplier? (I'll only join in if I don't have to talk to ...) I would like to participate in a choreography to sell paint such that the choreography supports a transaction model that is the same as mine? (can I steer it into my comfort zone) I would like to participate in a choreography to sell paint such that the choreography but I need to be sure that it understands the concept of "deferred payment"? (I understand the overall conversation but I never heard of that word before) The bracketed part at the end of each query tries to re-phrase what is being asked in party terms. To be able to support such queries requires that a choreography makes visible the semantics of liveness, roles and allows information about roles and behaviour to be inferred. The query about transaction models might require the use of clear separation into system and conceptual artifacts of a choreography. It might also require some form of behavioural equivalence such a bi-simulation over all of part of the choreography description. The last query, about deferred payment, might be a combination of behavioural equivalence and some form of ontology matching. The ontology matching might be used to find an equivalence relationship between "deferred payment" and "late payment". I am not claiming that we can achieve all of this. What I am trying to do is put some meat on what semantics might mean to a choreography description language. I do like the idea of Web Services locating a choroegraphies with which they can be shown to be compatible and then joining those choreographies in the right way. To do the inferencing example we would be well served if we utilised RDF or something similar. The use of concept matching, "deferred payment" vs "late payment", would probably necessitate the use of ontologies (and so OWL or OWL-light) to express the ontologies and the choreography would make some reference to the ontology that it uses which could then enrich the choreography description with the appropriate concepts. Summary So to summarise I think that there are two areas in which semantics play a role in choreography. Firstly to locate a choreography and check it matches the requirements of a requesting Web Service. Secondly to reuse ontologies and the concepts that they describe as terms in a choreography description linking the description to the choreography. Phew! That is as much as I can write on this topic without making it totally unreadable, which it probably is anyway. Cheers Steve T p.s. As a small anecdote on the importance of semantics and how easy it is to get it wrong: My father worked for the GEC in the UK for many years. He was responsible for QA (both hardware and software). He told me at the weekend that he has asked his PA to draft a letter to be sent to some sub-contracting company that GEC were using at that time. In dictation he said something like: "I would like to see your QA procedures so that I can assess the effectiveness of your QA system." Alas in production the letter sent turned out to be: "I would like to see your QA procedures so that I can assess the effectivemess of your QA system." As you can see even a single letter can change the meaning of things. This email is confidential and may be protected by legal privilege. If you are not the intended recipient, please do not copy or disclose its content but delete the email and contact the sender immediately. Whilst we run antivirus software on all internet emails we are not liable for any loss or damage. The recipient is advised to run their own antivirus software.
Received on Tuesday, 25 November 2003 19:58:12 UTC