Fwd: Semantics and choreography

Jack,

can you read it this time?

Cheers

Steve T


Begin forwarded message:

> From: Steve Ross-Talbot <steve@enigmatec.net>
> Date: Tue Nov 25, 2003  6:39:51  pm Europe/London
> To: public-sws-ig@w3.org
> Subject: Semantics and choreography
>
> These is my own thoughts submitted to the Choreography working group.
>
> We are interested in feedback, suggestions and the like so that we can:
>
> 1. Better understand the issues
> 2. Capture realistic requirements
> 3. Work with other groups to achieve 1 and 2
>
> I would appreciate a reply cc'd to
> 	public-ws-chor@w3.org
>
>
> Cheers
>
> Steve T
> co-chair W3C Web Services Choreography
>
> ----------------------------------------------------------------------- 
> -----------------------------
>
> 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.
>
> 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.